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MG MGB Technical - Modifying a 3 bearing 'b engine...

How much can you do?

The engine has already had a 30thou overbore, oversized pistons, rings and bearings when the engine was rebuilt.

I had thought of putting a new head and cam in, but have decided against a cam (due to mechanical ability, cost & I need the car for everyday driving). Would the 3bearing engine take it?

The book I was basing my thoughts on, Peter Burgess' tuning the B, only applies to the 5 bearing...

Bummer :(
Stefan Carlton 1

3 bearing engines have been raced, they are faster due to less internal friction .
S Best

General theory and many specific matters relating to cylinder head, valve train, and carburetion are equally applicable to both 3- and 5-main engines. Burgess' book is a great guide for both.
Marvin Deupree

Are they more prone to failure due to only having the three bearings?

I'm inclinded to stay away from modifying the engine now since I learnt it's a 3 bearing engine you see - I thought it was a 5 bearing model.

Stefan
Stefan Carlton 1

I have had 3 bearing engines since 1968 and have never had a failure on mine which have been moderately improved.

They are known to have the achilles heel, however it is wise to be discrete with RPM. Even in club competition I always kept mine down to 5,500RPM.

These 3BG engines are getting rarer these days and if you intend to use the engine to the limit you would be better finding a 5 bearing engine. The 5 bearing engine has improved con rods and floating gudgeons as well.


cheers
Ian Buckley

Stefen,

The 3 bearing engine revs better than the 5 as standard. However continued high rev use exposes its weakness.

As Ian says the con rods are a weak point on the 3 bearing engine. Years ago people used to put mga twin cam con rods in the 3 bearing if using it in competion. These are now very difficult and expensive. So carillo rods tend to be used now.

If you are modifying an engine on a budget the 5 bearing is a better bet.

David
David Witham

Stefan

As mentionned by S Best, the three mains were preferably used by the MG competition service to the five mains. Their cranks are made of steel instead of cast iron. You can convert 18G, 18GA engines to a 18V head provided you
modified the block for the larger valves openning. Only the speedo pinion makes difference between earlier and latest camshaft. You can shut the speedo drive shaft opening by a cover. But you have to provide an electronic
speedo and make some minors changes to the electric wiring.
Best regards.
Renou

You can rev a 3 main to 7000 (given sutiable valve gear (without modifying the vrank - that's something you sure can't do on a 5 main - you need to crossdrill it above 6500 or so.

The clamped wrist pin and rod isn't really a weak spot (except for full race tune) - I have a nice S shaped rod - the piston disintegrated but left the pin clamped in the small end.
Bill Spohn

This thread was discussed between 17/01/2005 and 19/01/2005

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