Rocker bushing specs

Walt Fricke · 12-06-2006, 11:41 PM

I can't seem to find any spec to tell me which of the several dozen rockers I have have bushings that need replacement, and which are still in spec. It doesn't help that my bore gauge is for cylinders and such and doesn't go this small (but I'm buying one that will, along with reamers). And my attempts to use T gauges were unsatisfying (it seems my guage of the proper size doesn't want to lock well enough). But a more fundamental problem is that I don't know what to look for, so to speak.

I have one brand new one I can use as a standard, but how worn is too worn?

Anyone know what the spec is for this?

Walt Fricke

hcoles · 12-07-2006, 06:06 AM

I went through this...I think the numbers are in the Wayne book... but yes it is hard to measure. I went down to the machinist and we took rocker shafts (after they were "buffed") and after making new bushings (I made some form I think it was 6160 brass) and pressing in we honed them to fit... the fit was very close..sort of like no binding and almost no noticable play...this may not help much. We used regular caplipers to get close and then honed to get the right "feel". I also ground a divot on the inside of the bushing so the oil would have more of a place to "pool". This area of the engine seems questionable to me...but probably ok if all is the correct size and finish. Seems a bit sketchy that we rely on oil drips falling into the little holes on the rockers...no direct lubrication.

Walt Fricke · 12-07-2006, 11:34 AM

Thanks, HC

You are right - Wayne has specs at page 207. How did I miss that!

Why didn't you just purchase replacement bushings?

I guess the fit issue after you have installed new bushings is straightforward as a practical matter: Hone until a good new shaft feels like it fits nicely.

So I suppose I could do the same in deciding which of my fleet of used rockers need new bushings? Slide a shaft in and compare that with how it wiggles compared to my one current new rocker?

But since I know what I at first missed in Wayne's compendium, I can at least see what my new bore gauge shows.

I agree that this is an odd appearing oiling system. The current 911 engines have positive rocker oiling, don't they? Did that perhaps coincide with the use of the cam area oil line restrictor? I wondered about that when rushing to install restrictors on my older engines. So far, so good, but it may take a while to see if rocker bearings that used to go for 200,000 miles now only last 150.

But here is my take on how it works in our older engines:

1) there is a lot of oil spray in there (ask anyone who pulled a rocker cover and started the engine just to see what was going on), so the oiling holes always have some in them, with gravity and capillary action ready to spring into service.

2) when the rocker is on the cam base circle, it can rise a little off its pivot on the shaft, and oil can move there. This may also answer what I see as a conundrum - since it appears that valve lash increases as the engine heats up (does on my engines when I have to change a rocker at the track, anyway), why have 4 thou lash - why not one? Maybe this relaxation is required for proper rocker oiling more than to account for thermal dimensional changes?

3) when it comes back on the cam lobe, that pressurizes the oil. The oil under pressure wants to escape sideways, which tests the shaft/cam carrier seal. Hence the fact that some of mine are always at least damp on the outside, if not outright leaky, due to reuse of the carriers.

But this makes a guy wonder if rocker arm axial play is also important (Wayne gives that spec also) - too much and maybe the issue is not wear per se somewhere, but the fact that the oil can too easily be squished out of the bearing surface because there is not enough resistance to its eventual escape? Like rod side play?

Walt

hcoles · 12-07-2006, 05:08 PM

Walt,
good points.

My guess is that by shimming to low axial play you won't be able to trap the oil..if that is what your thinking...but I don't know could work. Could be by the time you trap it...you don't get enough to flow...

From what I heard the stock type bushings are not easily available or are not available.

As a side note....on my 1971 BMW 2002 the rocker shaft wore out at low miles..and it had pressurized oil..maybe not enough pressure but that was the design.

About 10k or so after my rebuild I took a rocker shaft out to see what could be done about a leak (I used the o rings) and I noticed a bit of burnishing on the bottom of the shaft...not sure if it was expected wearing in of the surface or not...I turned it 180 and put it back in...no leaks... and no parts getting kicked out.

I've said this on this forum before..I don't like the cam chain setup and all of that... you see the complete redesign with the 964 engines and beyond.

-Henry

Walt Fricke · 12-07-2006, 07:10 PM

Henry

My rocker shafts have almost always shown some burnishing. I usually try to rotate them to an un-burnished surface. After several R&Rs such a spot is hard to find. But I'm so cheap I keep using the shafts until they look like they are about to crack at the sealing cone areas.

You make a good point about heat - I suppose these have to generate some, like the rod and other bearings do. So you want the oil to get out. A balance between pressure and flow.

And if pressure fed rocker bearings wear their shafts anyway, maybe this is not as much of a relative design weakness?

I'd love it had Porsche designed our cars as DOHC with tappets or something so there were no rockers. Did the 356 4 cam Carrera engines have rockers? But - - -

Walt

hcoles · 12-08-2006, 04:49 AM

Walt,

The story on the BMW at the time was that a batch of rocker shafts had incorrect hardening..one would think that the oil would still protect the surfaces but probably cold starts/etc. eventually ate the shafts up.

Yes, would have been nice to have DOHC's but we might be talking about a different set of issues at least the rockers can be taken out and dealt with farily easily. I may not take mine out again for 10 years... I hope.

-Henry

911pcars · 12-08-2006, 10:39 AM

I don't think there's a history of rocker shafts galling or bushings scoring, or I haven't been paying attention. If so, the current splash, dribble-into-the-orifice oil path seems to work fine. DOHCs and 4-valves would be ideal but a dream for an early engine, but I'd like to see a roller-tip on the cam lobe end of the rocker arm to help reduce the frictional loads a bit. That's probably dreaming as well.

Sherwood