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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?
|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.
|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.|
|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.
|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.
This thread was discussed between 12/06/2014 and 15/06/2014
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