Strange piston to valve clearance issues
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Teching In
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Strange piston to valve clearance issues Hey everyone, thanks in advance for taking a look. This has been a bit of a headscratcher for me so far. I'm putting together an LH6 5.3L for a 2,000bls Porsche race car. I have noticed that my piston to valve clearance on two separate cams are way less than what they are expected to be.
The first cam was a Comp Cams 228/230, .571/.573. With a 1.8 rocker the lift numbers were 604 and 606. This is essentially a stage 2 camshaft and should have no issue with clearance. However, after measuring with test springs on cylinder 1 and clay with cylinder 3 I found that my PVC was .005 on the intake and .037 on the exhaust on both cylinders. I flycut my pistons to .092 on the intake and left the exhaust until I had the new camshaft to measure. The second Cam is a Summit racing stage 4 - 238/246 with .625 intake lift and .605 exhaust. The 1.8 ratio rockers brings this up to .661 and .640. The clearance with this cam is .058 on the intake and .006 on the exhaust. Remember, the intake has already been flycut by .092. Without this flycut, I would have -.034 intake clearance on a camshaft that is supposed to clear with no problem.
I'm ok with flycutting pistons, I just want to know what's going on. Here are all the specs I can think of that would be helpful to know.
The block is an LH6 that has had .005 milled from the deck. I have confirmed this by measuring my piston to deck clearance to be .011 out of the hole which is .005 more than what stock should be. The head gasket is a felpro that has .041 compressed thickness. The heads are a 799 with a 2 inch intake valve that have not been milled. Both camshafts were degreed with a degree wheel and a dial indicator. The pushrods have been measured and are the correct length, giving the correct amount of preload on the lifters, I think this number was .025. None of the valves are being hung open. The head gasket was installed during the PVC tests and I confirmed the max lift for both camshafts and they were both correct. If you add the .005 from the deck job and .011 from the thinner head gasket, it’s not even close to making up the missing clearance. Thanks for your help, guys.
The first cam was a Comp Cams 228/230, .571/.573. With a 1.8 rocker the lift numbers were 604 and 606. This is essentially a stage 2 camshaft and should have no issue with clearance. However, after measuring with test springs on cylinder 1 and clay with cylinder 3 I found that my PVC was .005 on the intake and .037 on the exhaust on both cylinders. I flycut my pistons to .092 on the intake and left the exhaust until I had the new camshaft to measure. The second Cam is a Summit racing stage 4 - 238/246 with .625 intake lift and .605 exhaust. The 1.8 ratio rockers brings this up to .661 and .640. The clearance with this cam is .058 on the intake and .006 on the exhaust. Remember, the intake has already been flycut by .092. Without this flycut, I would have -.034 intake clearance on a camshaft that is supposed to clear with no problem.
I'm ok with flycutting pistons, I just want to know what's going on. Here are all the specs I can think of that would be helpful to know.
The block is an LH6 that has had .005 milled from the deck. I have confirmed this by measuring my piston to deck clearance to be .011 out of the hole which is .005 more than what stock should be. The head gasket is a felpro that has .041 compressed thickness. The heads are a 799 with a 2 inch intake valve that have not been milled. Both camshafts were degreed with a degree wheel and a dial indicator. The pushrods have been measured and are the correct length, giving the correct amount of preload on the lifters, I think this number was .025. None of the valves are being hung open. The head gasket was installed during the PVC tests and I confirmed the max lift for both camshafts and they were both correct. If you add the .005 from the deck job and .011 from the thinner head gasket, it’s not even close to making up the missing clearance. Thanks for your help, guys.
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My understanding is that you check PTV with a different (solid) lifter than the hydraulic lifter you run in the engine.
Is your checking lifter too tall, giving incorrect results when you use your intended pushrods?
Is your checking lifter too tall, giving incorrect results when you use your intended pushrods?
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@01CamaroSSTx
I first used a degree wheel and a piston stop to find top dead center, then zeroed out my degree wheel. After confirming the degree wheel was set at TDC, I then measured for .020 degrees before and after max lift on the intake lobe and divided in half to find my intake centerline. I used my adjustable double roller timing chain to fine tune the timing until the intake centerline is the same as the cam shaft manufacturer calls for.
