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MG MGB Technical - MGA flywheel on an MGB 3 sync

Hi All,

Does anyone know if the flywheel from an MGA (1600 Mk 1) is the same as a flywheel from an early MGB (1800 engine, 3 sync gearbox, 1967 car)?

Cheers
Iain
67 BGT
I D Cameron

No. 1. It is heavier, 2 it has a different pin configuration for the clutch.

The flywheel is, however the same diameter, and has the same bolt pattern for fitting to the crank.
dominic clancy

Iain

no they are completely diff as the MGA is a 3 bearing and yours is 5 bearing so will have to get one from a 5 bearing car only 1964 onwards

Steve
Ste Brown

Not to worry. I saw one on eBay and wondered if it would be the same as the gearboxes are very similar.

Steve, why does the bearings in the engine make a difference? According to Moss catalogue it is only the gearbox type that is mentioned. There is a single part number for the flywheel for all MGB's that have a 3 syncro box, whether they are attached to an early 3 bearing engine or a 5 bearing engine.

Iain
I D Cameron

Iain

moss are wrong again
12h713 is 3 bearing
12h1474 is 5 bearing

its to do with how it mounts and no rear oil seal

Steve
Ste Brown

Careful Steve!!!

Colin
Colin Parkinson

Ian-
Contrary to popular belief, the flywheels and ring gears used in the MGB are not completely interchangeable. The 10.75” (273.050mm) flywheel (BMC Part # 12H 713) and its ring gear (BMC Part # 1G 2874) of the three-main-bearing 18G and 18GA engines are not interchangeable with those of the later five-main-bearing engines, nor is its lockplate (BMC Part # FNX 506), nor are the mounting bolts (BMC Part # 51K 1022). This flywheel had three dowels. Both the 10.75” (273.050mm) flywheel (BMC Part # 12H 1474) and ring gear (BMC Part # 1G 2874) of the 18GB engine were special to it alone, although its lockplate (BMC Part # 12H 1303) and its six mounting bolts (BMC Part # 51K 1809) are common to the later, larger diameter flywheels. This flywheel also had three dowels. The engine backplate is also different due to the positioning of the electric starter assembly closer to the crankshaft in order to accommodate the smaller diameter flywheel. All 11.5” (292.1mm) 18GD, 18GF, 18GG, 18GG, 18GH, 18GJ, 18GK and 18V flywheels (BMC Part # 12H 2184) and their ring gears (BMC Part # 12H 2900) are interchangeable, although not with those of the earlier engines. These had two dowels. Interestingly, all of the engines used the same dowel (BMC Part # 1G 2984). Although the 10.75” (273.050mm) flywheels of the 18G, 18GA, and 18GB engines may seem more desirable to some due to their lower rotational weights, their matching ring gears are not interchangeable with that of the later 11.5” (292.1mm) flywheel of the later engines, forcing the use of both the matching engine backplate, inertia-type electric starter, and three-synchro transmission along with its appropriate length driveshaft (propeller shaft).
Steve S.

Never simple! So I have a gold seal (48G755) engine, which is wquivalent to an 18GD ir 18GG original unit. But I think the car would have had an 18GB engine originally (April 67 first registered). I am now building an engine based on an 18GG engine, with an 18GG crank. I have replaced the backplate with a 3 sync backplate, but I wasn't able to get hold of another flywheel, so part of the swap will be to take the old flywheel and transfer it over. The car is fitted with a late 3 sync OD box.

I assume that I am not going to hit any problems changing the flywheel over, but the above detial has left me a little confused, particualrly as I work with such a hotchpotch of parts!! I'm guessing that as I am swaping a flywheel that is attached to what could be a replacement for an 18GG engine then it will go straight onto another 18GG engine. I'm guessing any mods required will already be done
I D Cameron

Iain
Ifthe flywheel come off off the 5 bearing engine it will go onto any 5 bearing engine as long as you have a 5 bearing 3 synchro back plate as they are diff to 3 bearing back plates

Steve
Ste Brown

Hi steve,

Was not aware that there was more than one 3 syncro backplate! Any way of telling which one I have on my spare engine? What are the differences? The whole point of getting a spare one was to make the swap quicker and easier when I do it, but it is starting to look as though this could backfire!!

Cheers
Iain
I D Cameron

Iain, The difference between the two 3 synchro backplates is that up to '64 the rear main seal was in the block. From '65 to '67 the seal is in the backing plate. It should be easy to determine if you have the correct plate by seeing if it has a seal fitted. RAY
RAY

Oh good. In that case I will be OK!

Thanks
Iain
I D Cameron

Lain,

This is what happened to me.
To match a 18V engine onto a Three sync. gearbox
I had to sort out a 18GB flywheel. It's lower in diameter but a bit thicker than later 18Gx ones and exaclty the same diameter as 18G-18GA engines. Retaining the 3 main backplate is mandatory as well as keeping the very early inertia starter motor. You have to check for the gearbox spigot adaptability too, I can't verify any more this specific point by now. As long as 3 sync gearboxes were supplied, the 3 main backplate was matted onto engines. The screw template is quite compatible between two models.
I don't know exactly when the change occured, but surely at 4 sync. gearboxes time.
Hope this info relevant.
RG
Renou

This thread was discussed between 26/09/2008 and 01/10/2008

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