Posted by Scott
How is a leak down test performed and what purpose does it serve?
Thanks,
JSA
Leakdown test
Re: Leakdown test
Posted by Lee
Leakdown tests are more common in the aircraft business. The engine is rotated to TDC for the cylinder being checked, and an adapter is installed in the spark plug hole. A tool with an air pressure regulator and two gauges is hooked up to the cylinder and to shop air. The regulator is used to set a fixed value (usually 80 lb) on the first gauge. The second gauge will show a lower value. The difference is the amount of leakage in the cylinder. If air is heard escaping through the intake or exhaust, the valves are usually at fault. Air coming from the crankcase vent indicates bad rings. A reading of 70# or better is generally ok. This test is tougher to do on an auto engine, as there isn't a nice six-foot long handle (the propeller) to hang on to, and if the engine isn't exactly at TDC, the air will cause the engine to motor over. I feel that a cranking compression test is more useful for auto engines.
Leakdown tests are more common in the aircraft business. The engine is rotated to TDC for the cylinder being checked, and an adapter is installed in the spark plug hole. A tool with an air pressure regulator and two gauges is hooked up to the cylinder and to shop air. The regulator is used to set a fixed value (usually 80 lb) on the first gauge. The second gauge will show a lower value. The difference is the amount of leakage in the cylinder. If air is heard escaping through the intake or exhaust, the valves are usually at fault. Air coming from the crankcase vent indicates bad rings. A reading of 70# or better is generally ok. This test is tougher to do on an auto engine, as there isn't a nice six-foot long handle (the propeller) to hang on to, and if the engine isn't exactly at TDC, the air will cause the engine to motor over. I feel that a cranking compression test is more useful for auto engines.
Re: Leakdown test
Posted by Scott
I've read a lot of advice saying a leak down test is a necessary part of a prepurchase inspection but Ferrari mechanic here in Dallas only mentioned compression test.  
I've read a lot of advice saying a leak down test is a necessary part of a prepurchase inspection but Ferrari mechanic here in Dallas only mentioned compression test.  
Re: Leakdown test
Posted by Scott
I've read a lot of advice saying a leak down test is a necessary part of a prepurchase inspection but Ferrari mechanic here in Dallas only mentioned compression test.  
I've read a lot of advice saying a leak down test is a necessary part of a prepurchase inspection but Ferrari mechanic here in Dallas only mentioned compression test.  
Re: Leakdown test
Posted by Tom
Hi Scott,
Lee gives a good description, but cars in gear do a pretty good job in keeping the the engine from turning over.
I feel that both tests, leakdown, and compression, tell important information on the condition of an engine. Although some of the numbers may seem to tell the same thing, there are subtle differences. A compression check is far easier to do, so mechanics will often only give those numbers, but knowing where the compressed air is leaking from helps tell what may need fixing in a leak down test. There are also tricks mechanics can use to sway the numbers, like squirting oil in the cylinders to improve worn rings, so depending on who you trust, be careful!
Tom
Hi Scott,
Lee gives a good description, but cars in gear do a pretty good job in keeping the the engine from turning over.
I feel that both tests, leakdown, and compression, tell important information on the condition of an engine. Although some of the numbers may seem to tell the same thing, there are subtle differences. A compression check is far easier to do, so mechanics will often only give those numbers, but knowing where the compressed air is leaking from helps tell what may need fixing in a leak down test. There are also tricks mechanics can use to sway the numbers, like squirting oil in the cylinders to improve worn rings, so depending on who you trust, be careful!
Tom
Re: Leakdown test (long)
Posted by Alex
Scott,
Lee's description of the leakdown test is good however, the commercial leakdown testers I'm familiar with (Snap-on and Sun) will convert the pressure loss into a %leakdown value on the secondary gauge. Many good auto mechanics own leakdown testers (they aren't too expensive). I've spent many years developing engines in the auto industry and the leakdown test is a standard test we perform whenever we do a compression test on development engines (every maintenance interval).
