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MG MGB Technical - 18V vs earlier 5 main blocks

In a rebuild context, other than the eyebrow cutouts in the deck for the later head, is there any difference between a c.1974 18V block and any eariler 5 bearing block? Are the parts (crank, cam, rods, seals, bearings, studs, etc) essentially interchangeable?

Thanks in advance.
Steve Aichele
Steve Aichele

Early 18 G blocks have no mounting for a fuel pump and the block is a slightly different shape by the left engine mounting but otherwise no different.

They are all interchangeable between years - some studs changed lengths and bolts replaced studs for the main bearing caps - but as long as parts are individually compatable ( e.g. you can't put press fit pistons on a floating pin conrod or vice versa or long pushrods with barrel tappets or offset rocker gear with an early head with a central feed) any can be fitted.
Chris at Octarine Services

The other difference is that up until 68 on the block was recast with mounting holes moved forward about 2 inches to accomodate the shorter alternator. Ray
Ray

It seems that part of my thread was cut off. It should have read: Up until 68 the castings were moved forward about two inches to accomadate the shorter alternator. Ray
Ray

Once more: UP until 1967 the blocks had two castings for the rear generator bracket. In 1968 the castings were moved forward about two inches to accomadate the shorter alternator. Ray
Ray

Not quite, Ray! :o)

The pre-~67 blocks only had once set of bosses, as only the dynamo was used. From ~oct67 on, there were two sets, so you could use the alternator or the dynamo. On later alternator cars the rear set was used to mount the oil pressure sensor.

hth,
--
Olly
Oliver Stephenson

The head on my 67 was replaced at one time with an emissions head and since the engine # tab was probably eaten away when it was rebuilt I had no idea if my had an early block. When I to mounted an alternator I realized I had an early block. Someone in my MG club told me the earlier engines also came with forged cranks. Is this true?
Mike MaGee

'tis indeed true, we've got a steel crank in our heap of parts. Not sure of the year changeover though, anyone?

ttfn,
--
Olly
Oliver Stephenson

Don't be fooled into thinking the forged crank was better - actually the later cast crank is regarded as superior to the forged one, being better balanced and harder wearing.

The best crank to use is the flat sided one fitted to 1971 - 74 engines.
Chris at Octarine Services

Can I ask why the flat sided crank is considered superior?
Greg Fast

Strength, balance and wear characteristics.
Chris at Octarine Services

Chris, I mistakenly thought that the flat-sided one was the forged one. This is the type that we have in our spares area. What does the forged one look like?

Cheers,
--
Olly
Oliver Stephenson

Very large rounded counterweights with no notches in the sides - lack of the central ridge where the casting moulds joined.
Chris at Octarine Services

Pardon my ignorance but how are strength and wear characteristics evaluated? This is a bonafide question, I'm not trying to bait anyone.

Material chemistry, hardness or???
Greg Fast

Years of experience and examining crankshaft failure by the likes of Peter Burgess and other race / rally / road engine builders.
Chris at Octarine Services

Chris,
I have been told that the late B's used a fully counter-balanced cast crank and these are the best cranks. Did these come out at the same time as the switch to bolted main caps? What is your opinion of these cranks?

Thanks,
Leland
Leland Bradley

All MGB cranks are perfectly well suited to up to fast road specification - I only start worrying about the strength of the crank when building a rally/race spec engine.

The very late cranks are counterbalanced in the same way as the earliest cranks - these are not favoured by racers who prefer the early 18V slab sided crank with the counterweights knife edged to reduce windage.

The change from studs and nuts to bolts on the main bearing caps came with the 18V engines. The later bearing caps are stronger than the early ones and it is not unusual to see the later caps combined with ARP studs and nuts in high revving engines (7500 rpm).

Late flywheels were also only dowelled in one place, whereas early 18V and all 18G flywheels had two dowels.

Generally speaking the 1973 to 1975 early 18V engines had the best configuration of crank, flywheel and main bearing caps - just change the main cap bolts for studs & nuts if you wish, however I find the standard bolts adequate for all road engines.
Chris at Octarine Services

Thanks for the insight Chris
Greg Fast

So how does the original 3 bearing crank fit in to the scheme of things? Various MG books talk about the competitions department prefering this one but I expect they had a pretty much unlimited supply of the things.

Are they strong enough for fast road too or should they be treated with kid gloves? If I am to build a fast B should I try an older engine?

Mike
Michael

Michael,

The 3 bearing engine has a reputation for being a more willing revver thanthe 5 bearing. This may be in part due to the reduced drag on the crank. It is more fragile, with a reputation for week con rods. When used in competition the stronger twin cam con rods were used.

The factory's competion activity with the B was over by the mid to late 60s so they will have had far more experiance in working with the 3 bearing engine as the change over was for the '65 model year if I remember correctly.

David
David Witham

This thread was discussed between 28/10/2003 and 03/11/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.