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MG MGB Technical - Rocker pedestal shims

I'm just in the process of completing my head rebuild project and I noticed that my head does not have the shims under centre two posts,which from its year (1977 )it should have Does anyone know the true purpose of these shims, how thick they are and how vital!

Despite being rebuilt fairly recently, (before I got the car,) the shaft is quite worn with noticeable 'fretting' in the area going through the posts as well as the expected wear under the rocker bearings


Regards

Simon
S S Tew

>the shaft is quite worn with noticeable 'fretting' in the area going through the posts

It's my understanding that the shims are there to prevent that. I don't have any to measure, but I think they're only about 0.010" thick or less....
Rob Edwards

Yes, 10 k each and meant to slightly bend rocker shaft to stymie the fretting. Vic
vem myers

I think they're there to put some kind of pre-load on the shaft to stop it flexing? I've just put some in as mine didn't have any but there didn't seem to be any wear/fretting (I regularly run my engine to 6500 rpm). I reckon they're about 10 thou too, available from MGOC and/or Moss for pennies.
Neil
Neil22

Thanks guys , guess they're worth getting and fitting with the new shaft

Simon
S S Tew

I seem to remember that you only put a shim under the 2 centre pedestals. This gives the effect of bending the shaft!!

Can an engineer explain that one?/

CP
Colin Parkinson

Not so much actually bending as just pressing the top of the shaft to the top of the outer pedestals and the bottom of the inner two. Within the clearances between pedestal holes and shaft the bending effect is likely to be negligible.
Paul Hunt 2

There's not 10 thou of clearance in the bushes, therefore there must be a bending effect.
Neil
Neil22

Neil- Curious, as I understand these 10k shims were specifically not used in some years! Can Bigface or equiv. explain why? Vic
vem myers

Maybe it's a way to reduce fatigue?
N
Neil22

Bearings are 'loaded' in high speed machinery to provide improved oil film characteristics when shaft critical speeds are of concern - this was my experience in the design of some large 3600 RPM electric motors. However,this wouldn't be a factor in this application. I suspect that Neil has the answer - by preloading the rocker shaft you could be avoiding reversing loads and hence the potential for fatigue failure.

Barry
BJ Quartermaine

The shims were not originally fitted but the factory found that the shaft fretted in the pedestals during service so the shims were introduced under the centre two pedestals to put a load onto the shaft and stop it fretting.

Yes the shaft does get bent very slightly but well within its elastic limit.

The shims should be fitted to ALL B series engines.
Chris at Octarine Services

Chris does that apply with the tuftrided shaft?

CP

Colin Parkinson

Yup - all shafts, I daresay that the tuftrided shaft would stand up to the fretting better, but the pedestals won't!

These engines are tappety enough without introducing any more wear in the assembly ...
Chris at Octarine Services

Chris, what do you think of this idea? Drill through the side of the pedestal just into the shaft. Tap the hole, and put a grub screw into the hole, to lock the shaft into the pedestals?

Looks a better engineering solution to bending the shaft to tighten it up!!!

CP

Colin Parkinson

The problem is caused by having the shaft locked at one end rather than the middle. The shims are 4 1/2 thou and were factory fit on 18v engines and retrofitted when heads removed on pre-18v engines.

Careful drilling into shafts...you make a stress point and the shaft will snap!

Peter
peter burgess

Hi Peter, do you think that a competition engine should / should not have the shims?

Colin
Colin Parkinson

I can second Peter's comment - I have had shafts snap on the one grub screw hole!

Chris at Octarine Services

This thread was discussed between 02/07/2007 and 12/07/2007

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