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How to measure piston to valve clearance?
Posted May 14, 2009 - 06:54 PM
Posted May 14, 2009 - 09:21 PM
Posted May 14, 2009 - 09:56 PM
but not everyone has the tools for that.
After measuring the distance each valve moves until it contacts the piston, install the cams and setup the valve shims.
Now put the dial gauge on the cam lobe and measure how far the dial gauge moves from the valve fully closed (back of the cam) to fully open (lobe of the cam).
STOP if any valve contacts the piston. You won't hurt anything is you spin the engine slowly by hand near TDC.
Now, SUBTRACT the distance each valve moves from the cam from the distance the valve moves before it contacts the piston, and ADD the valve shim clearance.
As long as that value is larger than the minimum valve clearance spec'd by the cam mfr., you're okay.
Yes, it's a car link, but that's the technique I learned years ago sans dinging the pistons with a punch... and it works great on any engine from bikes to pushrod engines to DOHC import race engines. In any case, there's some useful info on "good" vs. "bad" piston and valve relief geometry. The benefit to this method is that if you're simply checking for "enough" clearance, you only have to install the head, cams, and shims once.
For those of us who have more time on our hands than cash and tools, here are some alternative methods:
Use some solder. Rosin core, .031" - .020" dia, larger if need be (if it's really thick, it's hard to squish, so use playdough if you have to - see below). Lay it out like a coil in each of the valve reliefs on the piston. (this is different than measuring "squish on a 2T). Be sure that the solder goes all the way into the center of the deepest edge of the valve relief. You'll need 5 of these coils.
lay the solder across the piston so that it crosses the valve reliefs at the "center" of the deepest part of the relief. Coils are much easier because they will "find" the center of the relief (imo).
Torque the head and install the cams as you normally would.
Rotate the engine through a complete cycle (2 full turns) so the cams turn 1 rev.
Remove the head, remove the solder.
Use a micrometer to find the thinnest point on the solder that has been squished by the valves. That will be your clearance.
Yes, the playdough way works too, You roll it into little "snakes" and drape it across the reliefs and in the squish (edge) of the piston. Measure it the same way. Solder is easier because it doesn't deform as much when you remove it from the piston.
You can use Plastigage too, but it's more expensive than solder although you don't need to measure it with a mic - it comes with a paper gauge that you match the squish to. Kinda nifty.
Put playdough on the piston in the reliefs. Install the head and valvetrain as above and crank the engine through a full cam rotation.
Use a depth micrometer to stab at the playdough in the thinnest spot so the part of the mic that sticks out just barely touches the squished surface of the playdough when the flat part of the mic is flat on the top of the piston. Then extend the mic until the moving part touches the piston surface. Subtract the smaller number from the larger and that's the clearance. Digital mics with thin depth tips are really useful for this method. Don't rock or shake the mic or you'll be way off on the measurement. A calibrated eyeball is needed when finding the top surface of the squished playdough...
Posted May 15, 2009 - 05:30 AM
Be sure to use gaskets (ideal your old ones) on the base and head. Torque the head just as you would normally. If you use new gaskets, they cannot be used for anything else other than rechecking V to H clearances.
Once torqued on, they cannot be used in a engine.
Posted May 15, 2009 - 08:48 AM