Piston To Valve Clearance Question # 258,325 - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-06-2008, 11:03 AM
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Piston To Valve Clearance Question # 258,325

Here's another piston to valve clearance question.............

Is there any way to know how much clearance I have with a particular setup "before" I purchase a new cam for it?

I've seen ways to check clearance - once you have bought everything, but I don't want to spend a small fortune on a roller cam setup and find out the clearance between piston and valve is to close.

I have a 454 block with factory Iron Heads from a 68-70 car. The heads are large oval port type with closed combustion chambers. My pistons are SpeedPro Forged with a .095" dome. Part # 2399F60. I was running a Hydraulic FT cam with about .575 lift for both exhaust and intake (1:7 ratio rockers). Duration was about 245 deg @ .050" lift. No problems with that - never smashed a piston to a valve.

Some of the roller cam setups I'm looking at have lifts of about .600 to .625. That's about .050" more lift than I've been running. Does anybody know from experience if I will be ok with that much lift or is that "pushing it".

Like I said, I dont' want to spend $800 for a roller cam kit with .625 lift and find out the clearance to tight - and then have to return all the parts and start over again.

Maybe I could or could have checked clearance with the old cam and use that as a baseline

The motor is already apart. Block is at the machine shop getting cleaned and honed.

Lee

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Last edited by leejoy; 01-06-2008 at 11:09 AM.
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Old 01-06-2008, 07:26 PM
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All you have to do is get the valve to go down .625. Then you will know what your clearance is.

You could take the spring off the valve, put the piston at TDC, set the head on the motor and see how far the valve will fall into the cylinder if it is .625+compressed thickness of the head gasket+.090 (for safety),
You should be good to go!
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Old 01-06-2008, 07:32 PM
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Wait...that isn't quite right The piston is never at TDC with the intake valve all the way open. You might be able to do it if you figured out what degree your valve was all the way open at, then drop the valve in.
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Old 01-07-2008, 05:56 AM
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There is no way to do it PROPERLY without the cam degreed in the block . You won't have to send everything back if it don't fit just get the pistons cut. I think you've asked the question before?
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Old 01-07-2008, 06:40 AM
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Generally the closest points the P/V get to is 10* atdc on the intake valve and 10* btdc on the exhaust valve. Put some light springs on the two valves, set the lash and put the engine at each point, then open the valves by hand and measure how far they go before they hit the piston and this is your approximate P/V clearance. The only thing to remember is that a roller will have higher ramp rates for the same duration/lift numbers as a flat tappet.
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