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MG Midget and Sprite Technical - 1098 gearbox on a 948 engine

Some help please - continued from previous identify this gearbox thread last month... despite reading everything I can find I cannot work out basically what defines which clutch pressure plate, clutch fork, release bearing, and slave cylinder should be used for which combination of engine and gearbox.

Car as received:
1960 modified frogeye sprite - modified in period - was a right old state - has gone through a nut and bolt rebuild apart from the gearbox which has now broken some teeth and also jumping out of reverse

Drive train as received and rebuilt
948 + 60" engine
948 crankshaft
948 flywheel - whole bottom end balanced
6.5" clutch
9 spring pressure plate - ? aftermarket
1275 throwout bearing
1098 clutch fork but appears bent and ? cut or snapped and welded back together (see photos)
3 1/2" slave cylinder pushrod
7/8" dual bore master cylinder
22g118 gearbox frontplate
ribbed 1098 gearbox with gearset that is identical to 1098 morris gears + bronze bushed and baulk rings (all marked HS14 rather than normal part numbers..)

I am concerned the clutch geometry is a mess and whilst have it apart want to sort it out
Could you help me fill in the ??? with what should be in the system:

948 + 60" engine
948 crankshaft
948 flywheel - whole bottom end balanced
6.5" clutch
???? what pressure plate ? standard 948
????throwout bearing
??? clutch fork
?? slave cylinder pushrod
7/8" dual bore mastercylinder
22g118 gearbox frontplate
1098 minor gearbox - have a spare that am rebuilding

I can't get my head around all the different changes and basically what is needed to mate a 948 engine to a 1098 box.

any help really appreciated.

(bizarrely have another 1098 goal seal spare box that has a 2a3087 i.e. 948 front plate on it with the correct mountings! incase this is what I should be using with this system)
DJ Paul

Something else to add to the mix is the engine backplate.

948 had a thin pressed steel item, whereas the 1098 & 1275 used thicker, cast items.
Dave O'Neill 2

The Factory recommended way to do the conversion to a ribcase gearbox on a 948 is to use
Backplate from 1098.
Early 1098 flywheel.
1098 7.5 inch clutch cover and driven plate.
1098 release bearing and yoke.
The info is to be found in the BMC Special Tuning Manual.
Alan Anstead

I assume reading that you have balanced the bottom end incl flywheel you still want to use it.

Years ago (1970?) I did the same in reverse - 1098 (van engine - dished pistons and high comp head - those were the days !) onto frogeye smoothcase box and ISTR using all the 948 bits (bigger clutch was obviously not possible etc) therefore used 948 flywheel but uprated the clutch to a 1200 Spitfire - all a bit hazy now - age thing ! but it worked fine.
richard b

After trying every combination known to man I settled with:

948 block
Standard flywheel drilled for 1275 clutch
1098 gearbox
1275 slave cylinder
Master cylinder sleeved with brass tube to 3/4", custom made piston. Small port in cylinder relocated to fit shorter piston.
1098 release fork
Roller bearing from Ford Sierra, in custom made carrier.

At last it all works fine. I could never get the 9-spring to work properly, the pedal pressure was horrendous. Diaphragm clutch is far better.

L B Rose

Sorry forgot to say that I am using the thin backplate.

L B Rose

Thanks you all so far.

@Alan, makes sense!
@Dave and richard: have thick backplate and the bottom end is all balanced, including the current 9spring pressure would be useful to still be able to use the 948 flywheel and 9spring pressure plate to maintain the balance.
@ LB Rose: where did you get your master cylinder sleeved and piston made? only place I can find is gerards harage in the USA>

is anyone able to explain to me why each bit is needed from which combination? I've never got a clear answer for 'why'

elsewhere been told this is the correct combination:

thick backplate
948 clutch
948 pressure plate( could downgrade to 6 spring as I agree L B Rose the 9 spring pedal pressure is horrendous!)
948 throwout bearing
1098 clutch fork
1098 slave cylinder push rod


anyone got a 1098 early flywheel for sale? as that combination makes far more sense and would be within my grasp.

DJ Paul

Where do you do your DJ-ing, Paul?

I'll get my coat . . . .
Nick and Cherry Scoop

Ha, hadn't noticed it was showing like that...

A life time of being called Paul is one thing....DJing is another...

DJ Paul

DJ, I did the master myself. I found a model engineering website that sold the brass tubing in exactly the right size. Sealed into the casting with high quality silicone. Most difficult bit is relocating the tiny port, as you can't get a chuck into the reservoir. However you could forget that if you keep the piston at standard length. I think the factory shortened it slightly because there is a rocking motion from the pushrod and with the smaller piston the counterbore is smaller and hence danger of binding. But on the brake side I just turned down the pushrod in the lathe. Of course, the clutch piston moves more so you might not get away with that. You will still need to drill the port through the brass tube, and then remove the bur on the inside. 3/4" pistons are difficult to find but I might be induced to make one for a couple of beers.

About 40 years ago I had a 9-spring clutch on my first Sprite and didn't find the pedal too tough - but I was a lot younger then!
L B Rose

Would you believe that not only is my name James Paul, but I am from Devon as well and I am DJ! My DJ name is DJfromDevon!

I also get called Paul all the time!
James Paul

Haha small world.
Just across the border nearish tiverton from you.

Im thinking the pedal pressure is because I've got a right old mishmash of bits through the whole hydraulic system

7/8 master
Front discs from later car
single piston rear plates from early car
9 spring clutching running on incorrect clutch fork geometry

Gonna work my way back see what can sort, have aset of twin back plates for the rear brakes, a 3/4 and 5/8 inch girling master cylinders so could make a sebring pedal box up etc

Gonna have a good old think.

Looking at the 9 spring clutches fitted to the late 948 cars it appears there is a slightly different part number for the clutch fork, I wonder if this is again different to the standard 948 clutch fork and the ribbed case clutch fork.
DJ Paul

This thread was discussed between 10/01/2018 and 14/01/2018

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