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P/V clearance measurement techniques

General engine tech -- Drag Racing to Circle Track

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donclark
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P/V clearance measurement techniques

Post by donclark »

Measuring piston to valve clearance:
Intake / Exhaust
Checking springs and dial indicator: 0.074" / 0.084"
Checking springs and clay: 0.095" / 0.120"
Heavy springs and clay: 0.130" / 0.140"

I would like to check with heavy springs and a dial indicator but do not have tooling to do so. I could probably make something but time is an issue., therefore would welcome any recommendations. Just wondering if anyone has made similar comparison measurements.
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Re: P/V clearance measurement techniques

Post by rebelrouser »

I prefer clay, it also gives a idea of the radius clearance of the valve pocket in the piston. And how parallel the valve head is to the pocket. Performance head manufactures it seems are always changing valve stem angles, and I have more issues with low clearance on just a small portion of the piston than the entire piston. If I have to cut deeper reliefs in the piston, I want to know if the angle is correct when I set up the cutter. Indicator will not tell you anything but the minimum clearance piston to valve. Also always screw in a spark plug when checking with clay, indicator will not tell you if the plug is too close either. Kind of embarrassing to have the motor pop and bang first pass down the track, because it closed 4 of the plugs under heat and rpm. I learned that in my younger days the hard way.
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Re: P/V clearance measurement techniques

Post by mag2555 »

For a quick check other then for radius clearance and if it's a solid Cam just keep stacking thicker and thicker feeler gauges inbetween the rocker arm tip and the valve stem until the motor looks up.

That feeler gauge stack height is your PVC.
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Re: P/V clearance measurement techniques

Post by donclark »

mag2555 wrote: Tue Nov 10, 2020 6:16 am For a quick check other then for radius clearance and if it's a solid Cam just keep stacking thicker and thicker feeler gauges inbetween the rocker arm tip and the valve stem until the motor looks up.

That feeler gauge stack height is your PVC.
I wondered about that, but Jesel cautions to "not bolt down or remove rocker arms under ANY spring load." The pushrods are the right length, which means I only have about 0.065" clearance when the adjusters are backed off. What I need is a very robust tool to pull the rocker against the spring as the valve is in the overlap position. Spring load is 400 lbs on the seat.
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Re: P/V clearance measurement techniques

Post by ClassAct »

The only way to know exactly what the P/V is requires the valve springs to be used. To me, that means you need to use clay. Checking springs will always show LESS P/V than a valve spring will.

I’ve seen guys notch the valve pockets because they used checking springs and when I assembled it I measured it with the springs and they just wasted time, money and compression.
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Re: P/V clearance measurement techniques

Post by donclark »

ClassAct wrote: Tue Nov 10, 2020 9:47 am The only way to know exactly what the P/V is requires the valve springs to be used. To me, that means you need to use clay. Checking springs will always show LESS P/V than a valve spring will.

I’ve seen guys notch the valve pockets because they used checking springs and when I assembled it I measured it with the springs and they just wasted time, money and compression.
Agreed. As my original post noted, when I used clay and the heavy springs that will be run on the engine, I got a lot different results than with the light checking springs. What I would like to do is verify with a dial indicator with the heavy springs installed. Just polling to see if anyone makes a tool that will grab the rocker arm and depress the spring at the overlap position to measure clearance. A pad to place the indicator stem would be a nice feature.
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Re: P/V clearance measurement techniques

Post by SupStk »

I use the soft spring and indicator method. Verifying angle and radial clearance can be done with bluing and a machinist combination square. The square is one of the tools used in setup for cutting valve reliefs.

A person will always see more clearance when checking with "real" springs.
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Re: P/V clearance measurement techniques

Post by tenxal »

If you have an LSM valve spring checker (everybody should have one...they are slick), part # PV100 is an anvil that lets you check piston-to-valve clearance with your race springs on. 8)
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Re: P/V clearance measurement techniques

Post by donclark »

tenxal wrote: Tue Nov 10, 2020 7:24 pm If you have an LSM valve spring checker (everybody should have one...they are slick), part # PV100 is an anvil that lets you check piston-to-valve clearance with your race springs on. 8)
Sweet, good to know. I already own two of their spring compressors. They make good stuff. Thanks!
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Re: P/V clearance measurement techniques

Post by Geoff2 »

I have checked V to P clearance with multi-grips on the rocker, gently pushing down until the valve contacts the piston. With the other hand, I use vernier calipers on the spring retainer to measure the installed height. After releasing the rocker, I measure the installed height gain; subtracting the two readings gives you the V to P clearance.
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Re: P/V clearance measurement techniques

