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MG Midget and Sprite Technical - How those little jobs do grow!

Little job: MOT coming up so jack up the back of the car and turn the rear brake adjusters in by 2 flats. Time: 10 mins, (max)

Nearside one done. Offside - ahh! soft click as it turns, and jumps its thread in the backplate. Remove adjuster, thread looks fine but try new one from the spares cupboard. "click" !

So, its the threaded bush in the backplate. No spare backplate. 10 mins has instantly escalated to days!

The job now entails removing the half shaft, and the hub - so new oil seals at the least,
- and check and possibly renew the bearing,
- and disconnect hydraulics to cylinder,
- so that's a brake bleed job too,
- and remove and replace the wheel cylinder!

But first find a new backplate !!!
Grr!

(actually I enjoy fixing stuff like this - just I prefer to choose when, and not be driven by an MOT date!)
Guy W

I don't know, abit of pressure ........!

And there I am the opposite; only working under pressure.

Takes all sorts I suppose.

Good luck with the mechanical half rear rebuild Guy.
Dave Squire - Notts

Sooo, it seems 1275 backplates are no longer available!

Looking at the thing, the only part which is damaged is the threaded bush that seems to be fitted into an irregular shaped hole in the backplate and then peened over at a couple of points to hold it there.

So I need a threaded bush if they can be bought.
Or I need to make one, but doubt if I have a tap to cut that fine adjuster thread - it looks like a 3/4" X 24tpi I think.

Edit: just found that Moss do something listed as:
17H7620 ADJUSTER REPAIR KIT £7.66
No explanation as to what it consists of, or even if it is that adjuster. Anyone come across this?
Guy W

Nope, but I'd be interested in what that rear brake adjuster repair kit comprises too.

Try Andy Jennings for a back plate. I was able to get a decent pair from him a few years ago and he's still breaking spridgets, so you might be lucky.

http://www.mgcars.org.uk/andyjennings/mg-midget-parts.htm

Also, on ebay ---

http://www.ebay.co.uk/itm/MG-MIDGET-REAR-AXLE-BACKPLATE-/230755784641?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item35ba1dd7c1
Lawrence Slater

Guy,

Also try Richard Woolley - Derby ?

He often advertises on the parts board.
I have his e mail somewhere - drop me a mail if you need it.

I had some good backplates from him a while back - if you are stuck I can probably find you one from my stash !

R.
richard boobier

that Wrexham lot seem to have stratospheric prices, I do wonder if anyone's daft enough to pay! I sold an excellent pair of backplates only a couple of weeks ago for about 20 quid.
The MOss adjuster repair kit IIRC is just the threaded adjuster and the two wedges.
David Smith

Yup that's right david, just found it with a picture on sussex spares.


Lawrence Slater

Pity. That is not what I would call a repair kit. Those are just standard replacement items. I was hoping it was some sort6 of replacement bush that could be fitted.

I have "bodged" it for now, but it isn't a permanent solution. Just went around half of the circumference with a drift to distort the bush such that the adjuster, when screwed in is now forced across to mesh tightly with the good threads.

The other possibility is to run some weld around the thread and drill and re-tap it. At least I could do that in situ and not have to disturb the hubs etc!

Thanks for the links, Lawrence.
Guy W

Helicoil

FRM
FR Millmore

Yes, I did wonder if I could get a Helicoil that size. I have used them before on other attachments and they are surprisingly easy to use - very satisfying too!
Guy W

I thought that also - however I could not find the size Guy quoted.

There is a fine thread 3/4 x 26 Cycle etc, but not found an insert to fit that either.

R.
richard boobier

Guy,
I've got a back plate sitting on my bench at the moment with the same problem. I was intending to make up a stainless threaded sleeve on the lathe, to weld in and replace the broken threaded insert, perhaps I should get on with it and make two? Is it 3/4? I only measured quickly and thought it was 5/8 24TPI.
I might also have a decent spare, I'll take a look tomorrow.
Paul (MkI

Hmmm, forgot about those fat boogers!
Can you weld a sleeve in it and use a 3/8-24 MGB adjuster?

FRM
FR Millmore

Paul, you are probably right about the size, my guess was just from the look of it.- l will measure it properly later.

Looks like either a helicoil or a sleeve and tap as FRM suggests is the easiest. Especially if there is more than l to do !
Guy W

If I can get a helicoil and tap of the right size, then I think that is the way to go. (I might even lend the tap to Paul!.

But just want to check with "those that know about these things" if helicoil is in fact appropriate in this application. I have used them before and think they do an excellent job. But how dependant is it to have a maintained tension on the coil for it to stay in position? What I mean is that I can imagine that with a bolt tightened into the coil, and under axial tension, it causes the coil to expand and the outer threads then maintain a tight interference fit. In the proposed application, the adjuster is moved along the thread as it is turned, but it doesn't really set up an axial tension in the same way. It is just an adjusting screw. So will the helicoil insert grip and remain fast? I guess I could always add some locktight when fitting the insert. Thoughts/ comments on this?

Second thing is what size am I looking at. (see photo) Pitch gauge confirms 24tpi. Calliper reads as 0.612" This converts to a bastard size of 4.896/8", slightly less than 5/8". So is that a 5/8" X 24tpi UNF or is it some other strange thread entirely?


