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MG Midget and Sprite Technical - Clutch Drag - Spigot Bush

I followed Philip Sellen's thread closely as my early Mk IV Sprite's clutch has always been less than perfect. When everything is stone cold manoeuvring out of the garage, there is no drag or crunching into 1st or reverse. Once things are warm, the gearbox input shaft obviously keeps spinning.

I'm about to pull the engine to address a few issues: rear main oil leakage, jumping out of 3rd gear and trying to get rid of the dragging clutch.

My main suspect is the spigot bush but I realise that the cause could be the clutch plate, especially if it's contaminated with oil, or possibly a bent release arm; the release bearing is one of Alan's roller-types.

I fitted a new Borg and Beck clutch when I rebuilt the engine 2 1/2 thousand miles ago. I bought a new spigot bush but did not change it (couldn't work out how to get it out! I know now).

A couple of years later I fitted a self-re-built 1275 gearbox to replace the Morris Minor one that it had when I got it. I couldn't find the spare bush so bought a new one from the same supplier, I now had three.

When it came to fitting the gearbox, none of them seemed to be quite right, either not wanting to go in the crank or not wanting to go on the gearbox. I ended up re-fitting the original one.

I don't know the dimensions of the one in the car but the two new ones are different.
One is similar to the one currently fitted with a length of 12mm, outside diameter of 15.85mm and internal 12.8mm.
The other is 15.85mm long, external diameter 15.8 and internal 12.74.
Which of these is correct?
Does anyone know what the official specification dimensions are?

I presume the bush is meant to sit firm in the crank with the gearbox input rotating within it. Is it just a fairly easy push-fit or should I use the 15.85mm one with the larger internal diameter and chill it in the freezer before trying to fit it?

Sorry if this is going over familiar ground again.

Colin

C Mee

I forgot to say that, in chasing the dragging clutch problem, this year I have re-sealed the far from ancient master cylinder, replaced the slave cylinder and changed the remote bleeder to a hose-based one to Alan's design. The last job was to replace the master cylinder with a new one which and that gave the biggest improvement but didn't quite eliminate the dragging.

C
C Mee

I just wonder if torque is consistently being applied via the clutch or spigot. But that it only actually starts to spin the input shaft when the gearbox oil warms up becoming less viscous. It could perhaps explain the odd feature of seeming to only be an issue once warmed up?
GuyW

Thanks Guy.

That seems quite plausible. Even spinning the gears for a short while would distribute it to some extent, reducing the oil's drag.

Before the new master cylinder, when things got well warmed up on runs gear selection would get harder to the point of impossible at times. I put that down to the clutch hydraulics but maybe the thinned warm oil could also have provoked it.

C
C Mee

Use grease to remove the spigot bush.

Fill the bush with grease. Hammer in a spare input shaft or anything the same diameter. That'll pop out the spigot bush.

Time honoured method.

https://www.mgexp.com/forum/mgb-and-gt-forum.1/removing-pilot-bushing-without-a-puller.2882799.2882806/

https://youtu.be/2TBZH5R4Cno?si=FU92pVnqh368doH3




anamnesis

Spigot bush.

If too loose, lock it in the crank with bearing fit.

If too tight to fit, slit it lengthwise, which will reduce the diameter.

It must spin freely on the input shaft.

It's only really a guide. There shouldn't be much pressure/force on it.

The input shaft bearings in the box mostly keep the shaft in line. The spigot bush is an 'aid'.

anamnesis

Thanks Anam.

I used the grease method when I swapped gearboxes - I was amazed how easy it was.

Thanks for the other suggestions.

C
C Mee

Some bush dimensions from MED.

They don’t sell the early bush. The bush that they sell is to replace the needle roller bearing, used in the later crank. They do give dimensions for both, however.

Nominal dimensions.

Dave O'Neill 2

Thanks Dave.

It looks as though the smaller one is what I need.

