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MG MGB Technical - Clutch rebuild; two questions

The clutch on my '79 B (with overdrive) slips sometimes when I hammer on the gas when shifting into first and, more frequently, second, so this winter I'm going to pull the engine and rebuild the damn thing with the Borg & Beck kit from Moss. Two questions:

1. While I've got the tranny out, is there anything else I should look at replacing, either for performance or reliability? Anything I should look at replacing because it's probably worn out in the 26 years that the car has been driven? (I'm just looking at the transmission here; I've got a plenty long list of might-as-wells since I'm pulling the engine.)

2. Are there any "gotchas" I should be aware of before getting into this project? I've never rebuilt a clutch before, so it'll be all new ground. Will my Haynes manual be sufficient to guide me through?

Chris St. Pierre

You could replace all the gearbox mounting rubbers since you will be removing that anyway. Replacing the clutch is pretty easy. You might also want to check the pilot shaft bearing while the clutch is off. I think it comes in the clutch kit but you should also replace the carbon release bearing with a new one.

Trickiest part is getting it all aligned to get the engine and gearbox to mount back together. I had problems until I borrowed an old first motion shaft I could use as an alighment tool. You can get universal alignment tools but I am not sure how good they are.

Simon Jansen

Brit tech sells a nice kit with trans mounts, Trans main shaft oil seal and gasket, clutch aligment tool along with clutch and pressure plate. What else I have found to need during replacement is u joints and engine mounts. Also handy to have on hand is exhaust stud kit, driveshaft bolts and intake and exhaust gaskets since you will be pulling the engine and these may need to be replaced. Be sure to get the flywheel resurfaced check the bushing in the clutch lever and get a rebuild kit for the slave cylinder and a new hose if it is original, cheap insurance.
Charles O'Brien

Chris - Go to the following web site: click on Cars, then MGB GTS and Gearbox 3 and scroll down to Crossmember modification. An hours work doing this modification will save you at least an hour of frustration under the car trying to bolt the crossmember to the transmission. Good luck - Dave
David DuBois

I've never had a problem with the gearbox crossmember and getting it back together. That's on both my roadster and GT when I had it. Is it only the overdrive ones that are difficult since there is less room?
Simon Jansen

When I replaced my clutch, a couple of years ago, I did the following:

Replaced clutch/pressure plate/throwout bearing (don't even think about not doing all three - false economy)

Cleaned/painted the engine

Replaced slave cylinder boot

Replaced slave cylinder pushrod and clevis pin

Replaced throwout fork bushing and pivot bolt

Replaced transmission front oil seal/gasket

Replaced rear crankshaft oil seal (requires removing flywheel)

Replaced transmission mounting rubber bushings

Inspected flywheel surface (it can be turned like a brake drum, if necesary)

Inspected flywheel ring gear for possible replacement (mine didn't need it)

Replaced engine oil pan gasket

Replaced pilot bushing

Inspected slave cylinder hose

Inspected driveshaft u-joints for possible replacement

There are probably other things that you could do, as well. This is case where the "might as wells" can run rampant.

Hope this helps.

Paul Noble
Paul Noble

Clutch slip is *more* likely to occur in 4th, gradually becoming apparent in lower gears as it gets worse. However OD slip is more likely to occur the other way round, it is all down to how the torque changes either side of the gears.
Paul Hunt 2

Holy crap. Moss Motors is going to *love* me this winter.

I was already planning on rebuilding both master and slave cylinders and replacing the hoses. As I'm replacing all of the suspension bushings in the car this winter as well, it makes sense to replace the tranny and engine mount bushings (if they need it; the PO rebuilt the engine [but forgot the $#!&*@ tranny while he had the engine out] about two years ago) as well. I'm also redoing the exhaust system, so I'll be replacing the hangers anyway.

The Brit-Tech kit sounds awesome -- I'll have to look into that. The Moss kit is just clutch, pressure plate, and throwout, IIRC. Moss also sells an alignment tool for $5, which I was planning on getting; has anyone used this?

Also, does anyone have any experience with the aluminum or lighter-weight flywheels? The car is my daily driver, so I don't want to damage the drivability or gas economy of it too badly, but if I can get some more pep off the line, I'd take it.

