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MG MGB Technical - 5 main engine on 3 main gearbox

I would like to replace the 3 main G engine in my MGB with a 5 main GB engine, but keep the gearbox. This is a Ford type 9 conversion, but of course the clutch housing to engine interface is identical to the MG transmission.

I know the spigot bearing is different, and have the right adapter for this. But are there any differences in the backplates between the 3 and early 5 main? Is for instance the starter in the same position?


Yes, the starter was in the same position until the full synchromesh gearbox came out. The holes in the backplate were all the same as well - after all they had to match the gearbox.
What is the problem with the 3 main bearing engine?
Paul Walbran

Thanks, Paul - then it will be an easy conversion!

The problem with the 3 main bearing engine is that it is now leaking too much oil through the rear crankshaft bearing. As you probably know there is no proper oil seal there, only a "scroll" type seal. I suspect gas is blowing by the piston rings and creating a lot of pressure in the crankcase, forcing the oil out. The cylinders have been bored out + .060, so I guess the walls can be a little flexible.

It has been working well for 8-9 years since it was overhauled, with only a "normal" oil leak for this type of engine, but now something is happening.

I will of course keep the engine, dismantle it and try to find out what causes the problem, but it will be comforting to have an engine with a proper oil seal in the car.

A 5 main will not run the 3 main mechanical Jaeger rev counter - they have no place for the drive cable to go.

You will need a later Smiths electric rev counter.

At the moment I am putting a 1964 Jaegar rev counter face on a 1969 Smiths gauge to keep my interior gauges looking the same because the supercharger camshaft in my 3 main has no rev counter drive.

I've not finished the job yet. The rev counter is fitted but I haven't even tried to get it reading the correct revs. At the moment I am running my rebuilt engine in at restricted speeds so I don't need an accurate rev counter. When I get my supercharger fitted I will do.

The Smiths rev counters were made as either positive or negative earth so that is another consideration.

John Prewer

If yours is the early 3 main bearing type without the PCV breather, a big improvement can be made to oil tightness by installing the PCV valve off the 18GA set-up. At the request of a customer, we fitted one to an XPAG engine recently which had been brought into us with a bad rear main oil leak - it left a CD-sized puddle each time it stopped. Once the valve was fitted, the leak stopped completely.
Paul Walbran

Yes, it is the earliest type of engine, without the PCV breather, so I am sure an improvement can be made as you describe. What worries me is that the oil leakage has increased so much from last year, so something mechanical is happening.

Fortunately my car already has a later electric Smiths rev counter, so that will not be a problem when installing a GB engine.


One other change you need to make is in the spigot bush in the end of the crankshaft - the 5 bearing one is 7/8" ID whereas the 3 bearing one is 5/8" ID.
A simple way to do this is to get a 1G765 3 bearing spigot bush which just haooens to be 7/8" OD. If you wrap a layer of shim around it, then it becomes a nice tight fit in the 5 bearing bush.
Alternatives are to machine an adaptor for the crankshaft, or to change the first motion gear in the gearbox.
Paul Walbran

Moss sells a special spigot bush for this conversion.

But frankly I'd try the PCV fix first. And you can also have the rear plate modified to take a seal - heck of a lot cheaper than a new engine.

Bill Spohn

This thread was discussed between 12/06/2014 and 15/06/2014

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