The leakdown test is a much better (more precise) test for detecting valve leakage. For a brand new engine, leakdown % of between 4%-8% is normal. We like to see values less than about 25% for a used engine (at 100,000 miles). When you start to see #'s up around 40% or higher, that's a good sign of a major valve leakage problem that will probably eventually lead to a burned exhaust valve (most leakage problems are usually on the exhaust side) or possibly a valve head failure. One reason that the leakdown test is more effective for valve leakage is that the compression test can often be well within acceptable limits and leakage could be 40 or 50%! For example, I've seen engines that only lost about 10% of their compression value from new, that had leakage above 40%.
As Lee pointed out, this test is much more time consuming so the typical garage mechanic is going to be reluctant to perform it (particularly on a V-12). This is even more difficult on the Ferrari because finding TDC compression through the spark plug hole can prove difficult with a dial indicator or simple probe (due to the angle of the spark plug hole relative to the piston top). I've heard that someone makes a "whistle probe" that you stick in the spark plug hole and it whistles while the piston is pushing out the air and that it changes tone and then goes silent as TDC is reached.
To Tom's comment about putting oil in the cylinder: This is actually a standard diagnostic tool that is commonly used by mechanics to determine if the leakage (or low compression) is due to ring leakage or valve leakage. The idea being that a couple of squirts of oil in the cylinder will help seal the rings for the compression or leakdown test. Of course this should only be done after a dry compression test.
As Lee pointed out, you have to be very careful that the engine is exactly at TDC when conducting the leakdown test, and it has to be on the compression stroke. It is easy for an inexperienced mechanic to do the test at TDC exhaust (with both valves slightly open) and report a 90+% leakdown!
I do highly recommend this test if you really want to find out the condition of the valves and seats. This test can sometimes be misleading and you have to be careful. So if you get an "oddball" high reading on one or two cylinders, it's a good idea to run the engine for a few minutes and repeat the test on those cylinders. The reason is that it's possible a small chunk of carbon could have flaked off the piston top or chamber and gotten caught between the valve and seat, hanging the valve open ever so slightly.
A compression test can also be somewhat unreliable as the results can change quite a bit with the temperature of the engine (should be warm) and the cranking speed. And of course, compression tests should always be run with the throttles open!
Scott,
Lee's description of the leakdown test is good however, the commercial leakdown testers I'm familiar with (Snap-on and Sun) will convert the pressure loss into a %leakdown value on the secondary gauge. Many good auto mechanics own leakdown testers (they aren't too expensive). I've spent many years developing engines in the auto industry and the leakdown test is a standard test we perform whenever we do a compression test on development engines (every maintenance interval).
The leakdown test is a much better (more precise) test for detecting valve leakage. For a brand new engine, leakdown % of between 4%-8% is normal. We like to see values less than about 25% for a used engine (at 100,000 miles). When you start to see #'s up around 40% or higher, that's a good sign of a major valve leakage problem that will probably eventually lead to a burned exhaust valve (most leakage problems are usually on the exhaust side) or possibly a valve head failure. One reason that the leakdown test is more effective for valve leakage is that the compression test can often be well within acceptable limits and leakage could be 40 or 50%! For example, I've seen engines that only lost about 10% of their compression value from new, that had leakage above 40%.
As Lee pointed out, this test is much more time consuming so the typical garage mechanic is going to be reluctant to perform it (particularly on a V-12). This is even more difficult on the Ferrari because finding TDC compression through the spark plug hole can prove difficult with a dial indicator or simple probe (due to the angle of the spark plug hole relative to the piston top). I've heard that someone makes a "whistle probe" that you stick in the spark plug hole and it whistles while the piston is pushing out the air and that it changes tone and then goes silent as TDC is reached.
