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MG Midget and Sprite Technical - King Pin Stub Axle Questions

Hi,

I'm currently fitting my king pins and stub axles for the first time.

When I took this all apart I found a large shim in both trunnions sitting on top of the trust washer. The shims are a similar diameter to the trust washer. I also found two of the smaller shims inside each of the trunnions, which I believe are what should be used according to Haynes manual.

I am thinking the large shims are not standard and that a prior owner fitted them due to excess wear somewhere.

I tried to fit everything without the large shim and once the top nut was fully tightened I could move the stub axle up and down by a very small amount.

As I understand it, if you tighten the top king pin nut without any shims and the stub axle becomes very stiff you should then add enough shims so that the top nut tightens normally but so that the stub axle moves with the right amount of resistance but without an vertical movement.

I guess this becomes trial and error until you have it just right?

So I am thinking new king pins or trunnions? This will be such a pain as I have had new bushes fitted to the stub axle and had them reamed to fit perfectly, only last week.

Any advice would be very welcome.

Thanks

James



James Paul

the large shims with the bronze thrust washer should not be there! Presumably a PO's bodge.Also AIUI there's an error in Haynes - it says if the stub axle is stiff or too tight then remove small shims whereas they should be added. If too loose you remove them, but I think you've tried that, did I read that on fb? Check carefully the recess in the trunnion as it's very easy to get shims jammed up in there without being particularly noticeable.
David Smith

It may be that the upper thrust surface of the stub axle or the surface in the trunnion has been faced enough to remove the possibility of correct adjustment without either the additional washer or a special thicker bronze thrust washer. I had to face the top of my stub axles as otherwise the surface was in poor condition but i only removed what was needed to clean up the surface and adjustment was still OK with standard bits.
David Billington

I am now wondering if this is linked to the bodged fitting of the stub axle bushes. There’s a possibility that the PO skimmed the top of the stub axles to get the bushes level as they weren’t installed correctly. Hence why I removed them and had new ones fitted.
James Paul

The only real areas for wear in the vertical plane are the top surface of the stub axle, or the bronze thrust washer, and unless the steel had become pitted and been machined, then most of the wear should be on the bronze bit. How thick is it?
GuyW

"I also found two of the smaller shims inside each of the trunnions, which I believe are what should be used according to Haynes manual."

"I tried to fit everything without the large shim and once the top nut was fully tightened I could move the stub axle up and down by a very small amount."

-----------

All above is good. At that point you should have tried removing one of the small thin shims fron the recess inside the trunnion. They come in different thicknesses. You try different sizes to suit.

But if your stub axle has been skimmed and the 'slack', has been taken up with a large steel thrust packing washer, I don't see that as a problem per se. It's just a surface for the oilite thrust washer to turn on. Devise a means to fix the steel packing washer in place to stop it turning.

It's also possible the you have worn (or machined) trunnions, that are now too deep for the oilite thrust washer. Again, salvage with a packing washer.

But first without the steel packer, assemble and deshim in the correct way. Measure the thickness of your thin shims. And as Guy asked, how thick is the oilite washer? Report back.


anamnesis

That's really the direction I was heading.
If the phospher bronze washer has taken the wear, it may be simply the PO fixed it by adding a steel backing washer as a packer as he didn't have any spare replacements. No real problem with that. Replacing a worn phospher bronze with a new, correct thicknes might restore the slack so that you can then shim it as normal. If that doesn't work, possibly because the top of the stub axle carrier has been machined, then a good flat steel washer as a backing for the phospher one shouldn't be a problem.
GuyW

There is another possibility. The lower stubaxle bush may have been pressed in too far. If it has been, and it doesn't have to be much at all, just equal to the thickness of the top thin shims, then no amount of deshimming at the top will help. Because if the lower bush is in too far, it has the same shortening effect as skimming the top of the stubaxle.

Solution, drift the lower bush down a tad.

Most of the thrust is up to the trunnion, most of the time. But over time with all the rise and fall of the suspension, that lower bush opens up abit at the bottom, and sits further down on the radius at the bottom of the kingpin. I've drifted mine down on a few occassions to solve exactly the problem of not being able to de-shim enough.
anamnesis

I don't know whether this will help with the issue James is having, but it might be worth checking anyway.

When I had my front suspension apart a couple of years ago (to rebuild the Frontline arms) I bought new phosphor-bronze thrust washers. On close inspection (confirmed with a digital caliper), it was apparent these were not of uniform thickness - they were thicker at the centre than the outsides. I very carefully filed them flat before installing them and adjusting the fit with shims.

I seem to remember that different suppliers sold different thicknesses of shims and I had to buy from multiple sources to get what I needed.

Good luck.
Jon
Jonathan Severn

I had the opposite problem (to you James) when fitting new kingpins. They were going too tight, however many smaller steel shims I added and, when measuring the kingpins carefully, found they were both too short up to the upper shoulder. On one side I filed down the bronze thrust washer until the correct play resulted (with a few steel shims). On the other side I made a thicker steel shim from a washer which solved the problem.

But it seems you have the opposite problem to me James, so why not get two more bronze washers and combine with the existing older ones, filing down the older washers as required.
Bill Bretherton

Hi,
Thanks for your replies.
The trust washer is new from Moss left one in the 1st picture. . I don’t have a very accurate measuring tool, but they look 4mm to me, same as the old ones.

I’ll try some more combinations and see if I can get sorted.

Thanks




James Paul

Looks like there is a slight difference between the two, so I’ll use the large shims where necessary. See how that goes





James Paul

Update:

So left hand side need the thicker large shim and three 0.008 smaller shims

The right and side didn’t need any shims at all.

I wanted to check if it is ok to have this without at least one small shim at all?

i.e. is it designed to have at least one?
James Paul

Yes it IS ok, not to have any thin shims.

In as much as the thin shims allow you to remove one or more to compensate for wear, it designed to have shims. But only in that sense does it matter.

Did you see my coment about the lower bush? If you drift that down by the 8 thou you mention, then you will need 8 thou of shims on top, or an 8thou packing washer behind the oilite thrust washer.

How much will you use it? It won't need doing again for quite a while if you only do a thousand miles a year.
anamnesis

Thanks it’s good to know.

Yes I did see that about the lower bush, as everything seems correct now I’m happy and I guess at some service in the future will see what further adjustments need to be made.

For now thanks and tine to crack on with fitting the springs. Long bolt method!

James Paul

This thread was discussed between 22/05/2021 and 23/05/2021

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