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MG MGA - Rocker shaft pedestals

A precautionary tale - the aluminium version fitted to the cylinder head on my 1500 have failed which means the head has not been held down tight on the block and.... you guessed it - water loss.

If yours has aluminium pedestals, you might want to put some steel ones by. There are some new ones about.

Roger

Roger Walker

I think the MGB pedestals are a direct replacement.
George G.

Be careful that any replacement pedestals are identical.. Some of the rearmost ones have a diiferent offset for the oil feed hole.
dominic clancy

Roger, what do you mean by 'failed'? Did they somehow shrink under the pressure of a head stud?
MAndrus

Yes, the nut on the head stud had "dished" the pedestal when tightened on an earlier occasion but when tightened last week the pedestal started to collapse with cracks parallel to the stud.

One supplier suggested the pedestals were an aftermarket repro cast in a more easily workable material. I simply don't know if this is the case (the engine was rebuilt in Sweden some years back) but it makes eminent good sense that the material of the pedestals should be as strong as the material of the head.

George ; I think you are right (different parts no. ?)but Dominic's advice very much to mind (thanks for that too). I have been lucky in finding some new OE pedestals from NTG motor services (Ipswich) sales@mgbits.com

I suggest, it's worth checking all our As and perhaps early Bs.

Good wishes,

Roger
Roger Walker

The aluminum pedestals are original MGA, not repro as your supplier suggests. As noted, they are prone to failure and should be replaced with the MGB units which are cast iron. Not sure if the switch away from Al took place with the model change to MGB, or at some later point.

You can usually pick up an entire MGB rocker shaft assembly at reasonable cost from Ebay. There is some value to using your MGA rocker arms with the MGB pedestals, because they have an additional oil passage to the adjustment screws. Of course, that's only of benefit if you still have the original type adjustment screws with the oil holes. Currently available replacements do not have them to the best of my knowledge.

-Del
D Rawlins

Del
Very useful - thanks. FWIW the steel/iron replacements came in a BMC box with the MGA part numbers suggesting the factory may have woken up to the weakness before the end of the MGA straight four production run.
Good wishes,
Roger
Roger Walker

More likely the factory was consolidating part numbers for replacement parts after MGA production had ended, putting the MGA part number on the MGB parts and discontinuing production of the alloy pedestals for replacement parts.
barneymg

Barney - quite possibly - perhaps the important thing is that at some point, the Factory seems to have recognised the weakness. Have you come across other Aluminium pedestals that have failed ?

Not connected with this event, I am most grateful for the advice given on MGB disc brakes and suspension on the A - negative earth and Alternators - Removal of the foot dipswitch and sensible indicator lights that will be recognised by today's drivers. The advice has been invaluable. Whatever the originality purists think, these cars would not have captured the imagination on the scale they have without the interest and imagination of you and so many particularly in North America, who have kept these cars on the road and made them drivable in modern traffic.
Many thanks
Roger

Roger Walker

Okay, now I will give you a hard time. I don't believe there was ever anything wrong with the design or strength of the alloy pedestals. And I don't believe the factory changed to iron pedestals for MGA for any reason other than to consolidate inventory. In fact I am not yet convinced that they ever did change. The fact that you found a box with the MGA part number for alloy pedestals and it had MGB parts inside might be a one-off fluke.

I have not seen any other evidence that the alloy pedestal was ever superseded to an iron part under the same part number. If they wanted to change the MGA to iron parts they would have simply changed the SPL to show the part number for the iron pedestal as replacement for the old part number. It is very unlikely they would ever supersede an alloy part with an iron part without changing the part number. If there was any reason to change MGAs to iron pedestals, they would surely want to avoid such confusion, and avoid the chance of accidentally installing alloy pedestals as replacement parts.

The problem with alloy pedestals is not design or materials, it is misuse and field service abuse. The SPL clearly specifies standard steel flat washers (part number PWN105) and Heavy Section spring washers for use on top of the alloy pedestals, which will prevent deformation of the alloy part which causes the fractures. The problem arises when someone installs a thin steel washer (even a hardened steel thin washer) which is prone to deformation when tightening the nut. If the thick washers were retained there would never be any deformation of the alloy pedestal. I have seen lots of fractured alloy pedestals, but I have never seen one fractured that was not also badly deformed under the nut due to removal of the original thick washers.

PWN105 Flat Washer - Standard
LWN305 Spring Washer Heavy Section (not medium section lock washer)
2K5319 (2K5319 not available use LWN305)

For sure in recent years (decades) many of the Brit car parts suppliers have been supplying thinner steel flat washers (sometimes very thin hardened steel flat washers) and Medium Spring Lockwashers in place of the original stronger washers.

This is one of those lessons we may have to learn the hard way. I saw someone else with this problem more than 25 years ago and shortly later went through this myself once before I figured out what was causing the problem. See the very clear picture here: http://mgaguru.com/mgtech/engine/ch104.htm
barneymg

This thread was discussed between 23/09/2015 and 25/09/2015

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