@grubinski
Each time I tested PTV I used the link bar hydraulic roller lifters and pushrods that I’m intending to run on the engine. When testing PTV I had test springs on the intake and exhaust valves of cylinders 1 and 3 and performed the tests concurrently. On cylinder 1, I used a dial indicator to find the minimal clearance of both the intake and exhaust. I did this by compressing the valve by hand and recording its overall travel until the valve made contact with the piston. I measured every degree from 5 degrees after TDC to 25 degrees after TDC on the intake and 25 degrees before TDC to TDC on the exhaust. On cylinder 3, I used the clay test. When I tested for max lift, I got the same numbers that the manufacturer said, after adjusting for the 1.8 rockers. And, the valves aren’t being hung open, so I’m not sure how to tell if the lifters are sitting too tall.
All the tests were done multiple times and the numbers were very repeatable.
I first used a degree wheel and a piston stop to find top dead center, then zeroed out my degree wheel. After confirming the degree wheel was set at TDC, I then measured for .020 degrees before and after max lift on the intake lobe and divided in half to find my intake centerline. I used my adjustable double roller timing chain to fine tune the timing until the intake centerline is the same as the cam shaft manufacturer calls for.
@grubinski
Each time I tested PTV I used the link bar hydraulic roller lifters and pushrods that I’m intending to run on the engine. When testing PTV I had test springs on the intake and exhaust valves of cylinders 1 and 3 and performed the tests concurrently. On cylinder 1, I used a dial indicator to find the minimal clearance of both the intake and exhaust. I did this by compressing the valve by hand and recording its overall travel until the valve made contact with the piston. I measured every degree from 5 degrees after TDC to 25 degrees after TDC on the intake and 25 degrees before TDC to TDC on the exhaust. On cylinder 3, I used the clay test. When I tested for max lift, I got the same numbers that the manufacturer said, after adjusting for the 1.8 rockers. And, the valves aren’t being hung open, so I’m not sure how to tell if the lifters are sitting too tall.
All the tests were done multiple times and the numbers were very repeatable.
To double check what’s going on here, set the adjustable timing set back to zero. At the same time, go back to a 1.7 rocker and then see where you are with PTV. The 1.8 rocker is not only adding lift, but it’s also altering the duration numbers a tad, and duration affects PTV far more than lift does. I prefer the clay method when checking also. This will give you a baseline set of numbers that you can compare your other data with. Report back with findings please.
You can also do the valve drop method. Take the spring off put at TDC #1 and see how far the valve falls on #1. Say it falls .250. Now go look at the comp cam catalog and in the lobe section they give a TDC lift a the #6 (when you are at number 1 the number 6 is in the both valves are open scavaging mode. This is where you run into piston to valve clearance is the opposing cylinder TDC when both valves are open. Say the comp catalog says it is .100 lift at TDC 6 (on a 114 centerline they give you some chioces) you take that and multiply that by 1.8 that is .180 that the valves are open - the intake and exhaust will vary off of that but you will see that in catalog (maybe exhaust is .160). So based on this you are at .070 clearance (.250-.180 = .70). Now you could also measure the number #6 pushrod or lifter to determine what the valve lift is also. Now this is at TDC many times the valve hangs up there and the piston is moving so I would add say another .030 for clearance - to the .100 and .120 so you would want .130 and .150 in that position. You can also get some dummy 10psi Moroso springs and map it out that way to by measure PTV. Make sure you are not off a tooth either. This is a good way to test when a motor is all together.
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Hey everyone, thank you for all the replies. Here’s something I’ve been thinking about -
I’ve been doing all these PVC tests with hydraulic lifters and checking springs. The lifters are supposed to have a a preload of about .040 which I believe is half a turn. Without this preload, could that .040 be what’s throwing the numbers off? Also, that would be .040 at the lifter which would be .072 at the valve. This, plus the thinner head gasket and the deck job would add up to .088 which is exactly what I seem to be missing.
The problem with this theory is the fact that the valves are all closed with the checker springs installed and I’ve confirmed the max lift to be the same that camshaft manufacturers says that it should be (.661 and .640.) So how could the numbers be off if the valve is still starting in the closed position and still traveling the same distance?
thanks for the advice about the valve drop test, double06. I’ll need to look into that.
I’ve been doing all these PVC tests with hydraulic lifters and checking springs. The lifters are supposed to have a a preload of about .040 which I believe is half a turn. Without this preload, could that .040 be what’s throwing the numbers off? Also, that would be .040 at the lifter which would be .072 at the valve. This, plus the thinner head gasket and the deck job would add up to .088 which is exactly what I seem to be missing.
The problem with this theory is the fact that the valves are all closed with the checker springs installed and I’ve confirmed the max lift to be the same that camshaft manufacturers says that it should be (.661 and .640.) So how could the numbers be off if the valve is still starting in the closed position and still traveling the same distance?
thanks for the advice about the valve drop test, double06. I’ll need to look into that.