Thanks for all the suggestions! Keep 'em coming.
Chris St. Pierre

Paul is correct. Check the clutch carefully before you proceed. Apply the handbrake and select 4th gear start the engine and slowly release the clutch whilst feeding in more power. The enging should stall. If it keeps going then either the clutch or the overdrive is slipping. If the drive slip is taking off in first gear then die to the vastly greater amount of torque being transmitted it is likely the overdrive that needs replaced. If the slip takes place when accellerating hard from sya 30mph in 4th gear then it's almost certainly the clutch. Either way you'll have to pull the engine and if you decide it is the clutch then this will be confirmed by the friction material being flush with the rivets and perhaps oily as well. If its first or second gear slip then pull the box and replace the overdrive as well.
Iain MacKintosh

Ugh, that doesn't sound like fun. It's definitely slipping in first and second; I've never had it slip in fourth. If I do have to replace/rebuild the overdrive -- as it's appearing -- what needs to be replaced? Moss doesn't seem to have a rebuild kit, just a zillion small, expensive parts. I don't have my Brit Tech or Victoria British catalogues handy, so I don't know if either of them would have a kit of some sort, either.
Chris St. Pierre

I'm a bit out here, but if it were mine I'd have a go at rebuilding the O/D. However I think I would be for advising you to get an exchange unit just for safety and to be reasonably confident that the whole lot didn't have to come out again.
Iain MacKintosh

Chris are you driving with the overdrive engaged all the time?
J A Kelly

This is a possibility, but I don't have an MG so if it doesn't apply, please correct me.

If there is a flexible clutch line (I assume there must be), it could be swollen closed. The pressure from pushing the pedal will disengage the clutch fine, but engaging will be slow. The swollen line won't let the fluid back fast enough, letting the clutch slip. The only thing is, it should be worse changing up to 3rd or 4th gear. But it's worth a look.


No, I'm not. I engage the overdrive when I go over 40 mph, and that's about it.

Tom and others--

The clutch does not slip at all in 3rd or 4th. I did the test suggested by Iain this weekend, and the car stalled. So it's definitely the OD.


I'm willing to rebuild it myself, but unfortunately none of the "usual suspects" appear to have a rebuild kit. Does anyone know what parts would be necessary to rebuild it? Do any smaller vendors have rebuild kits? Are there instructions anywhere? Thanks!
Chris St. Pierre

Chris, at least you have proved to yourself that it is the overdrive that is at fault. The Haynes (Bentley ??) manual gives a fair description of overhaul procedures. I reckon that you would need to replace the main annulus and also the sliding mechanisms if they are shouing signs of wear but perhaps only the centre anulus complete with friction materials. Check all parts foe wear all gears, pivots etc fit new circlips (don't skimp on this) and check that their retaining grooves are perfect with sharp edges. Replace all O rings and gaskets. Check the pump bore and all pistons and cylinders very carefully. Be scrupulously clean, wash all parts in kerosene and blow dry and through all passages with compressed air. Assemble everything with 20/50 oil.

Keep us informed how you proceed also consider lifting the engine and gearbox forward only and remove the overdrive from the rear but without actually splitting the transmission from the engine or removing it from the car.
Iain MacKintosh

I've had a read through of the above and no-one has mentioned the roller clutch (one-way clutch, sprag clutch, whatever you want to call it) in the overdrive. If the overdrive is slipping under accelleration, then the roller is broken and will require replacement. I'm not sure if they damage other parts of the overdrive when they fail - anyone had one go?

Oliver Stephenson

You're spot on Olly. I was thinking about it when I wrote the post but seem to have forgotten to mention it. My concern about all this is that overdrive parts are expensive and this little lot could well equal or exceed the cost of an exchange unit although that could be far more expensive in the States than in the UK.
Iain MacKintosh

I just got my clutch kit for my midget from the "B" Hive. Way cheaper than MOSS.
J.T. White

Iain, I'm not concerned about that, as a rebuilt O/D tranny goes for upward of $2000 here. The parts do look pricey, though -- those I can find. I've only done the most cursory searching, though, so hopefully some local parts dealers or other smaller dealers will have what I need. Still, I'm a bit worried about things like the circlips -- those are the sorts of parts that are made out of unobtainium.

I'm intruiged about your suggestion to *not* pull the engine, but just work on the O/D from the underside. I'm a bit worried about accessing everything -- will I need rubber arms, or should all of the parts be easily reachable from beneath?
Chris St. Pierre

Circlips shouldn't be any problem at all. They are general fixings used in many branches of engineering and so are standard and easily located.