To Tom's comment about putting oil in the cylinder: This is actually a standard diagnostic tool that is commonly used by mechanics to determine if the leakage (or low compression) is due to ring leakage or valve leakage. The idea being that a couple of squirts of oil in the cylinder will help seal the rings for the compression or leakdown test. Of course this should only be done after a dry compression test.
As Lee pointed out, you have to be very careful that the engine is exactly at TDC when conducting the leakdown test, and it has to be on the compression stroke. It is easy for an inexperienced mechanic to do the test at TDC exhaust (with both valves slightly open) and report a 90+% leakdown!
I do highly recommend this test if you really want to find out the condition of the valves and seats. This test can sometimes be misleading and you have to be careful. So if you get an "oddball" high reading on one or two cylinders, it's a good idea to run the engine for a few minutes and repeat the test on those cylinders. The reason is that it's possible a small chunk of carbon could have flaked off the piston top or chamber and gotten caught between the valve and seat, hanging the valve open ever so slightly.
A compression test can also be somewhat unreliable as the results can change quite a bit with the temperature of the engine (should be warm) and the cranking speed. And of course, compression tests should always be run with the throttles open!
Re: Leakdown test (long)
Posted by Tom
Hi Alex,
As always, good information!
My point about oil in the cylinder should be clarified. Know what numbers came back before and after the oil is added. You can imagine how someone can be tricked to see better numbers, especially when a shop is motivated to sell a car! Again, and I can't stress this enough, use a third party mechanic to inspect a car!!!
Tom
Hi Alex,
As always, good information!
My point about oil in the cylinder should be clarified. Know what numbers came back before and after the oil is added. You can imagine how someone can be tricked to see better numbers, especially when a shop is motivated to sell a car! Again, and I can't stress this enough, use a third party mechanic to inspect a car!!!
Tom
Re: Leakdown test. Original Engine Concerns
Posted by Scott
Guys:
I am now thoroughly informed about the leak down test. My Ferrari education on this board continues!
I talked to someone who is selling a 1966 330 with 60k miles. The engine is completely original - it has never been disassembled at all - cylinder heads never removed. He says it smokes "a little" at idle. Without knowing any more than this, would the fact that the engine has never been opened be a red light for concern? I have been reading here and else where about cylinder heads bonding to the engine block on old V12's. Does this always happen?
Thanks
JSA
Guys:
I am now thoroughly informed about the leak down test. My Ferrari education on this board continues!
I talked to someone who is selling a 1966 330 with 60k miles. The engine is completely original - it has never been disassembled at all - cylinder heads never removed. He says it smokes "a little" at idle. Without knowing any more than this, would the fact that the engine has never been opened be a red light for concern? I have been reading here and else where about cylinder heads bonding to the engine block on old V12's. Does this always happen?
Thanks
JSA
Re: Leakdown test. Original Engine Concerns
Posted by Scott
Guys:
I am now thoroughly informed about the leak down test. My Ferrari education on this board continues!
I talked to someone who is selling a 1966 330 with 60k miles. The engine is completely original - it has never been disassembled at all - cylinder heads never removed. He says it smokes "a little" at idle. Without knowing any more than this, would the fact that the engine has never been opened be a red light for concern? I have been reading here and else where about cylinder heads bonding to the engine block on old V12's. Does this always happen?
Thanks
JSA
Guys:
I am now thoroughly informed about the leak down test. My Ferrari education on this board continues!
I talked to someone who is selling a 1966 330 with 60k miles. The engine is completely original - it has never been disassembled at all - cylinder heads never removed. He says it smokes "a little" at idle. Without knowing any more than this, would the fact that the engine has never been opened be a red light for concern? I have been reading here and else where about cylinder heads bonding to the engine block on old V12's. Does this always happen?
