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MG MGB Technical - 3 main or 5 ?

other then dropping the sump pan is there any way of telling if the engine in the 67 GT that i am endevoring to renavate is 3 or 5 main (there is no plate on the block and i am led to beleive it was a gold seal replacment at some time)many thanks in advance for any information, Vin
Vin Rafter

Vin-
Your engine, if it's original to the car, should be a five-main-bearing engine. The first version of the five-main-bearing engine was the 18GB, used in the MKI models. If the engine identification plate is missing, there is a way to date the age of the engine block. On the right hand side of the engine block, in the area between distributor and oil filter, there are three numbers that form a circle and that are slightly raised, e.g., 30 12 71, which tells the day, the month, and the year during which it was cast (in this case 30th day of December, 1971). The top one is the day, the left one is the month and the right one is the year. In the early 1970s the month code changed to a letter, thus 12 G 3 would be 12th day of July 1973.
Stephen Strange

Hi Vin

well you got the top man to answer your question. Stephen is the expert on the MGB engine. Have you a copy of his amazing article on tuning the MGB engine? Its known as "the book" and it is fantastic.

If you have the log book for the car it may have the engine plate number in it. They used to be really keen on keeping these up to date.

The 3 bearing engines tag should start with either 18G or 18GA

The 5 bearing engine tags start with 18GB, 18GD, 18GF, 18GG, 18GH, 18GJ, 18GK, 18V

If you have a goldseal replacement engine fitted their ID tags mostly started with a 48G number.

The 3 bearing engines all have cable driven mechanical rev counters whist all the 5 bearing ones have electrically driven ones.

Finally, most 3 bearing engines drip lots of oil from the backplate as they have an oil thrower and no oil seal as such. The 5 bearings have proper oil seals an usually are more oil tight.

PS Dont make the mistake of taking Les out for a run out in the MGB, it is a really comfortable and civilised car and you will never persuade her to ride in the MGA afterwards!

Colyn

Colyn Firth

The take off for the mechanical tach is at the rear of the camshaft.

Herb
Herb Adler

The input shaft from the gearbox into a five bearing engine is more or less a constant diameter shaft, whereas the one for a three bearing is a smaller, narrower diameter for the first inch or so.

John.
JW Prewer

Fastest way is look for the tach drive.
Bill Spohn

I had an MGA with a 1500 B series engine that was not drilled for a mechanical rev counter drive so I had to fit an MGB electronic rev counter. The engine prefix was listed in BMC literature as a "miscellaneous special" so it wasn't from an Austin, Morris, Wolseley etc.

I'm not suggesting for one minute that there are any 1800 3 bearing engines like this, only pointing out that there are more variations than you might imagine.

John.
JW Prewer

The use of the 3 main 1800 was very limited, John, so I don't think there are any non-drilled blocks. They were used on MGBs and possibly some small production cars, though not, AFAIK, the TVRs, which skipped right from 1622 to 5 main MGB. Never say never, I agree. The minute you do someone will pop up with a photo of some oddball Australian use no one has ever heard of, or some such. ;-)
Bill Spohn

This thread was discussed between 11/09/2012 and 18/09/2012

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