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Old 02-08-2019, 07:29 AM
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Piston to valve clearance.

I’m building a 383. It’s is zero decked and I’ll have a over all lift of 594/594. Duration at 50 is 242/240. Flat top pistons with 5cc valve reliefs. Should I worry about piston to valve clearance ? Or do I not have enough lift for it to be a main concern at the moment?

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Old 02-08-2019, 08:18 AM
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It's your engine and I wouldn't take someone else's experience or suggestions that it would be okay. An effective way to determine your valve clearance is the modeling clay method. Put just a touch of WD40 on the piston valve relief area, put a lump of clay on the same area and bolt your head on without a head gasket. Setup your valve train hardware and slowly - I SAID SLOWLY - rotate your engine over at least 2 full revolutions. Pull it back apart, cut the depressed clay in half with a razor blade and measure the thickness. That is the valve clearance that you have under normal running conditions + the measurement of the crushed head gasket. I have also seen engine builders that will set valve lash at "zero clearance" before performing this operation which in turn will give a tighter clearance measurement but is also giving some assurance that the lash setting will be providing additional clearance.



Keep in mind that high RPM in a situation where you miss a gear or the trans/convertor goes bye-bye, the valves may go into a float condition and touch the pistons. Rev-limiters can be your best friend that day.

Last edited by chasracer; 02-08-2019 at 08:24 AM.
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Old 02-08-2019, 12:29 PM
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To add more to what has already been said...

Peak lift numbers don't mean squat as far as piston-to-valve clearance iis concerned...at peak lift the piston is roughly 1/2 way DOWN the bore and moving AWAY from the valve.
Piston-to-valve clearance is far more dependent on duration and intake lobe centerline, as the point the valve is closest to the piston is roughly between 20 before TDC and 0 TDC on the overlap period, that time when the piston is coming up to TDC pushing exhaust out and the exhaust valve is getting ready to close, and the intake valve is starting to open to take advantage of that outward going exhaust pulse suction to start airflow into the cylinder, Piston and valve are moving TOWARDS each other at this point, just before the piston reaches TDC starts back down the bore to inhale more mixture.

As cam duration gets bigger, the intake valve is starting to come open sooner at the same time the piston is coming up to TDC to meet it and then stop and dwell at TDC while the valve is still moving open....this is the danger area, valve isn't even off the seat .200" so peak valve lift of .594" is meaningless art this point.

Bottom line is every build is different, and at over 235@.050" duration on the intake, you sure better check your p-to-v clearance and not be guessing or taking someone else's word for it.
Chances are, from my experience, you will just be okay on the intake side and have a mile of clearance on the exhaust....but if you don't check and verify you deserve every problem you get if it breaks parts on fire-up.

With hydraulic roller lifters, you'll need to bottom out a lifter with no oil in it to make the check, to eliminate plunger travel from altering your results.
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Old 02-08-2019, 10:22 PM
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The smaller the intake centerline, the less intake PV clearance you will have. Not usually a problem at common 106 IC and moderate duration. Move that IC to 100 with same lift/duration and things change a lot.

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Old 02-09-2019, 07:32 AM
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Quote:
Originally Posted by ericnova72 View Post
To add more to what has already been said...

Peak lift numbers don't mean squat as far as piston-to-valve clearance iis concerned...at peak lift the piston is roughly 1/2 way DOWN the bore and moving AWAY from the valve.
Piston-to-valve clearance is far more dependent on duration and intake lobe centerline, as the point the valve is closest to the piston is roughly between 20 before TDC and 0 TDC on the overlap period, that time when the piston is coming up to TDC pushing exhaust out and the exhaust valve is getting ready to close, and the intake valve is starting to open to take advantage of that outward going exhaust pulse suction to start airflow into the cylinder, Piston and valve are moving TOWARDS each other at this point, just before the piston reaches TDC starts back down the bore to inhale more mixture.

As cam duration gets bigger, the intake valve is starting to come open sooner at the same time the piston is coming up to TDC to meet it and then stop and dwell at TDC while the valve is still moving open....this is the danger area, valve isn't even off the seat .200" so peak valve lift of .594" is meaningless art this point.

Bottom line is every build is different, and at over 235@.050" duration on the intake, you sure better check your p-to-v clearance and not be guessing or taking someone else's word for it.
Chances are, from my experience, you will just be okay on the intake side and have a mile of clearance on the exhaust....but if you don't check and verify you deserve every problem you get if it breaks parts on fire-up.

With hydraulic roller lifters, you'll need to bottom out a lifter with no oil in it to make the check, to eliminate plunger travel from altering your results.

My intake centerline is 107 ATDC and exhaust is 117 BTDC
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Old 02-09-2019, 10:57 AM
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Quote:
Originally Posted by Cameronluster View Post
My intake centerline is 107 ATDC and exhaust is 117 BTDC
Like Eric said - check it. You can use a checking spring and dial indicator if heads are to stay on.

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Old 02-15-2019, 02:20 PM
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Quote:
Originally Posted by 2001Blazer4x4 View Post
Quote:
Originally Posted by Cameronluster View Post
My intake centerline is 107 ATDC and exhaust is 117 BTDC
Like Eric said - check it. You can use a checking spring and dial indicator if heads are to stay on.

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Tapatalk

What’s the best way to check it with the heads on? Like let’s say I install the heads and and go to removed the valve spring with my compression tool and the locks still want to hold on to the valve as I’m compression the spring so the valve opens inside the cylinder. Will it hurt anything if I use the piston for the valves to press against inside the cylinder to allow just the spring to move and release the locks?
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Old 02-15-2019, 03:35 PM
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Quote:
Originally Posted by Cameronluster View Post
Whats the best way to check it with the heads on? Like lets say I install the heads and and go to removed the valve spring with my compression tool and the locks still want to hold on to the valve as Im compression the spring so the valve opens inside the cylinder. Will it hurt anything if I use the piston for the valves to press against inside the cylinder to allow just the spring to move and release the locks?
Give one side of the retainer a semi-sharp downward angle blow with a rubber or rawhide mallet, or a dead blow hammer, or a lead hammer. This will generally unstick things. Don't have the piston at TDC so you don't bounce the valve off the piston and chance bending it.

You can use the top of the piston, but don't lean on it hard if it is stuck good as you could bend the valve if it is hitting the edge of the relief eyebrow putting all the force out at the edge of the valve.
Just go easy with it.
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Old 02-15-2019, 04:35 PM
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I usually use an old socket slightly smaller than the retainer and give it a good whack. That usually un-sticks any keepers.
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Old 02-15-2019, 04:44 PM
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As mentioned, check springs and a dial indicator. I'd turn the crank slowly and push the valve down to check distance. Did this for 4 revolutions of the crank to verify.

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Old 03-07-2019, 05:55 PM
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Quote:
Originally Posted by ScojoDak View Post
As mentioned, check springs and a dial indicator. I'd turn the crank slowly and push the valve down to check distance. Did this for 4 revolutions of the crank to verify.

Attachment 440547
Agree, if you don't want to take heads off, dial indicator is the way to go. If memory servers me correct, you want to take measurements 5, 10, 15 after TDC and 15, 10 , 5 BTDC. Can't remember what minimal readings you'd need to look for but am sure its on the internet.
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Old 03-07-2019, 07:04 PM
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Originally Posted by rdobbs77 View Post
Can't remember what minimal readings you'd need to look for but am sure its on the internet.
0.080" on the intake and 0.100" on the exhaust.
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