Thanks
JSA
Re: Leakdown test. Original Engine Concerns
Posted by Tom
Hi Scott,
I watched Francois rebuild four V-12 engines in the last two years, and two of them, that had original miles and had never been apart, had stuck heads. There is a method, and special tool to use to remove this head, so when it's time for a rebuld, there's hope! I'm assuming you read Jonathon Brent's story about his head puller? http://www.tomyang.com/cars/ferrari.html?headpuller.htm
Some engines get a few more miles that others before the valve guides begin to wear, but unfortunately, 50K miles is where the first signs of smoking begins to rear its ugly head. Please note however, that this was design flaw in the original material used for the guides, and the modern replacement should last longer!
If the seller says it smokes "a little," I can translate that as well as when a seller says "a little rust!" I would go and see for myself what "a little" smoke looks like!
In NY, where I live, we don't have emissions testing for these old cars, but other States may vary. Considering which State you live in, the burning oil may not pass an emissions test, if your State requires one. One owner I know can register his car in CA, but actually gets pulled over by the police because his car smokes so badly! Damn Tree Huggers!
Tom
Hi Scott,
I watched Francois rebuild four V-12 engines in the last two years, and two of them, that had original miles and had never been apart, had stuck heads. There is a method, and special tool to use to remove this head, so when it's time for a rebuld, there's hope! I'm assuming you read Jonathon Brent's story about his head puller? http://www.tomyang.com/cars/ferrari.html?headpuller.htm
Some engines get a few more miles that others before the valve guides begin to wear, but unfortunately, 50K miles is where the first signs of smoking begins to rear its ugly head. Please note however, that this was design flaw in the original material used for the guides, and the modern replacement should last longer!
If the seller says it smokes "a little," I can translate that as well as when a seller says "a little rust!" I would go and see for myself what "a little" smoke looks like!
In NY, where I live, we don't have emissions testing for these old cars, but other States may vary. Considering which State you live in, the burning oil may not pass an emissions test, if your State requires one. One owner I know can register his car in CA, but actually gets pulled over by the police because his car smokes so badly! Damn Tree Huggers!
Tom
Re: Leakdown test. Original Engine Concerns
Posted by Tom
Hi Scott,
I watched Francois rebuild four V-12 engines in the last two years, and two of them, that had original miles and had never been apart, had stuck heads. There is a method, and special tool to use to remove this head, so when it's time for a rebuld, there's hope! I'm assuming you read Jonathon Brent's story about his head puller? http://www.tomyang.com/cars/ferrari.html?headpuller.htm
Some engines get a few more miles that others before the valve guides begin to wear, but unfortunately, 50K miles is where the first signs of smoking begins to rear its ugly head. Please note however, that this was design flaw in the original material used for the guides, and the modern replacement should last longer!
If the seller says it smokes "a little," I can translate that as well as when a seller says "a little rust!" I would go and see for myself what "a little" smoke looks like!
In NY, where I live, we don't have emissions testing for these old cars, but other States may vary. Considering which State you live in, the burning oil may not pass an emissions test, if your State requires one. One owner I know can register his car in CA, but actually gets pulled over by the police because his car smokes so badly! Damn Tree Huggers!
Tom
Hi Scott,
I watched Francois rebuild four V-12 engines in the last two years, and two of them, that had original miles and had never been apart, had stuck heads. There is a method, and special tool to use to remove this head, so when it's time for a rebuld, there's hope! I'm assuming you read Jonathon Brent's story about his head puller? http://www.tomyang.com/cars/ferrari.html?headpuller.htm
Some engines get a few more miles that others before the valve guides begin to wear, but unfortunately, 50K miles is where the first signs of smoking begins to rear its ugly head. Please note however, that this was design flaw in the original material used for the guides, and the modern replacement should last longer!
If the seller says it smokes "a little," I can translate that as well as when a seller says "a little rust!" I would go and see for myself what "a little" smoke looks like!
In NY, where I live, we don't have emissions testing for these old cars, but other States may vary. Considering which State you live in, the burning oil may not pass an emissions test, if your State requires one. One owner I know can register his car in CA, but actually gets pulled over by the police because his car smokes so badly! Damn Tree Huggers!
