252 heads on 231

[Stephen Soulsby]

I'm still chasing the stumble and general poor running of my car I pulled off the left valve cover to take a look around and here is what I noticed. With the motor at TDC both valves are supposed to be completely closed right? When you take out the bolts for the rocker shaft, the #1 (and other) valves lift about 1/4" and are sealed against the head. When I installed the rocker arm shaft, the valves went back down about 1/4" into the combustion chambers. If I'm not mistaken this would mean the valves are floating all the time, correct? My car only pulls 12" of vacuum at idle and 10" in gear. There are NO vacuum leaks and the Snap-On scanner shows nothing unusual except the unstable idle, and O2 sensor readings. The car acts as if it's rich and lean at the same time. If I lean it out to 36 it knocks severely and won't rev past 4,250, but if I richen it up to 40psi it still knocks and my O2 is at about 950 and still knocks. Every manual I have looked in says what I already know about the valves being non adjustable.

The reason for the subject title is I'm using heads, rocker arms, and pushrods from a 252. Could there be some type of conflict between the block and heads?

Or are my observations of the rocker arms and valves normal?

 

[KendallF]

Originally posted by Stephen Soulsby
With the motor at TDC both valves are supposed to be completely closed right? When you take out the bolts for the rocker shaft, the #1 (and other) valves lift about 1/4" and are sealed against the head. When I installed the rocker arm shaft, the valves went back down about 1/4" into the combustion chambers. If I'm not mistaken this would mean the valves are floating all the time, correct?

The reason for the subject title is I'm using heads, rocker arms, and pushrods from a 252. Could there be some type of conflict between the block and heads?

Or are my observations of the rocker arms and valves normal?

The 252 heads/rockers/pushrods are unlikely to be the problem unless something's broken. They're the same as 231 parts. Make sure you're at TDC compression stroke; verify by pulling #1 spark plug and having someone crank until you feel pressure pushing out. Then align precisely to TDC. At TDC compression stroke, both valves on #1 cylinder should be closed.

I don't know the background on your problem, but it's possible that the timing chain jumped time or that it was installed incorrectly. This is best checked by inspection with the front cover removed.

Good luck,

 

[crazygn]

are the push rods the same as the 231 , could they be a little longer then the 231 push rods. just a guess.

 

[Stephen Soulsby]

Oddly enough I looked up pushrods for my 231 and the 252 and there were three lengths listed (and a fourth for the TTA).

8.693

8.723

8.753

8.4xx (Turbo Trans Am)

I think these were the sizes. That to me is not different enough to get the valve pushing in the head as much as mine is.

 

[Buick From Hell]

Both valves being closed at the same time only happens for a short span of crank rotation! Like Kendall says, you MUST be on the compression/power stroke. If you're on the intake/exhaust stroke, the exhaust valve won't be all the way closed yet, and the intake valve won't be all the way open yet. And if ANY of your valves are being held open AT ALL, due to wrong pushrods for example, it's a 99% chance they're ALL being held open. If this were the case, you'd barely have enough compression to even start the engine!

SO--get a helper to turn the crank clockwise with a 1-1/8" socket. You may want to put a crayon mark on the damper straight across from the timing mark, so you can estimate BDC
Watch the valves on the #1 cylinder as the timing mark approaches TDC. On the intake/exhaust stroke, the exhaust valve will have started to open at roughly BDC, and should be closing. (it should be fully closed just after TDC) When the exhaust valve is almost closed, the intake valve will start to open. As the crank nears BDC the intake starts to close. It should be fully closed at roughly BDC. As you again reach TDC, both valves should be fully closed.

If your valves are not following this basic pattern, you may have a cam-timing problem.

NOW--if the valves appeared to open and close as they should, THEN loosen the rocker shaft while at TDC. If you find the valves are being held open as you described, then you may have a pushrod length, or other valve-train geometry problem!

Let us know!