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MG MGA - MGA and MGB engine backplates

I am asked to clarify the differences between a MGA high starter backplate and a MGB 5 bearing backplate. AFAIK the only differences are the height and the area around the crank seal. Am I correct, and can a standard 5 bearing backplate be used to mate a five bearing B engine with a high starter MGA gearbox without modification?

I know the front plate has to be swapped for the engine mounts to work, and that the 3 bearing flywheel (B or A depending on clutch) has to be used

Dominic Clancy

Hi Dominic.
Knowing how much work you do on these cars yourself I’m not sure what follows will be of any help.
I think it depends on which five bearing engine you are using. When we fitted an 18V engine to my MGA Mk II (and therefore of course “high starter motor”) we used the 18V backplate but the cut out area for the starter had to be modified (enlarged downwards from memory) to fit the starter in the original MGA location. (We machined the flywheel down to MGA diameter, but an 18GB flywheel would have been easier if we had the luxury of finding and using one).

I can’t commment about the 18GB (first 5 bearing MGB engine) starter location but would not be surprised to discover it was the same on the 18GB MGBs as the preceding “high starter” MGAs, but I simply don’t know.

I presume you would know anyway that the 18GD engines onwards (Mk II/ GHN4 and onwards) had a larger diameter flywheel + ring gear and altered location for the later pre-engaged starter motor. The backplate for these would need to be different to the earlier cars.
T Aczel

No - the 1800 plates carry the starter slightly lower than the high starter MGA plate so the plate doesn't match the gearbox.

Best solution is to use the Highgear plate.

Pic below shows the MGA plate on top of the 1800 plate - everything lines up EXCEPT the starter mounting holes.


Chris at Octarine Services

Just to add - the 18GB flywheel can be used but not the 3 bearing 18 & 18GA ones - the flywheel mounting on the crank is different.

Also front plates only need swapping if the B engine is an 18V847 - up to 18V 780 the front plates were the same as MGA.
Chris at Octarine Services

Thanks Chris. I knew someone was bound to have more accurate and extensive knowledge about this than me, but after close to a day, I thought someone better contribute something. Patience on my part would have been a better virtue!!
T Aczel

Long time ago for me to be precise but Bob West supplied me with an MGA high start back plate. I took it down to the local machine shop with an MGB 5-bearing seal and they machined it to fit. Been working beautifully for 18 years.

Steve
Steve Gyles

You have to use the engine back plate that matches the gearbox.

MGA early and mid 1500 = low starter.
MGA 1600 and 1600-MK-II = high starter.
MGB G and GA engines = high starter in different location.

All of the above have 120-tooth 10-inch flywheel.
MGA has two locating pins, 28 pounds for most, 20 pounds for the MK-II.
MGB has three locating pins, 20 pound flywheel.
You can relocate the pins to change from MGA to MGB clutch.
For MGB clutch use gearbox front cover, release arm and release bearing from MGB 3-ynchro gearbox.

MGB GB engine 1965-1967 was first of the 5-main engines with different bolt pattern for the flywheel. The rear plate has different center hole to accept the crankshaft rubber rear seal. To mate this engine to an MGA gearbox, use the MGA rear plate and bore out the center hole to accept the rubber seal. Also plug the drain slot in front side of the MGA rear plate.

All MGB engines from 1968 and later have larger flywheel and different back plate to suit the larger 4-synchro gearbox. To mate these to MGA gearbox use the flywheel from 1965-1967 MGB.
barneymg

Re the late backplates, they are not a straight swap - one of the gearbox bolts is out of place and the starter hole and mounting is in a totally wrong place - would need major surgery to make it work.


Chris at Octarine Services

Chris

Sounds like another order on Friday!

Dominic
Dominic Clancy

This thread was discussed between 16/09/2018 and 17/09/2018

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