piston valve clearance measurement

tadd · 12-30-2016, 10:21 AM

What is the 'best' way to check for the piston/valve clearance?

I am conversant with the assemble with clay, then rotate everything process. However, I wonder if that is the 'smart' way. It is the 'damn for sure' way though.

Part of the reason I ask, is that is a lot of assembly, and there are a few parts I don't have yet (rocker shafts and lash caps)...plus its a ton of work.

So I was wondering if it might not be 'equal' to measure (map) out the intake and exhaust cam lobes, put those values into a spread sheet and multiply by the rocker ratio. Then over plot the piston travel by degrees. One can than then see where closest approach is...

Then using just the cylinder/head stack, some 'checker springs', and a degree wheel...measure piston to valve clearance at a few calculated values in the closest approach window??

chris_seven · 12-30-2016, 12:15 PM

position the piston at its closest approach and screw in the adjuster until the valve just touches the piston with each turn being 1mm.

I like to see around 2mm for the exhaust valve.

m42racer · 12-30-2016, 04:28 PM

Can I offer another way. Especially if you are missing some parts. This is based upon all parts being measured and measuring the same.

Install 1 Piston, Cylinder with both upper rings. This is to ensure the piston stays as straight as possible. Fit a degree when to the front of the engine and zero the TDC position. Obviously make sure the deck height etc., is as you will run it. This is a good time to CC the engine too. Put the piston at TDC. Now fit 1 head without the valves. No springs etc. With a center punch with a diameter the same as the guide ID ( so it fits) gently tap to leave a small witness mark on the piston for both intake and exhaust pockets. This just has to leave a very small witness mark. Now remove the head and with the valve diameter known trace a path from the witness mark to the inside of the pocket. Hopefully they are bigger than the valve diameter and at the correct angle. You can measure the included angles of the valves in the head to make sure. If everything is ok, refit the head with the valves loose. No springs, just the valves.

Let them drop and touch the piston while you tighten the head down. Only needs snugging up. Hold 1 valve up against the seat with a rubber band of sorts to a Cam deck stud. While holding the other valve against the seat with your hand Turn the engine clockwise until you come to 20° BTDC. Allow the 1 valve to drop and touch the piston. With a dial indicator, measure the distance from the seat to the piston. Do the same every 5° BTD and ATDC to 20° ATDC. Record these numbers.

Now remove the piston and re fit the head with valves fitted with soft springs, cam deck and time the cam. Repeat the same procedure with an indicator, at the same degree points. Record these numbers. These are the cam lifts. These same numbers can be found in a Cam Pro or similar device or from the cam supplier. Just tell them the timing positions you wish to run and they can calculate the cam lifts for you with having to do this second step. The cam lift numbers want to be less than the distances measured just with the valves. The differences want to be positive and the differences are your clearances. If the cam lift numbers are more than the distances measured with just the valves, you have interference at this cam timing and either have to move the cam or cut the pockets deeper. This way may seem lot more work than with just the adjuster screw, but its intended to show another way when new pistons are fitted and interference is found. This way the interference is "on paper" and the pocket depths can be calculated, and cut one time only.

boosted79 · 12-31-2016, 08:00 AM

Screw in adjuster 2 turns (2 mm) from zero lash at TDC, turn engine over slowly with plugs out. If no contact you are good to go.

TVRLOTUSTR3 · 06:25 PM

Mine hit the exhaust valve at 1 1/2 turns 1.5mm. I did not want to change my deck height so I advanced the cams a bit. Solex cams from 4.6 to 4.75 just made the 2mm clearance on the exhaust valve. Plus I should have better lower end torque, still pulls strong up to 6800 rev limiter.

strictly · 01-01-2017, 12:56 PM

I like the checker spring approach, some great advice above of course. Just my personal take but I'm still learning, the checker spring makes it easy to push the valve down by hand.

I dislike just screwing both intake and exhaust valve scews, two turns in and then just turning the crank over 720, as you could kiss the edge of a valve and not really notice while turning, possiably damaging the valve/sealing.

Perhaps I'm too cautious, but I found the "checker" spring approach was easy, and gave a good "map". It was also way quicker, then moving 5 deg at a time, and screwing in valve to measure contact depth.