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MG MGB GT V8 Factory Originals Technical - 3.9 Interim Engine

After talking to Glen Towery I've found that my 3.9 engine is an interim model with 10-bolt heads, 28cc combustion chambers (vs 36cc), composite head gaskets, 9.35 CR, 4.0/4.6 type mains but without the cross bolts and a crank-driven oil pump but with a distributor. It also uses a camshaft thrust plate.

What kind of CR would result if shim type head gaskets were used on this engine?

What about using this type head on an SD1 8.13:1 engine? What would the CR be?
David

Are you sure that block is not cross-bolted?

28cc heads and tin gaskets on a 3.9 litre would give about 10.1:1 CR and on the SD1 3.5 about 9.19:1

Geoff
Geoff King

Nope. No cross-bolts. The undrilled bosses are there, however.
David

Just for the benefit of the archives, what is it that identifies this interim model, other than taking it apart?
Is it a serial number? Casting number? Was this from a Range Rover? What year?

Derek Nicholson

The so called interim engine has a crank driven oil pump and a distributor – the obvious difference is the front cover. And I’m reasonably sure, but not certain, that it will have the small main bearings of the 3.5/3.9/4.2.

Earlier engines had a cam driven oil pump and a distributor, later engines had cross-bolted main bearing caps, larger main bearings, crank driven oil pump, no distributor and an alloy sump.

Geoff
Geoff King

It has, according to a book I own, the same crank journals and bearings as the 4.0 and 4.6. The crank is the same as the 4.0 but with a shorter snout like the early engines. The block is the same as the 4.0 with the robust main caps but the bosses for the cross bolts are not drilled. A dead giveaway is a serpentine accessory drive with distributor and no outer row of head bolts. Quite a hybrid. I intend to use an early front cover from a 215, GM distributor and high volume oil pump kit.
David

If you are interested in selling the serpentine front cover, please let me know.

Jim

jimbb88@comcast.net
Jim Stuart

Geoff, I read further and you are absolutely correct about the size of the main journals. The first interim engines even used the early type main caps from January through April, 1994 along with 14 head bolt holes drilled in the block but still using 10 bolt heads. After April, the main caps were the same "full contact" type as the 4.0/4.6, but not side drilled.The outer head bolt holes in the block were not machined at that point. The interim engines were discontinued in October, 1998 for the 3.9 and April, 1996 for the 4.2 with the last of the LSE Range Rovers. As you pointed out, the crank used the smaller diameter journals throughout.
David

If I sell it Jim, it's yours.

Des Hammill's book, "How to Power Tune Rover V8 Engines" has a great section on this version.
David

This thread was discussed between 13/12/2006 and 14/12/2006

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