Anyone telling you that the 3 main is a lemon is either parroting what he
heard someone else say, or simply doesn't know what he is talking about.
The 3 main B engine has larger diameter main bearings than the earlier MGA
engines, Twincam included, and a good stiff crank. A 3 main will give better
performance and generally feels stronger than a similar stock 5 main engine
with nominally the same power output.
I have seen 3 main engines last for a 7 hour endurance race without problems,
and you can in fact rev a 3 main higher than an unmodified 5 main - the 5
main will trash itself above 6500 rpm unless it is crossdrilled. The trade
off for the higher revving 3 main is that it is not as stiff and does not
have as long a life in use as the 5 main, before fatigue sets in.
A cross-drilled 5 main is superior to the 3 main for racing and really high
rpm use, but remember that the 3 main Twincam crank was used with a 7000 rpm
stock redline, and another 500-750 for racing, so as long as you have a good
crank and build the 3 main engine properly, you should have no problem.
Also remember that they are 36 years old now, that every crank has only so
many turns in it, and that some used cranks you will buy will have most of
those rpms allotted to it at 'birth' used up. If you get a low mileage one
and treat it as you would any crankshaft (and magnaflux it before use), you
should have no problems at all. I use 3 main B engines in my TVR race car,
as we don't need to use over 6500 rpm in vintage, and it does perform better
- I practice what I preach.
Some of the bad rep of the 3 main comes from a mistake the factory made, BTW.
The bigger engine runs hotter than the 'A's did. You will recall that the Mk
2 MGA had an oil cooler. Some bright light (probably in accounting) decided
that based on tests in English weather, likely in the depths of winter, the 3
main B didn't need an oil cooler, and the first half year of production was
shipped that way. When the heat of the American South met the inadequate
cooling, they predictably ran bearings, gained an unwarranted reputation for
unreliability, and the accountant no doubt finished his career counting sheep
in the Hebrides.
Bill S.
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