Piston to valve clearance ( Chevy article, but still applies to Mopar)

[Mark Barnes]

Read so many threads on this subject I thought this may help...

https://www.hotrod.com/articles/predict-piston-to-valve-clearance-before-ordering-a-cam/

 

[66Satellite47]

I would not follow the method in this article. I have degreed many cams and checked piston to valve, the minimum is not near TDC on any of my cams. In general the larger the the overlap, the closer the piston to valve will be. Careful measuring is the way to go. Pick a cam based on your engine combination and desired performance level. Then do the measurements for piston to valve.

 

[Mark Barnes]

I would not follow the method in this article. I have degreed many cams and checked piston to valve, the minimum is not near TDC on any of my cams. In general the larger the the overlap, the closer the piston to valve will be. Careful measuring is the way to go. Pick a cam based on your engine combination and desired performance level. Then do the measurements for piston to valve.

Only reason I posted this is because it somewhat mirrors the process I'll have to use in the very near future. Flat top pistons at zero deck, bigger valves, small chambers and a pretty fair size lift and duration SFT cam. So I guess I'll be using these- https://www.lindytools.com/ihpc

 

[Wietse]

Seems like the article is more on predicting the PV clearance with a given cam spec.
What I did was rotating the engine 10* at the time and record the PV clearance, after that you can see where you find the tightest clearances and redo that section but then with 1* at the time and you get a nice chart of your PV clearance.
Doing it like this you will need checking springs, adjustable pushrod and a solid lifter to match your setup you are planning to use.
Set up a dial gauge on your spring retainer and at every measuring point you just push the valve down till it hits the piston, the gauge will show you the travel for cam lift to piston.

 

[66Satellite47]

Seems like the article is more on predicting the PV clearance with a given cam spec.
What I did was rotating the engine 10* at the time and record the PV clearance, after that you can see where you find the tightest clearances and redo that section but then with 1* at the time and you get a nice chart of your PV clearance.
Doing it like this you will need checking springs, adjustable pushrod and a solid lifter to match your setup you are planning to use.
Set up a dial gauge on your spring retainer and at every measuring point you just push the valve down till it hits the piston, the gauge will show you the travel for cam lift to piston.

I agree, I start with 10* increments, then narrow it down. I use the push rods I will run.

 

[Wietse]

I agree, I start with 10* increments, then narrow it down. I use the push rods I will run.

Not sure what you are running, but since I was going to use hydraulic lifters I had to calculate the right length of the adjustable pushrod to use it with a solid lifter and have the same length as my hydr. lifter and correct length pushrod incl. pre-load of .050"

I believe the tightest spots were around 10* before & after TDC.

 

[66Satellite47]

My cams have been solid lifter or solid rolllers. The overlap is what varies the TDC location. Most of mine have been beyond 10* TDC. But is the measuring that tells the tale.

 

[451Mopar]

Article seems like an ad for Comp Cams. I don't think the cam companies will give out their cam profile data, if they did you could calculate the P-V clearance too.

Knowing engine stroke and rod ratio, you can calculate the position of the piston movement from TDC for any degree of rotation.
Knowing the cam profile, cam installed position, and rocker arm ratio, the valve lift can be calculated at any degree of rotation.
The valve drop test just fills in the missing part (of equation) of how far the valve is from the piston at TDC.

I guess it is a nice service from Comp, and they may even suggest a custom ground cam (less overlap, less duration, or slower intake ramp rate / quicker exhaust ramp rate?) for engines with limited P-V clearance?

If you have the cam on hand, you could check lift at each degree of rotation and have decent cam profile data to do the calculations, but installing it and measuring is easier and more accurate.

In general: More duration = less P-V clearance, More Overlap (tight LSA) = less P-V clearance, Advancing cam install position = Less intake / More exhaust P-V clearance, The reverse for retarding install position = More intake / Less exhaust P-V clearance.

 

[BSB67]

At overlap condition, check the exhaust valve at 12°, 10°, and 8° BTC, and the intake at 8°, 10°, and 12° ATDC. But clay, when done properly is better.

 

[66Satellite47]

I believe in a dial indicator rather than clay.

 

[Challenger340]

We do Valve drop checks and record the clearance every few degree of Crank Rotation on Cam overlap for BOTH Intake and Exhaust Valves... and every 5* well after that...
which,
can then also be utilized later in Computer modelling in conjunction with the exhibited Cylinder Head Flow data we saw.... to see what's going on with Cylinder Scavenging(5th cycle) on the Engine Dyno results....
as well as determine potential 'room' left for Lobe Profile changes .
Also why some very low lift/low pressure drop Flow Testing can open doors with cam profiles.

just saying....
well worth doing and recording during assembly to work with your Head/Cam guy to maximize your combination..... works for street/strip efforts and can be especially helpful in restricted classes.

 

[BSB67]

I believe in a dial indicator rather than clay.

Of course that does not check radial clearance to the valve pocket.

 

[66Satellite47]

Of course that does not check radial clearance to the valve pocket.

My dial caliper does. I always make sure the pocket diameter is adequate for the valve size. I have never had a set of pistons that did not have the valve pockets correctly aligned with the cylinder head valve location.

 

[33 IMP]

My dial caliper does. I always make sure the pocket diameter is adequate for the valve size. I have never had a set of pistons that did not have the valve pockets correctly aligned with the cylinder head valve location.

I have. I have some B1 stroker pistons I'm gonna try to use with Victor heads. Definitely will need to clay them. I suspect they won't work without some mods.
Got the pistons (10, new in box) for 12 cents on the dollar.

 

[66Satellite47]

I trust my dial indicator and caliper far more than clay.