Post by Jeff Lee »

tenxal wrote: Tue Nov 10, 2020 7:24 pm If you have an LSM valve spring checker (everybody should have one...they are slick), part # PV100 is an anvil that lets you check piston-to-valve clearance with your race springs on. 8)
This is what I do also. I now have a roller cam and I use actual race springs which are 1,050 # open. The handle supplied is 2-piece so you can go short or long handle. Even long handle is a chore with this much pressure!
How to get your dial indicator on something steady with full valve train in place and valve moving up & down? Stick a single edge razor blade between the retainer and spring. Use razor blade as your dial indicator pickup point.
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Re: P/V clearance measurement techniques

Post by hoodeng »

I do two tests that are quite simple. first some plasticine in the piston reliefs, o ring over valve stems to hold in place, final fit second hand gasket thickness gasket on, head on, couple of bolts, then push the valves into the plasticine @tdc then pull back up and hold with o ring, head off then cut plasticine in half with sharp blade and check radial clearance before proceeding with any set up work. Once radial clearance is confirmed to spec. Marking your flywheel or balancer for all the tdc's will get the job done in one rotation.

Then the next step.

Plasticine out and refit head then do a drop test, you can easily check the total tdc clearance with a vernier or stem protrusion gauge. To ensure tdc put a dial indicator on the top of the valve and rock slightly [ you need to be pretty close already to tdc but that is not hard].
The little bit of variance you will find doing it with or without springs is negligible considering if you build with V to P in the TDC lift + .060"-.080" range.

I am interested in the comment that there was a large variance between testing with a hard and soft spring if that is the way you are doing it, i would not expect any more than a few thousandths variance, something is stretching or the valves are being pulled into seat alignment?


Cheers
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Re: P/V clearance measurement techniques

Post by CGT »

tenxal wrote: Tue Nov 10, 2020 7:24 pm If you have an LSM valve spring checker (everybody should have one...they are slick), part # PV100 is an anvil that lets you check piston-to-valve clearance with your race springs on. 8)
Yeah, that are homemade variations of that work for checking with actual running springs. :lol:


A technique I used before I really owned a dial indicator, and still use sometimes and recommend to others sometimes. Place head on engine with no springs on at all, put orings or tape or whatever on stems to keep the valves from falling through. Adjust lash or zero lash with cam on base circle, by holding valve up on seat. With piston well before tdc, let the valve rest into valve pocket or piston top. While holding rocker etc steady, turn engine over and observe.

You can see easily the valve tip, taking up the space between itself and the rocker tip as the piston pushes the valve up. Measure the closest point with feeler gauges at that point. That is your ptv. I know it doesn't take into account radial clearance, and if you got one on the ragged edge of close you would likely want to go another route. But I just thought Id mention it. It gives a nice visual of whats going on through those tight area's.
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Re: P/V clearance measurement techniques

Post by ClassAct »

hoodeng wrote: Wed Nov 11, 2020 3:01 am I do two tests that are quite simple. first some plasticine in the piston reliefs, o ring over valve stems to hold in place, final fit second hand gasket thickness gasket on, head on, couple of bolts, then push the valves into the plasticine @tdc then pull back up and hold with o ring, head off then cut plasticine in half with sharp blade and check radial clearance before proceeding with any set up work. Once radial clearance is confirmed to spec. Marking your flywheel or balancer for all the tdc's will get the job done in one rotation.

Then the next step.

Plasticine out and refit head then do a drop test, you can easily check the total tdc clearance with a vernier or stem protrusion gauge. To ensure tdc put a dial indicator on the top of the valve and rock slightly [ you need to be pretty close already to tdc but that is not hard].
The little bit of variance you will find doing it with or without springs is negligible considering if you build with V to P in the TDC lift + .060"-.080" range.

I am interested in the comment that there was a large variance between testing with a hard and soft spring if that is the way you are doing it, i would not expect any more than a few thousandths variance, something is stretching or the valves are being pulled into seat alignment?


Cheers

Every rocker arm I know of, if correctly manufactured should measure over nominal rocker ratio with no load on them. All rockers flex. The design engineer should have this built into the rocker ratio so that with no load on them the ratio is greater than nominal and with load the ratio is nominal.

I had a set of rockers that the intake was 1.73 on the intake and 1.66 with no load on them. They were nominally 1.6 ratio, so I was not too happy because they were way too high.

So I called them and they said quit checking them with checker springs and put the springs on them you will use. And both came in at exactly 1.6 and, with a head that uses an offset intake rocker I expected some loss of nominal lift because of that.

With the rocker engineered for the loss of lift from spring load and the horrible pushrod side geometry my lift was right where it should have been.
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Re: P/V clearance measurement techniques

Post by MadBill »

A couple of points:
With heavy springs and slim stems (and maybe even without), if it's the rim of the valve that contacts the piston (say because the eyebrow is very small or the relief angle doesn't match the valve guide angle) you could easily bend a valve due to not noticing the relatively small force increase as the two make contact.

Also, reference has been made to checking P/V at TDC. For most engines the closest approach is ~10° BTDC for the exhaust valve and ditto ATDC for the intake.
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