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MG MGA - Swivel pins

For $99.00, I bought a rusted pair of frt Mga disc brake units (minus the calpers). It is to be a project in salvaging important parts, rather than them becoming a boat anchor. I have already saved one disc brake backing plate, two from hubs, four trunnions, two disc caliper adaptor plates and two swivel pins. Part of the project is also to try ou my new powder coating unit.

I am rebuilding 1600 frt swivel pins, as a possible disc brake install on my Mga Coupe. The top section of one of the swivel pins (the upper threaded section) has the threads flattened. I think this was due to the previous owner trying to remove the trunnion without first removing the distance tubes.

Has anybody ever tried to rethread these Acme threads. Does anyone know if it is possible to repair/rethread them with a die. Are they savable or should I just buy a repalcement. If I need to purchase are there any available used, among the members.

Gordon Harrison

Gordon. The upper link looks as if it is badly bent.
Sandy
Sandy

I have not removed it from the bracket yet. It can be straightened out later. More worried about the upper threads at the moment. Do you think they are repairable ?
Gordon Harrison

I wouldn't like to drive a car with that amount of swivel pin damage, however it had been repaired. It's going to fail under stress at some point, probably in a corner, which will leave you in a very interesting predicament on three wheels, no steering, and heading for the apex of the bend in a sparky straight line.

That swivel pin and possibly the top trunnion to match it should be heading for the scrap bin, just be glad you got the other parts cheap!
dominic clancy

Given the documented instances of the swivel pins cracking and even failing completely, I'm not sure that I would want to drive on one that was bent and re-straightened. One of mine was bent, and I substituted a spare assembly from a parts car, but I would look to replacement of the pins in the absence of a straight, original unit. I went so far as taking the swivel pin assemblies to a machine shop, which checked them for cracks using magnetic particle inspection.

It cost me a bit, but I felt it was worth doing for peace of mind. I wouldn't tear a car down just to do NDI on the swivel pins, but if you are rebuilding the suspension I think it's worth doing.

-Del
D Rawlins

I don't see the swivel pin in the pic
The threads are flattened from new.
Art Pearse

All four trunnions came off very easily.Very well greased. The inner bushing were pressed out and the machine shop verified (visually) their integredy. They will be media blasted, as will the other parts, in order to detect any flaws. Should I suspect or determine a need for further inspection I will have them profesionally tested.

The caliper brackets and hubs are simply rust covered and will clean up very well. The link pins will absolutely be professionally tested, and should they fail they will be replaced. Remember that Mr. Gaylords website sites the cases of BRAND NEW link pin failure. Damded if you do and damed if you dont. If by chance ,one of you has some good link pins let me know. But as I mentioned, this was more of a project to salvage safe usable parts and practice with my new powder coating unit.

The sale of just the caliper brackets will recoup the cost of the whole assembly.

But my principle question and meaning of this thread was more to understand ACME threads and to find out if they can be repaired. You will find Acme threads in your bench vice screw, table saw blade adjuster and other mechanical things.

When I look at the thickness of the Acme threads I get the feeeling that they are designed for heavy duty work and stress. They have a much thicker crown and seem higher and wider. Apparently built for heavy duty work.

Gordon Harrison

Don't media blast the pins themselves. It will make them more prone to corrosion, and also tend to mask any defects, making detection more difficult. Best to thoroughly degrease, and remove any corrosion on the exposed parts (that weren't covered with grease) with emery cloth.

There have been instances of known replacement pins failing, and also pins in service where it was not possible to know whether they were originals or replacements. In the case of the new pins that failed, IIRC, it was clearly due to improper strain relief at the ends of the acme threads. This can be visually inspected for, and corrected by properly machining the pins where the threads end, to provide for strain relief.

The links/trunnions and spindles can definitely be powder coated successfully... It is what I did. But you will need to have every bit of grease removed first... I recommend heating them slightly to soften any hardened grease. If there is grease left in the passages, it will run out when you are curing your powder coat, and possibly ruin the finish. It's also very important to plug/mask the holes to the interior of the trunnions to keep any blast media from getting in there. After coating it is best to again thoroughly flush the trunnions to be sure there is no blast media in there.

I'm certainly not saying you HAVE to have the pins magnafluxed, and most people don't, but it seems to me to be cheap insurance that could possibly even be life saving. It makes for one less thing to worry about, anyway. If you aren't the ultimate user of the parts, and are just going to sell them, I probably wouldn't do it either, unless I had a buyer lined up who was willing to pay for it.

-Del
D Rawlins

I was given to think that
Acme threads are not threads but a clever way to increase contact area within a given length.
Visualize each valley and crest being flattened out and you will see that it has a much larger contact area.
Sandy
Sandy

This thread was discussed between 26/06/2015 and 27/06/2015

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