Guy W

In a similar situation, I've had some success with cutting 4 slots at right angles down the length of the spare adjuster and filing a slight taper at the end, so converting it into a "tap". Use it like a proper tap with oil - forward a bit, back a bit and it may be sufficient just to reform the thread in the bush.
M J Chapman

No, that won't work. The thread in the bush has worn very slack such that the adjuster screws in very freely, but jumps the thread.
Guy W

Well if it is a 5/8" X24 then I am struggling to find a Heliocoil at that size. UNF is only 18 tpi. Nothing that I can find is as fine as 24tpi.
Guy W

Maybe its back to the idea of running some weld around the thread and re-tapping. Or maybe braise would be easier? I can get a 5/8" X 24tpi tap for a tenner.
Guy W

OD of threaded things is normally a bit less than nominal, so I'd say 5/8". The /24tpi is a special, UNEF probably, not likely to find a Helicoil unless it is used for something more common - like an aircraft application. Helicoils are substantially larger than the hole they go in, and are greatly compressed when installed. That is what holds them in place, even when the Helicoil has the distorted threads to make it a prevailing torque locking fastener.

FRM.
FR Millmore

Guy,
If you put (weld or other means) a plate over the original hole, and then drilled through, so that the "new" hole is just smaller than the original, you could then use the modified adjuster described by M J Chapman.

Save you a tenner on a special tap, would definitely be the correct size and thread, and I think it would be easier to tap than weld, which is often very hard.

Thoughts?
Lawrence Slater

Weld a flatened cone onto a bolt (thread-end to blunt part of cone); and use 2 clamping nuts either side of the strap across the backplate to determine how far cone pushes the wedges apart.

No problems then with dissimilar metals...

Or weld one of the nuts onto backplate external to replace theads in backplate.

A
Anthony Cutler

This is why you shouldn't play with things unless they need playing with.
C L Carter

True, I absolutely agree with you Christian,but on this occasion the brakes did need tightening up for the MOT. It wasn't really optional!
Guy W

Not sure I agree with that Christian. I've had the same problem as this, and it could have occured at a much the worse time, like on a long trip, instead of close to home.

The crap and muck on the rear of the backplate, eventually finds it's way into the threads I reckon, and grinds away each time you adjust the brakes. You can't avoid adjusting the rear brakes, if you don't like a crap pedal.

So it's just a matter of time --- a long time as it happens -- before the adjuster is forced out by the strength of the brake springs, and the hydraulics. That's what happened to mine. Very suddenly my pedal went much further. When I went to adjust the brakes, the adjuster just kept popping backwards. Luckily, I got lucky with Andy Jennings having a couple of good spare backplates.

Anyway, Anthonys suggestion sounds a good one.

Other than that, I'm kind of surprised that there are no new ones for sale. There's obviously a market (growing) for them, as the ones on peoples cars are all getting to the same point when the threads give out.



Lawrence Slater

Only problem with Anthony's suggestion is that the steadying strap across the rear of the backplate has long since gone. They are pretty flimsy affairs and I think there would be too much spring in them to work effectively in supporting the brake shoe pivot point anyway.

I made enquiries today and it seems that no one stocks "helicoil"- type kits for this fine UNEF thread (UNified Extra Fine) So I have ordered a 5/8 X24 tap instead.
What I actually do will depend on what I find in my Bitsa Box (AKA garage) I may try the weld idea, but as Lawrence points out, weld can be quite hard for tapping. Or I may drill out the old bush and weld a new slice of thick walled tube in its place, and then tap a thread into that. Innovation needed!
Guy W

With the strap gone anyway, maybe you could just weld a correctly threaded nut or bush to the outside of the backplate then?

You could leave the existing bush in place as a guide. Insert the adjuster so that enough thread is exposed at the rear, to allow you to screw on the correctly threaded nut or bush, and then just tack weld it on. Then remove the adjuster, and then complete the weld.

But whatever you do, when you perfect the method, maybe you can go into the Spridget brake backplate renovation business Guy ;)
Lawrence Slater

I'm having a quiet sob here when I think of all the axle casings I've chucked out over the years that might have yielded some good backplates :-(
David Smith

I could join you in that quiet sob.

When I think of all the years of visiting breakers yards after 1977 (when I got my sprite), and being told "take whatever you want" from various spridgets, and only ever "taking what I needed".

Now there are no more breakers yards like that, and parts from braking spridgets go on ebay for a premium.

If only I'd have known back then ------- :)
Lawrence Slater

Ditto sob!
Dave O'Neill2

Guy, a tenner for a tab is well worth it compared to thread cutting on the lathe. Do you want any sleeves making up so you could just machine out the old one & weld/braze them in and then tap? That would be very simple.
Paul MkIMkIV

If you do decide to look for a new backplate, you could try this guy:

THE MG MAN
Tel: 07855740024
The Old Saw Mill, Back River Street,
Congleton, Cheshire, CW12 1HJ

I just bought a rust free (yes, absolutely rust free) drivers door from him for £45 - which I thought a good price given the amount of work I was in for with the old door. He told me he was breaking several Midgets. He might be closer to you as well although he offered to courier my door (I collected - I prefer to find any problems with the seller present).

I have no connection with the MG Man.

Chris


Chris H (1970 Midget 1275)

This thread was discussed between 12/09/2012 and 17/09/2012

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