Easy way to check piston to valve clearance

CobraEd · #1 (**permalink**) 01-16-2008, 06:48 AM

I have been wanting to upgrade my 1.6 roller rockers on my SBF to 1.7 for more lift. I was thinking that an easy way to check the piston to valve clearance without taking the heads off would be to just remove the valve cover, bring the piston up to TDC with the valve in the most open position, and then simply push the rocker open all the way and see how much farther the valve goes down before hitting the piston. I can use a dial indicator to measure. This seems so easy, why don't you ever hear about it. Everyone always says to take the heads off and use clay. Am I missing something??

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DAVID GAGNARD · #2 (**permalink**) 01-16-2008, 07:02 AM

I guess that would work just fine, but, you ever tried to push a rocker all the way open before???????? Spring pressure depending on your springs could be from 120 to 200 pounds closed........at least double that open!!!!!!!!!! So,to push the rocker all the way open you may need to exert 500 pounds of pressure on it!!!!!!!!!!!! There in lies the problem.............

David

CobraEd · #3 (**permalink**) 01-16-2008, 07:05 AM

I have done it in the past just fooling around . . . when I was young and strong with heads off the car..

If necessary, I could make a little lever that hooks under the poly-lock and do it.

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DAVID GAGNARD · #4 (**permalink**) 01-16-2008, 07:12 AM

Ed;
If you make a lever or tool to do the job, then I see no reason it would not work........

The spring pressure on my street car is about 130 lbs. closed and on the race motor it is 200 closed on the seat, just thinking the open pressure would be tremendous............ I have seen a tool designed to change valve springs without taking the head off, and maybe something similar would work.............

David

CobraEd · #5 (**permalink**) 01-16-2008, 07:14 AM

I will see if I can find that tool. Thanks!

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Jerry Clayton · #6 (**permalink**) 01-16-2008, 07:25 AM

The reason is-----the point of closest contact is not at tdc or max valve opening---it is a few degrees before tdc as the exhaust valve opens and a few degrees after tdc as the intake opens---the piston is chasing the exhaust valve as it closes and the intake valve is chasing the piston as the piston starts down

A little rocker ratio change really doesn't amount to much at those points so unless you are really near the limits you will probably be ok, but I would check anyway

DAVID GAGNARD · #7 (**permalink**) 01-16-2008, 07:35 AM

Ed;

It's a little late for this now, but I always check my piston to valve clearance, even on my street motor which has a puny little 484 lift cam.....reason being, if anytime in the future I want to upgrade my cam or rocker arms or anyhting else, I pretty much know how far I could go without getting into trouble......

Also, when I build a motor, I keep records of everything.....Rod and main cap torque #'s,piston to cylinder clearance, ring end gaps,head bolt torque, part numbers of rings,piston,oil pump,etc.,etc.....

It may be overkill, but when and if I need to go back into the motor to freshen it up or make changes, it saves me a lot of time and head scratching..............

David

Excaliber · #8 (**permalink**) 01-16-2008, 08:07 AM

I've heard of special light weight springs that you install to run such a check without removing the heads.

blykins · #9 (**permalink**) 01-16-2008, 08:25 AM

Most builders use that method. I use it in addition to the clay.

You'll find that the closest clearances are going to be in the overlap period of the cam. I start checking 15BTDC to 15ATDC. Your exhaust valve is going to be the closest BTDC because the piston is chasing the exhaust valve down.

You won't be able to open a valve with the stock spring on there. You'll need a set of checking springs.

I check both valves on #1 cylinder by putting the pushrods in and using a pair of checking springs. Mount your dial indicator. You set your lash and let the cam open the valve. Check it every 5 degrees of crank rotation. When the cam opens the valve as much as it's going to, zero your dial indicator then take your hand and open the rocker arm while watching the dial. When the valve bumps the piston, then note the reading.

You should also clay it up to check radial clearance. Depth clearance isn't all there is to it. Sometimes the valve relief tapers down and the valve will get closer as it opens up more.

undy · #10 (**permalink**) 01-16-2008, 08:28 AM

Quote:

Originally Posted by Excaliber

I've heard of special light weight springs that you install to run such a check without removing the heads.

....FE rocker shaft spring works well. If it's a hydraulic cam run the rocker adjusting nut down to where the lifter becomes a "solid". Set up a dial indicator to check if you have that 0.050" to 0.100" clearance that's usually desired.

Rickster289FIA · #11 (**permalink**) 01-25-2008, 11:32 PM

You can run the piston to TDC and then remove the springs and retainer (with a valve compressor). Once they are off, wrapp a thick rubber band around the valve stem until really tight to make sure the valve does not drop into the cylinder. Then you can add a lighter checking spring and start checking witha dial indicator in 2 degree increments from 15-20 degrees before to 15-20 degrees after TDC. Once you get your measurements, you can replace your spring and retainer by using a compressed air adapter that screws into your spark plug hole, or others have used a piece of soft rope pushed into the cylinder through the spark plug hole to hold the valve completely closed while replacing the spring and retainer.