Tom
Re: Leakdown test. Original Engine Concerns
Posted by Scott
Hi Tom:
Yes, I have read the article about the head puller. That cleared it up for me - I thought it was some kind of long term electrolytic process involving the head gaskets, rather than stud corrosion.
The mechanic selling the 330 told me $2k to $3k for a valve job. That sounds way on the low side to me unless only the absolute basic
reconditioning work were done, most of the parts were reuseable, and there were no other complications. Mechanic here in Dallas told me $10k for a valve job - I was too shocked to ask what was included (or rather excluded).
In Texas, if your car is over 25 yrs. all you have to do is get original Texas license plates in good condition for the year of your car and you can register it as a classic - no yearly inspections or emission test required. I don't
know what ex-Gov. Bush had to do with that but it sounds like something he would approve of.
Later,
JSA
Hi Tom:
Yes, I have read the article about the head puller. That cleared it up for me - I thought it was some kind of long term electrolytic process involving the head gaskets, rather than stud corrosion.
The mechanic selling the 330 told me $2k to $3k for a valve job. That sounds way on the low side to me unless only the absolute basic
reconditioning work were done, most of the parts were reuseable, and there were no other complications. Mechanic here in Dallas told me $10k for a valve job - I was too shocked to ask what was included (or rather excluded).
In Texas, if your car is over 25 yrs. all you have to do is get original Texas license plates in good condition for the year of your car and you can register it as a classic - no yearly inspections or emission test required. I don't
know what ex-Gov. Bush had to do with that but it sounds like something he would approve of.
Later,
JSA
Re: Leakdown test. Original Engine Concerns
Posted by Scott
Hi Tom:
Yes, I have read the article about the head puller. That cleared it up for me - I thought it was some kind of long term electrolytic process involving the head gaskets, rather than stud corrosion.
The mechanic selling the 330 told me $2k to $3k for a valve job. That sounds way on the low side to me unless only the absolute basic
reconditioning work were done, most of the parts were reuseable, and there were no other complications. Mechanic here in Dallas told me $10k for a valve job - I was too shocked to ask what was included (or rather excluded).
In Texas, if your car is over 25 yrs. all you have to do is get original Texas license plates in good condition for the year of your car and you can register it as a classic - no yearly inspections or emission test required. I don't
know what ex-Gov. Bush had to do with that but it sounds like something he would approve of.
Later,
JSA
Hi Tom:
Yes, I have read the article about the head puller. That cleared it up for me - I thought it was some kind of long term electrolytic process involving the head gaskets, rather than stud corrosion.
The mechanic selling the 330 told me $2k to $3k for a valve job. That sounds way on the low side to me unless only the absolute basic
reconditioning work were done, most of the parts were reuseable, and there were no other complications. Mechanic here in Dallas told me $10k for a valve job - I was too shocked to ask what was included (or rather excluded).
In Texas, if your car is over 25 yrs. all you have to do is get original Texas license plates in good condition for the year of your car and you can register it as a classic - no yearly inspections or emission test required. I don't
know what ex-Gov. Bush had to do with that but it sounds like something he would approve of.
Later,
JSA
Re: Leakdown test. Original Engine Concerns
Posted by Tom
Hi Scott,
$2K-3K is a little low, considering how much you should have done while the engine is out. I would also assume for that kind of money, he may not even pull the engine, and try to do it in the car. It's possible to do, but a real pain in the ass!
Just to remove the engine, the transmission needs to come out - from the inside of the car, which means at least the seats, and center console comes out. The usual things will need to be removed from the engine compartment for the engine to come out from fuel and electrical lines, to radiator and air cond (if it has it).
With the engine out, and the heads off, they'll need to machined to get the new guides to fit. On a 50K mile motor, inevitably some of the valves will show wear from the tappets, and should be replaced. The rest of the valve train will need to be inspected as well and replaced if needed. These days, Francois recommends simply replacing all the valves with new ones because with the labor involved in removing them all to fit new guides, it doesn't make sense to put the old ones back.
