MG Sports Cars

What are the difference between these motors when it comes to performance, reliability and any other possible pros and cons?

You might have to clarify that question a little Jason, as to my knowledge either a 3 bolt or a 5 bolt main engine would be a pretty rare bird. If however you are referring to the 3 main or 5 main bearing 1800 MGB engine that's a different matter. Technically at least the 5 main would be stronger as it has better support of the crank but the 3 main has it's adherents. Something about revving quicker due to less rotating mass as I recall. However, just about any 6 or 8 cylinder engine will vastly improve the car.

JB

Yes I am talking about the MGB 1800, sorry.

First of all... take the word "bolt" out of the question. It's the number of main bearings that's the issue, not the number of bolts that secure main bearing caps. (There are two bolts per cap in either case.)

I'm no expert on these engines, and to be honest I'm not really much of a motor guy at all but I've asked this question myself a few times. From what I've been told, there's no question that the five bearing crankshaft is better supported and that the resulting assembly is more robust. It cost MG/BMC money up front to make the change because there's more machine time, assembly time, and more parts in a five bearing engine. Presumably, they calculated that there would be a payoff in terms of warranty expense and/or customer goodwill.

The change went into production in early 1965, so as a practical matter there simply aren't that many 3-main engines still available. However, if you're a purist and you're vintage racing an early-model MGB then I suppose you MIGHT want the 3-main engine simply due to authenticity.

What do most racers prefer? My impression, based on a completely unscientific sampling, is that the 3-main engine tends to be preferred in U.K. whereas the 5-main engine tends to be preferred in North America. Now, if you take a census of the MGB racecars on BritishRaceCar.com you will note that at least four of the seven early MGBs shown have been converted from 3-bearing to 5-bearing engines! Specifically: Mike Adams', Mike Kusch's, John Targett's, and Alan Tosler's cars. I said "at least" because I strongly suspect the other three early MGBs have 5-bearing mills too.

I have heard it argued that there's less friction in a 3-bearing engine, but I'm skeptical about that argument.

The 3 main engines have the advantages of revving more freely (based on personal experience) and on being able to run to 6500 rpm+ on the stock crankshaft without modification.

The 5 main engine has the advantage of having a stiffer crank, and is the engine of choice if you are going to use it up in the 8000 rpm range (routine for race cars) but you either have to use a special crank, or have the stock one crossdrilled for additional oil supply if you intend to use it above 6500.

So for most street use, the 3 main is just dandy, but as stated the supply is getting low.