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MG Midget and Sprite Technical - Rocker bushes

Is it possible to replace the bushes in the midget 1275 pressed steel rockers? I’ve got 2 or 3 full sets spare and would like to build up 1 good reconditioned set. The Moss catalogue lists the bushes as only for the forged or sintered rockers.
John Payne

I’ve often wondered that myself.

The only thing I can think of is that the OD of the bush may be different. Otherwise, I can’t see why you shouldn’t replace them.
Dave O'Neill 2

I believe its not possible to renew the bushes in the pressed steel rockers.Its worth checking the ones you have are worn. I have found its the shaft that goes rather than the bushes. I managed to create a good spare assembly using a couple of sets of old rockers.
Do get a properly ground and hardened shaft. I got mine from Peter May. If you are really keen Swiftune do a stronger shaft which has a smaller bore, not cheap though!!
Bob Beaumont

ISTR that replacement bushes need to be reamed in sets for correct clearance to the rocker shaft, which suggests that it would only be worth doing if the shaft is essentially unworn or if you were fitting a new one.
But it would be interesting to know if the pressed rockers will accept the new bushes.

Interestingly, bushed rockers with new shafts are available these days for 1500s, which the factory didn't trouble with.
They're on my wishlist. I have a birthday coming up...

Edit: Oops, crossed with Bob. Interesting.
Greybeard

It is possible to replace the bushes in the pressed steel rockers as I've done the set in my engine even though i was told it wasn't possible. IIRC standard steel backed bronze bushes were used and pressed in and reamed, the only issue I had was that a few bushes were slightly loose in the boss so I had to swage the rocker bosses down slightly by pressing on a suitably sized socket as that's all I had at the time. This went with the use of a new gas carbo nitrided rocker shaft as that process didn't leave possible contaminants in the shaft.
David Billington

If you get a new shaft, check it carefully.

I bought a new one and found it was 0.003" smaller diameter than the old one.
Dave O'Neill 2

Well I’ve just removed the bush from one and it came out ok with a couple of sockets and the vice. The bush is a steel backed one that looks very much like the ones Minispares etc sell.

The rocker and bush both have Vandervell markings on them so at least they were well made.

Shafts are another contentious subject. I’ve got a heavy duty Minispares one fitted - before I read about the issue with crud in some of them - and also have a new spare standard one. There have been previous discussions about hardened shafts not being such a good idea as they wear the bushes - easier to replace the shaft than the bushes. Who knows? I try not to get too worried about such things!

I might order a set of bushes and see how it goes. But then I might also order the set of 1.3:1 roller tipped ones I’ve just enquired about at Minisport!

Thanks all
John Payne

John,

Have you got the dimensions of the bush handy?
David Billington

AFAIK the issue of shaft wear is that the steel swarf gets embedded in the softer bush metal and will rapidly wear a new shaft fitted without changing the bushes.

It is possible to get new shafts 5 thou oversize so that the existing bushes and pedestals can be reamed out to suit, getting rid of any embedded swarf in the process - I think this is what is done to most of the reconditioned assemblies you can buy.

I always fit the tuftrided standard shafts and new bushes, reaming them on the lathe.
Chris at Octarine Services

John

Something else to check is that your rockers are all from 1275s. 1098 engines also had pressed rockers, but the lift was different and the pads were narrower, which means that they aren’t suited to 1275 heads.
Dave O'Neill 2

My 1275 has about 300,000 miles on it. I believe that the rocker shaft was changed once. I recently had the cylinder head off and there was still no noticeable play. Oil pressure is also high.

Even though the shaft and rockers are "wear parts" and should technically be replaced together, the rocker bushings are so robust that they still do not warrant replacement.

I suggest cobbling together a viable rocker set from your collection by trying them on a new, hardened rocker shaft and selecting the best of the lot.
Glenn Mallory

Glen, unfortunately that is what I have already done! The best 8 are on the car and with a new shaft. There is still more play than I would like though so I thought I’d see how much better they would be with new bushes.

David, OD of bush is around 0.695”. Because it is a split bush it isn’t exactly round so it ranges from .692 to .697.

Not sure what the arrow means on the bush - I must take more notice of the orientation when I remove the next one!





John Payne

"The best 8 are on the car and with a new shaft. There is still more play than I would like "

Check the shaft diameter.
Dave O'Neill 2

Good point Dave, going back to your second post.
John Payne

John,

The arrow, MOD property? ;)

From your dimension I would assume the bush is nominally 11/16" OD so I presume the rocker shaft is 9/16", likely why I have a 9/16" reamer in my inch reamer drawer. It was noticeable when looking earlier for that style of steel backed bronze bush that they're far less common than they used to be as I haven't found anyone in the UK selling them in different size series like used to be the case, just lots of DU, DX, and oilite bushes.
David Billington

Anyone ever seen the special service tools - 18G226 and 18G226A - for removing and replacing the valve rocker bushes (not sure which type though)? See: https://www.scribd.com/document/132766893/Sprite-Service-Tools

Cheers
Mike
M Wood

There's a copy here http://www.spritespot.com/Downloads/Workshop%20Manual/S%20Service%20Tools.pdf that doesn't require you sign up to view or download. May have to copy and paste due to embedded spaces '%20'.
David Billington

This thread was discussed between 24/05/2020 and 29/05/2020

MG Midget and Sprite Technical index

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