With the heads off, it's also a good time to replace the timing chain. Even on a push rod V-8, it makes sense to replace this part from chain stretch, so you can imagine how much a single overhead cam V-12 chain can stretch!
There are a whole buch of other wear items at this point of your "valve job" you should consider addressing since it took so much trouble (and labor) to get here. What kind of oil pressure did the engine have before it was taken out? Should the bottom end be inspected before everything is returned. There is only worry when you don't know the condition of the rest of the engine. Are you sure the smoking problem was only from the valve guides? Worn rings will also contribute to this problem. How many miles over 50-60K will you get on this engine after this "valve job" before the rest of the motor needs attention, and this process is repeated (have you priced head gaskets?).
Another point to consider is how will the mechanic "time" the cams in if he doesn't remove the engine from the car on this $2-$3K "valve job." These engines, though built in the 60s, are basically race engines requiring a certain amount of accuracy when being assembled. The timing marks are on the flywheel, which are hard to see when the engine is in the car. I've watched my engine being "timed" in on the bench, and there is a certain "art" to doing it, and leaning over a fender, peering into an inspection hole can only upset the accuracy of the work. Of course the mechanic can just line everything up to the factory marks, but there are reasons why these marks might not be the best place to set the cam.
I hope this sheds some light on the descrepancy in pricing!
Tom
Hi Scott,
$2K-3K is a little low, considering how much you should have done while the engine is out. I would also assume for that kind of money, he may not even pull the engine, and try to do it in the car. It's possible to do, but a real pain in the ass!
Just to remove the engine, the transmission needs to come out - from the inside of the car, which means at least the seats, and center console comes out. The usual things will need to be removed from the engine compartment for the engine to come out from fuel and electrical lines, to radiator and air cond (if it has it).
With the engine out, and the heads off, they'll need to machined to get the new guides to fit. On a 50K mile motor, inevitably some of the valves will show wear from the tappets, and should be replaced. The rest of the valve train will need to be inspected as well and replaced if needed. These days, Francois recommends simply replacing all the valves with new ones because with the labor involved in removing them all to fit new guides, it doesn't make sense to put the old ones back.
With the heads off, it's also a good time to replace the timing chain. Even on a push rod V-8, it makes sense to replace this part from chain stretch, so you can imagine how much a single overhead cam V-12 chain can stretch!
There are a whole buch of other wear items at this point of your "valve job" you should consider addressing since it took so much trouble (and labor) to get here. What kind of oil pressure did the engine have before it was taken out? Should the bottom end be inspected before everything is returned. There is only worry when you don't know the condition of the rest of the engine. Are you sure the smoking problem was only from the valve guides? Worn rings will also contribute to this problem. How many miles over 50-60K will you get on this engine after this "valve job" before the rest of the motor needs attention, and this process is repeated (have you priced head gaskets?).
Another point to consider is how will the mechanic "time" the cams in if he doesn't remove the engine from the car on this $2-$3K "valve job." These engines, though built in the 60s, are basically race engines requiring a certain amount of accuracy when being assembled. The timing marks are on the flywheel, which are hard to see when the engine is in the car. I've watched my engine being "timed" in on the bench, and there is a certain "art" to doing it, and leaning over a fender, peering into an inspection hole can only upset the accuracy of the work. Of course the mechanic can just line everything up to the factory marks, but there are reasons why these marks might not be the best place to set the cam.
I hope this sheds some light on the descrepancy in pricing!
Tom
Re: Leakdown test. Original Engine Concerns
Posted by Tom
Hi Scott,
$2K-3K is a little low, considering how much you should have done while the engine is out. I would also assume for that kind of money, he may not even pull the engine, and try to do it in the car. It's possible to do, but a real pain in the ass!
Just to remove the engine, the transmission needs to come out - from the inside of the car, which means at least the seats, and center console comes out. The usual things will need to be removed from the engine compartment for the engine to come out from fuel and electrical lines, to radiator and air cond (if it has it).