Now, removing the unit from underneath I did suggest a little tongue in cheek. Obviously you will have to remove the rad and oil cooler in order to do this but you can remove this complete with the diaphragm panel without taking the oil cooler hoses through the panel. With a minimum of disconnection and dismantling you will be able to lift the front of the engine over the crossmember whereupon the O/D will clear the rear fixed crossmember. and you will be able to lower the tail of the unit down. From memory you have to remove the remote housing but you can then remove the circle of bolts and withdraw the overdrive.

This may all be useless information as you obviously need a pit and also a hoist but on the assumption that you are not goiong to replace the clutch there is no need to seperate the engine from the transmission and also no need to remove the whole thing from the body. You will have to break the joint between the collector pipe and the remainder of the exhaust and undo a few wires
Iain MacKintosh


Beware that often rebuilt trans/OD means that the box has been rebuilt and the OD bolted back onto it (after a clean of course!!). Chris Betson at Octarine Services ( will send you a properly rebuilt OD for about 465 GBP, including shipping to US. (about $820 USD). Another place with a good rep for properly rebuilding units is Overdrive Repairs in Sheffield ( but I don't know if they ship to US and last time I enquired they needed mine sent first to rebuild as they had no stock. If your box is good there is no point spending on another box as well.

Mine only slips after a long motorway blast (50 miles at 80mph) and so I haven't yet got round to repairing it, but it is on my list!

Iain C
I D Cameron

Iain M--

There are a few guys in the local club who have hoists, and I've got to replace the exhaust system this winter as well, so those aren't big deals, and I'd certainly rather pull the radiator than the engine!

However, pulling the engine doesn't sound too bad taking Iain C's suggestion into account, as I bet I could get a decent price for an O/D transmission here, even one that needed a rebuild. I'm certainly not going to buy a rebuilt O/D from the States, given the price, but it looks like Octarine's price is actually pretty competitive with the price for just the parts Stateside.
Chris St. Pierre

An overdrive from Octarine will be a very good investment and if the price is competitive I'd be inclined to go for it. I wouldn't be happy about fitting an unknown transmission though and I would be much happier with the one you know and presumably works well. I know you could rebuild a second unit but I always like to rebbuild a transmission that I've run and that way I know exactly where the faults are and can go straight and fix them. You wont know this in an unknown unit.
Iain MacKintosh

Just a quick test I would do before ripping the overdrive to bits...

Get thee up to reasonable speed in third gear, so you're on peak torque - 3000 rpm or so. Plant the throttle and use either a hill or your left foot on the brake to keep the car (and engine) at a constant speed - you said it doesn't slip in 3rd so this should be okay.

Then, flick the overdrive switch whilst keeping your feets where they are. If the roller clutch is broken, the revs should rise breifly before the overdrive clutch engages, as the pistons move the overdrive between the two positions.

Does this make sense to anyone? I'm thinking that if the revs don't come up in the transition, the roller clutch must be holding torque.

Probably not worth much, but I thought I'd just put it out there anyway :o)
Oliver Stephenson

I knew we would be in trouble here when Chris said TWO questions!!!

Don't think I have anything to add because in aggregate it looks like the list I ended up with when my clutch went.

Almost organic, these cars, aren't they!

Roger T

Some sort of sense Olly especially bearing in mind that the main clutch has already been tested and seems OK. However I'm not sure that this test will be conclusive and may indicate a fault in either friction material or the roller clutch ??
Iain MacKintosh

My only reason for doing it would be to really really make sure that it is the overdrive that is at fault - if the roller clutch is not broken, then I would be suspicious about the overall diagnosis of slipping overdrive when acellerating.

I'm not suggesting that the diagnosis is incorrect - just that this is what I would do if I had the same symptoms.

Oliver Stephenson

This must be an overdrive problem Olly. Chris says that it only slips in first and second gear. Now I suspect that slipping is worse in first (confirm please Chris) as this is the gear in which maximum torque will be transmitted through the overdrive. Torque in the O/D will be progressively reduced in the O/D in the higher ratios so the unit won't slip (but it soon might ) In fourth gear O/D torque is much reduced but the torque through the main clutch will still be the same and as the overdrive is not now reaching its limits the engine stalls. This at least prooves that the clutch is capable of withstanding full engine power which effectively rules it out of this equation. I still suspect the friction cones in the clutch and can't immediately think on a way to prove this. Maybe the ultimate decision will be on inspection of the roller clutch which if faulty I think might just tear itself apart instead of slipping.
Iain MacKintosh


