Rocker shafts clearance specs?

[peter915]

Can someone who knows, or has a factory manual, tell me the specs for the clearance (fit) between the rocker shaft and rocker bush? Any other details would be gratefully appreciated.
Peter

[Jim Williams]

Rocker shaft is 17.992 - 18.000. Bush is 18.016 - 18.027.

What other details did you need?

[peter915]

Jim,
Great stuff - thanks for the info!
Do you have any detail on max tolerance (wear) on the rocker pad face - where the rocker rubs against the cam? This part (the face) needs to be parallel with shaft.
I'm going to re-condition my rocker shafts, new bushes, and shafts.
thanks again,
Peter

[Jim Williams]

Peter,

I've never seen any tolerances on wear of the rocker arm faces. You're right, obviously all these working parts need to be parallel, rocker bores with respect to the faces that ride on the cam, and of course, the surfaces of the lobes on the cam with respect to the centerline of the cam.

But I don't know how you would measure any of this parallelism with the tools available to the average mechanic. I think about all you can do to inspect the faces of the rockers and make a judgment that there are no ridges or pits present. I don't find anything in the spec books with any tolerances on this. I guess one has to pretty much trust the machine shop to hone the rocker bores to be parallel with the rocker faces.

If you are going to disassemble the parts in the cam housing with the intent of putting it all back together, it is reconmmended that the rockers be bagged with the location written on the bag, then the rockers returned to the original position. I think this is just considered to be good engine rebuild practice. if you happened to be putting in different cams, then I'm not sure why this would be required.

Maybe someone else will have a different take on this.

[camgrinder]

The important things to measure for is taper and out of round. The rocker arm shaft can also be tapered. After replacing the bushing the pad needs to be reground. A misalignment of over just .0008" can cause wear on the cams and rocker arms. If there is excessive clearance in the rocker shaft to bushing, the misalignment can be greater.
One of the reasons to keep the rocker arms in order is the wear pattern. Each individual camshaft lobe profile has its own print on the rocker, some of the higher velocity profiles will have a longer wear pattern on the rocker face. Other profiles can have a shorter pattern. If you mix up these rocker arms you can have a situation wear the wear pattern is short and the new profile runs off this pattern onto the unworn section. If the rocker arms are old and worn, there will be a slight ridge at the end of the pattern. This can cause an accelerated wear problem on new or used cams. An example would be a 964 intake (high lift, high velocity and long print) and a 930 turbo exhaust lobe (Low lift, low velocity and shorter pattern)

[peter915]

Jim & John,
Excellent replies, thanks guys.
Yes, I have had some of my rockers out of the mtr, and without any measuring - a couple have definite excess clearance ( you can rock them whilst on the shaft). (new shafts)
I have always kept the rockers numbered and back to their original location.
Mtr has done around 160k miles, with a rebuild at 121k. (previous owner) and I doubt very much if rockers where checked. Suspect head with loose rockers is also one with worn valve guides?
I wonder if this is just coincidence?
My plans for now are to make all rockers new again, with mtr in car. Then maybe next year plan a top-end re-build.
Thanks again guys,
Peter