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MG Midget and Sprite Technical - Lower Fulcrum Pin Fit

Hi

I am currently upgrading from 8" drums to standard discs with a kit of new parts including uprights and kingpins from one of the well known suppliers. The kit arrived today and I have started trial building and inspecting the parts.

I have found the new lower fulcrum pins (2A4020) are a loose fit in the king pins which would mean the soft cotter pin taper is solely responsible for locating the pin in the bore. I would only expect a very close slide fit here.

Pins measure 0.560" O/D and the bores are 0.5625"

I am interested to know if this is correct and if not whether there is a source for pins of the correct diameter.

Thanks,

James
J Barratt

I've just been through this and can confirm the loose fit and that the secure location is due to the cotter pin.I tapped mine in with a pin punch before putting the nut on with a ss spring washer. The last owner only relied pressed the cotter in by hand (well finger) and secured with the nut. When I dismantled it the nut wasn't even finger tight and I could press the cotter pin out with my finger. No doubt this was the cause of the clicking and knocking noises coming from the front suspension and the reason for my rebuilding the front suspension.
MG Moneypit

Conversley, in the past I have had fulcrum pins where the flat was barely machined sufficiently so that the cotter pin wouldn't go far enough in to even get the nut to start on the thread.
GuyW

Yes same experience. I checked with Moss and it does say on the website that the cotter pins should be filed to suit.

So did it with mine and got it to fit nicely, but not convinced on the strength of the thread. Seemed a bit cheap to me.

https://www.mgmidgetrestoration.com/2021/06/front-steering-rebuild.html
James Paul

As with your old bicycle you tap the cotter pin in with a punch, put the washer and nut on and tighten. Then tap the pin in some more and re tighten the nut, it will usually be loose again. Repeat until it seems like it won’t go much more!!

Sometimes you need to put a washer on that is larger than the thick part of the cotter pin or the nut bottoms out before it is tight. Never try and ‘pull’ the pin in with the nut, it will just strip the threads. The original pins if still in good order are much better than the new ones - where have we heard that before?!
John Payne

I had to file my cotter pins too. The kingpins were a fairly loose fit on the fulcrum pins so are held firm by the small cotter pins. A pretty crude design really. I also had problems with the stub axles going tight with the top nuts tightened and ended up having to grind down the bronze washers plus making a thick spacer/ shim. I'd found that the kingpins were too short as measured to the upper shoulders.
Bill Bretherton

Thanks for the feedback. I'm surprised if the fulcrum is meant to be loose in the kingpin, if this is so it is the only instance of this type of joint being so reliant on the cotter I have come across. My Austin 7 has cotter pined joints everywhere and they are all close slide / tap fits. It was the same on the pre-war GP cars I used to work on as well.

Is there any chance someone on here may have a genuine BMC fulcrum pin they could run a micrometer over?

Thanks

James
J Barratt

No James, you are rght. The fulcrum pin should be a close fit in the kingpin. 25 thou does sound a bit excessive to me.

The cotter pin is a bit of a thread diversion as its primary purpose is simply to stop the fulcrum pin from rotating and in so doing, unscrewing itself from the wishbone bushes. It does also lock the fulcrum pin but shouldn't be the (very feeble) thing to rely on to take up that much slack between pin and king pin.
GuyW

Guy,

He wrote "Pins measure 0.560" O/D and the bores are 0.5625"" which is 2.5 thou.
David Billington

For context here is a video of the fulcrum in the kingpin
https://photos.app.goo.gl/aoUHCKyp6zwYGHYK6

It is all academic now; the kit is being returned. Thankfully the supplier has never had a problem with any parts before and I am just an angry customer - so everyone else is okay!

Other problems I imagined were a seized king pin, bushes full of swarf, wheel bearings with the wrong radii and a upper trunnion that had nearly half a mil clearance on the kingpin.

Ready to fit stub axle assy
https://photos.app.goo.gl/E4wMkk71MH7HZXrKA

And minus the upper trunnion (my thinking was it was just incorrectly shimmed)
https://photos.app.goo.gl/K6PpFkHou1pSm6ku6

Upper trunnion
https://photos.app.goo.gl/KgCeyxF7vFYAC4sr5






J Barratt

Thanks David! I never could count!(decimal places)
GuyW

James,
Found an old pan/ pin under the bench so pretty old - can’t say if it’s BMC or not.

The micrometer says 0.561 and the metric one agrees at 14.25 mm.

I must get one of those workshop magnifiers!

The Imperial mic was my uncle’s who was a precision engineer - it’s probably 80 odd years old Moore and Wright and when I was a lad I would not have been allowed even to touch it ! Seems strange using it now !

HTH.

R.
richard b

Fit for such parts would have a minimal clearance typically around 1/2 thou as achieved using a precision ground pin in a reamed hole. This was the case with the Quinton Hazell kit that I sourced. The cotter pin is only intended to prevent rotation as otherwise the trunnion pin will fret in the kingpin hole. In extreme cases it would be possible to fit a thin wall sleeve into the kingpin after resizing the hole.
S G KEIL

This thread was discussed between 19/04/2022 and 20/04/2022

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