You are correct. Under normal driving conditions, slipping is worse in first.
Chris St. Pierre

The comment above by Paul Noble about turning a flywheel like a brake drum implies to me that a lathe with a bit will do the job. I have seen this done and it is a BAD idea. To resurface a flywheel you grind it with a stone not cut it with a bit. The reason we can't use a bit is that a flywheel gets hot spots. (The hot spots are caused when the heat from friction changes the cast iron of the flywheel to to a harder form of iron like steel). A bit will bounce over the hot spots and leave a worse surface for the disc. A stone on a proper flywheel machine grinds off the hot spots and leaves a smooth and even surface. That is why I bought a Van Norman model 9000 for over $10,000 instead of an adapter for my brake lathe for next to nothing. Paul if I read your comment wrong i apologise,I just thought people should know the difference between a grinder and a lathe. Pauls list of things to do and check is excellent advise.
R J Brown

Looks like its end of story then Chris and you have to take thew overdrive out for repair / exchange. Please let us all know how you get on as there is not too much information around about this and I'm sure we will all be waiting just to see how easy and successful the rebuild was. Maybe you can go into the O/D overhaul business !!
Iain MacKintosh

Just a point on the sprague clutch - in a non-OD gear i.e. 1st and 2nd both halves of the clutch are turning at the same speed as they are locked together. If the friction material is slipping then the clutch will also slip as that is the direction in which it is designed to slip. In a forwaqrd gear the physical design means that as acceleration torque is applied the friction surfaces are pushed harder together which tends to resist slipping. However in reverse the opposite is true but as soon as the friction material *does* start to slip the sprague clutch locks to transmit the drive. In OD the one half of the sprague clutch is being overdriven relative to the other half, but in the direction that allows it to slip. But if OD should happen to be engaged in reverse the sprage clutch will be overdriven again, but in its reverse direction, so you get the force of the friction material fighting against the locking action of the lutch, and that is what destroys the rollers and carrier.

If the OD is slipping in 1st and 2nd i.e. no OD then it is the friction material or return springs that are at fault, or blocked passages allowing oil pressure to remain in the cylinders which will reduce pressure on the friction material.
Paul Hunt 2

Paul Hunt makes a very good point as normal and John Twist says that 90% of all overdrive problems can be cured from the outside without removing the unit for repair. I wonder if it is just worth removing the O/D sump and replacing all o rings and the ball valve and filter, removing the gauze filter and pump piston and cleaning and blowing the whole thing out before resorting to a complete strip down. Seems worth the hour it would take. What do you think ?
Iain MacKintosh

Paul... I think I must be mis-reading your post. When in a non-OD gear, with a slipping direct friction but a healthy sprag clutch, the OD as a whole will not slip during accelleration, but will slip on over-run. This is the opposite of the meaning I get from reading "the clutch will also slip as that is the direction in which it is designed to slip".

The OD in my roadster is holding hydraulic pressure when hot, making the direct friction slip, so I get this exact behaviour and can reliably demonstrate after a half hour thrash. I get no engine braking unless overdrive is engaged (3rd/4th only) and cannot reverse unless I leave the car to sit for a while, so that the pressure can bleed down.

I'm not sure how strong the sprag clutch is, but I have launched hard in 1st (165 tyres, stock motor) with the OD in this condition and I haven't broken it yet!

Apologies if I've mis-read your post, I have had my head in electronics datasheets all day so I'm a bit out of focus.

Oliver Stephenson

Don't have the workshop manual to hand but I *thought* I had it round the right way, I'll have another read tonight.
Paul Hunt 2

R J,

I don't really know the proper method of resurfacing a flywheel. I just know that it is often recommended as part of a clutch replacement. At least, one should look closely at the clutch surface. I guess I did assume that a lathe would be used.

The point is that it can be rough or wavy and chew up a new clutch disk in short order. Like brake drums or disks, it may need to have its working surface renewed.

When I did mine a couple of summers ago, the flywheel was very smooth, so I opted to skip that step. I did check with a reputable machine shop about the cost. I think they wanted about $28 dollars to resurface it, although I didn't ask if they were going to use a bit in a lathe or a grinder.
Paul Noble


Yes, I'd much rather clean the sump than tear apart the whole car! I'll have to arrange some time on a lift and give that a look. I'd love to save myself $900 and/or a week of work!
Chris St. Pierre

This thread was discussed between 30/09/2005 and 06/10/2005

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