Many Thanks

Colin
C Mee

Make sure your new bush is a self lubricating sintered bush and make sure it's fully loaded up before you fit it up- a plain brass/bronze bush will pinch up when it gets hot.
William Revit

My 1971 came with a small diameter bushing. Over the years when the engine would be removed for clutch replacement, I found a bushing with a crushed cone shaped end (probably due to a transmission reinstallation issue), no bushing at all, just bronze flakes in the bell housing, seized bushings stuck on the input shaft or in the flywheel pocket and several that appears to be OK.

During a recent rebuild, the machinist damaged the crankshaft when grinding it and supplied a replacement. When I installed the transmission, I found that this crank used a larger diameter bushing that had to be driven into place. I always pack the pocket with grease and make sure that all the bearing surfaces are coated. The thicker bushing seems to be holding up well, with no input shaft drag. Apparently two diameters were used over the years.
Glenn Mallory

I don't know what BMC recommend re spigot bush lubrication but using grease with a sintered bronze bush is not recommended as it blocks the pores and prevents oil circulation. On the subject of are they pre-lubricated or not, they're supposed to be and I've never had one that hasn't been. I've had plenty that didn't seem to be but when checked they were lubricated. A few years ago I had one that seemed dry but I had to machine it to length and once spinning in the lathe a neat annular ring of oil formed on the OD and disappeared again once it stopped turning as it was reabsorbed. No harm in letting it soak up a bit more though.
David Billington

No engineering training but I thought you were supposed to soak them in hot oil and then leave them in the oil to cool, drawing the oil into the pores.
GuyW

A video here https://www.youtube.com/watch?v=7-Dsii-WxKY covers some interesting points about them.

I have seen a video of their production where to fill with oil they're placed in a vacuum chamber which is then evacuated and then flooded with oil so filling the pores in the sintered bronze, the oil is then allowed to drain and the bushes are ready to go.
David Billington

Just remembered sintered bearings aren't just bronze other metals are used as well such as iron. I was amazed when I went to braze a Ford clutch release bearing carrier and saw the amount of oil that came out when it was heated, it was sintered iron and oil impregnated.
David Billington

Well, that's that theory out of the window.

The engine is out and nearly dismantled. The gearbox separated easily and the spigot bush came straight out using the grease method. It is an easy sliding fit on the input shaft with a tiny amount of play.

One reason for having the engine out is to (try to) deal with the rear-end oil leak. There was next to no oil in the bellhousing and the clutch plate is perfectly dry with no sign of oil contamination.

In the past I've tried different length push rods with no effect so, while there's the faint possibility that the lower viscosity of the warm gearbox oil might have an effect, I guess it's back to bl**ding bleeding again.

I've managed to lift the engine out without the clutch slave cylinder so I'll be able to orient it so the bleed point is as high as possible.

In the mean time, the spigot bush has been heated and I was pleased to see oil bubbling out. it went hot into a 35mm film canister full of oil where it will remain until I start reassembly, probably several weeks from now.

Thank you for all your comments, advice and suggestions.


Colin
C Mee

Colin when you bleed it do first push the piston as far back up the bore as it will go and clamp it firmly there. Firstly to reduce the volume of fluid in the slave, which can harbour lurking air bubbles that get bypassed in the bleed process. And secondly so that you are pushing against a solid resistance rather than the possible movement of the clutch springs. It makes clutch bleeding akin to brake bleeding where you are pumping against solid brakes so that the air is forced out.
GuyW

Guy, yes I've tried several variations: zip-tying the slave's push-rod fully in has worked well in the past but more recently I've been using a telescopic prop against the seat back to hold the pedal on the floor.

I have also tried parking the car across the slope into the garage to tilt the driver's side up. More recently I've been jacking it up as high as possible on the driver's side.

A while back I welded up and re-drilled the worn clevis hole on the pedal and added an oilite bush.

None of the methods have completely eliminated the problem, hence looking for another possible cause.

With the cylinder 'loose' in the engine bay I should be able to orient it with the bleed point upper-most (there's a brake-hose based remote bleed arrangement). That, combined with a zip-tied push-rod just might do it, beyond that I'm stumped.
C Mee

This thread was discussed between 19/10/2025 and 27/10/2025

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