Simple way to measure piston to valve clearance
 

I think I have an easy way to measure piston to valve clearance. It occurred to me when I had a head disassembled on the bench that it would be easy to use this as a piston to valve clearance tool. One could rotate the engine to TDC. Then use a head with valves but no springs.

Bolt this semi-naked head onto the engine and mount a dial indicator to the end of the valve stem. Next, simply push down on the valve until it bumps the piston and measure the distance. Subtract out the lift of your cam and you should now have the piston to valve clearance.

What do you all think?

The only issue I see is how much material was removed from the head when refinished. You should get this spec when you get head work done and this could easily be factored into the equation.

Using the method you just stumbled upon, any machine work is discounted! Because you are measuring "as-built" clearances.. no need for to account for machine work...

Ive used a similar method for years..

Is the piston to valve clearance the tightest at TDC? From what I understand that may not be the case. However, one could use this method if they checked at different crank angles.

Cheers

Jeff, wouldn't this way of measuring give a 'worst case scenario' clearance. in a running engine different points of rotation may have tighter clearance than TDC, but this method gives you the least possible clearance. only issue I can see might be that you're giving up a little clearance if a different point than TDC is the tightest valve clearance.

The valve is safely in it's pocket when the piston is at TDC. The piston and valves are closest several degrees before and after TDC. The cam timing and the cam profile determine where this occurs and how close they are.

-Chris

You could also check it with the head on the engine ready to go, find the spot where the interference may be TDC? and using the rocker adjusting screw count the turns until the valve hits the piston. The screws are 1mm pitch so each turn is .03937

I think that's the process that Wayne describes in his 911 engine rebuilding book, but as Chris says it's important to check clearances through the various degrees of crank rotation and positions.

Brian

I have used the "rocker screw adjustment" method many times. I kept turning it 1/4 turn and rotating the motor around until I got to a number I was happy with knowing that I had at least that much clearance at all times.

Cheers

rocker screw as said is best...cause evry thing is closer b4 TDC

Hello all.

As has been correctly stated, valve to piston clearance is least slightly off TDC.

So the method I use is a variation of the one posted.

I use a very light spring, and assembl;e it all witha wide tappet clearance.rotate and press down on the valve to check the free play frequently.

Close up the tappet a bit repeat..very soon you will work out where the clearance is least..

I don't use any measuring tool, just my fingers and eyes..

And then when you get real close to clash, just work back and fore about that point using the dial guage or the tappet screw thread pitch to get the actual number..

Kind regards
David

Hell, you could cut a pie-shaped wedge in the valve, run the piston up to TDC, chuck the valve stem in a cordless drill and cut your way to clearance!

Just kidding of course, that seems like a decent method, you would use the Z-block on the tip of the stem and wiggle it between zero clearance in the valve pocket and the valve seat.

Do you have to position the gauge on the tip of the valve stem (most accurate I would assume)? Would it be acceptable to use the edge of the retainer?

Jamie,
I think your method would work well if it was a noninterference motor because you're assuming that valve is always at maximum lift, but I expect if you actually run the "experiment", you'll find you'll come up with a negative number! Dave and I have thought about this before as well and never came up with a good solution other than to have the head/cam tower assembly all bolted together. There's just too many variables in machining to do anything other than the lash adjuster method, or clay method.

There's a really good graph in Wayne's book showing the relationship of the valve to the piston at various crank positions, that visually explains why TDC is not the highest-risk position for interference. It is probably around here on the forum somewhere too. I like the lash-screw method for most applications. Really aggressive/race applications, have used the solder method. Seems like the best insurance on the investment.

Thanks Dave. I'm going to be spending some quality time on P to V clearance making sure it's accurate. This is the part that has me most nervous.

I was told that with the modifications I did (New Mahle Euro p's and c's and 964 cam grind and only .003" taken off the heads to clean them up) that I shouldn't have to worry much. Wayne's book even calls this out as an easy bolt-on exchange for more performance without other modifications.... but I guess it's always good to double and tripple check! What do you think?

You put the (flat) tip of the gage on the edge of the retainer.
(The rocker's "elepant foot" would be in the way of the top of the valve stem.)
-Chris

Doh! Thanks for correcting me, Chris :) I thought about that after sending.