Rocker Shaft Installation

keynsham1 · 02-08-2014, 11:28 AM

I have a 911SC and I have just installed six of my rocker shafts. when tightening the through bolts it was noticeable that although most became very tight in the bore very quickly such that holding the nut end was not required, whilst others needed the nut end to be held right up until the bolt was torqued up, otherwise the whole rocker shaft would rotate. Is this normal?
I would have expected that if the coned ends were expanding the shaft ends out to make a seal, then it would clamp up in the bore quite quickly.

panama911 · 02-09-2014, 03:41 AM

It may be due to worn out bores in the cam housing - you may check by moving around the shafts into different bores to see if the 'looseness' of the fit is because of the bores or the shafts...

cgarr · cgarr's Garage

Are they new shafts? I have noticed some of the aftermarket brands measure .001 undersize.

keynsham1 · 02-09-2014, 11:03 AM

They are the original shafts which came out of the engine. I cannot see how the bores in the cam housing can be worn as this is not a moving part. I am reluctant to over tighten them, but I think I will just give them a little tweek to see if a small amount of extra tightening will stop this. I don't know if it is a problem or not really. They are certainly not loose but it doesn't really feel right. I would expect them to clamp up.

cgarr · cgarr's Garage

Make sure the caps on the shafts are clean and the threads so they can expand the shafts. I have seen rusty ones that were not seating well inside the shafts making it not expand well.

panama911 · 02-09-2014, 11:29 AM

IMHO the bores can be worn/damaged by frequent previous rebuilds and/or punching them out with too much force... but you can check that out. If the bores are OK, all the better!

Walt Fricke · 02-09-2014, 08:19 PM

This hardware can take quite a bit more torque than the factory spec.

But as Craig points out, if the threads on nut, bolt, or both are rusty or otherwise have a higher friction coefficient, more of the torque will go into turning the nut, and less into stretching the bolt. And bolt stretch is what draws the ends together and expands the shaft ends to lock the shafts in place.

The issue isn't that the nut end isn't holding by itself toward the end of the tightening. It is whether or not the shaft ends have been adequately expanded against the bores, so the shafts don't walk while the engine is running. You might put force on them to see if you can push them. I'd not go overboard with a miniature porta-power or sledge hammer.