With the engine out, and the heads off, they'll need to machined to get the new guides to fit. On a 50K mile motor, inevitably some of the valves will show wear from the tappets, and should be replaced. The rest of the valve train will need to be inspected as well and replaced if needed. These days, Francois recommends simply replacing all the valves with new ones because with the labor involved in removing them all to fit new guides, it doesn't make sense to put the old ones back.
With the heads off, it's also a good time to replace the timing chain. Even on a push rod V-8, it makes sense to replace this part from chain stretch, so you can imagine how much a single overhead cam V-12 chain can stretch!
There are a whole buch of other wear items at this point of your "valve job" you should consider addressing since it took so much trouble (and labor) to get here. What kind of oil pressure did the engine have before it was taken out? Should the bottom end be inspected before everything is returned. There is only worry when you don't know the condition of the rest of the engine. Are you sure the smoking problem was only from the valve guides? Worn rings will also contribute to this problem. How many miles over 50-60K will you get on this engine after this "valve job" before the rest of the motor needs attention, and this process is repeated (have you priced head gaskets?).
Another point to consider is how will the mechanic "time" the cams in if he doesn't remove the engine from the car on this $2-$3K "valve job." These engines, though built in the 60s, are basically race engines requiring a certain amount of accuracy when being assembled. The timing marks are on the flywheel, which are hard to see when the engine is in the car. I've watched my engine being "timed" in on the bench, and there is a certain "art" to doing it, and leaning over a fender, peering into an inspection hole can only upset the accuracy of the work. Of course the mechanic can just line everything up to the factory marks, but there are reasons why these marks might not be the best place to set the cam.
I hope this sheds some light on the descrepancy in pricing!
Tom
Hi Scott,
$2K-3K is a little low, considering how much you should have done while the engine is out. I would also assume for that kind of money, he may not even pull the engine, and try to do it in the car. It's possible to do, but a real pain in the ass!
Just to remove the engine, the transmission needs to come out - from the inside of the car, which means at least the seats, and center console comes out. The usual things will need to be removed from the engine compartment for the engine to come out from fuel and electrical lines, to radiator and air cond (if it has it).
With the engine out, and the heads off, they'll need to machined to get the new guides to fit. On a 50K mile motor, inevitably some of the valves will show wear from the tappets, and should be replaced. The rest of the valve train will need to be inspected as well and replaced if needed. These days, Francois recommends simply replacing all the valves with new ones because with the labor involved in removing them all to fit new guides, it doesn't make sense to put the old ones back.
With the heads off, it's also a good time to replace the timing chain. Even on a push rod V-8, it makes sense to replace this part from chain stretch, so you can imagine how much a single overhead cam V-12 chain can stretch!
There are a whole buch of other wear items at this point of your "valve job" you should consider addressing since it took so much trouble (and labor) to get here. What kind of oil pressure did the engine have before it was taken out? Should the bottom end be inspected before everything is returned. There is only worry when you don't know the condition of the rest of the engine. Are you sure the smoking problem was only from the valve guides? Worn rings will also contribute to this problem. How many miles over 50-60K will you get on this engine after this "valve job" before the rest of the motor needs attention, and this process is repeated (have you priced head gaskets?).
Another point to consider is how will the mechanic "time" the cams in if he doesn't remove the engine from the car on this $2-$3K "valve job." These engines, though built in the 60s, are basically race engines requiring a certain amount of accuracy when being assembled. The timing marks are on the flywheel, which are hard to see when the engine is in the car. I've watched my engine being "timed" in on the bench, and there is a certain "art" to doing it, and leaning over a fender, peering into an inspection hole can only upset the accuracy of the work. Of course the mechanic can just line everything up to the factory marks, but there are reasons why these marks might not be the best place to set the cam.
I hope this sheds some light on the descrepancy in pricing!
Tom