Differences between the Marina and MGB B series engine?

[martinclan]

I have now pulled the engine from my Marina based Marlin. It's done 130k as far as I can tell from the paperwork so I am expecting to have to rebuild it.

However I noticed that used MGB B engines frequently turn up on Ebay which would be a cheaper route - assuming the seller is not telling porkies about condition.

So the question is what are the differences between the B as fitted to the MGB and the Marina? As a start I have discovered:

1. The crankshaft spiggot bush is different as the Marina gearbox has a much smaller input shaft diameter.
2. I guess the rear endplate must be different
3. ???

Cheers, Robin

[balmy]

MGB has electric fuel pump.
Camshaft may not have lobe to run a mechanical pump
Think the breather is different but can’t remember how.

[MarinaCoupe]

The MGB engine has the same camshaft as the TC Marina rather than the slightly lower power Marina Super & Delux single carb version, so don’t expect a standard Marina single carb type distributor to release TC type power with the MGB engine, the advance curves and maximum advance are entirely different. Get the MGB distributor with the engine.

The MGB engine has a duplex timing chain, the Marina a single timing chain.

I’m not sure the MGB flywheel is the same diameter as the Marina one, so it’s best to swap it over to ensure that the starter motor engages correctly.

[balmy]

Is it the same camshaft? I can remember some going over it in the past but couldn’t remember the outcome.

[MarinaCoupe]

Yes David,

The standard MGB camshaft has the same part number as the Marina TC / HL / GT - ‘88G 303’. The timing of the cam varies slightly between the MGB and Marina but the components are the same.

There is an earlier MGB cam 88G 252 - but was only fitted up to 1964.

The rule of thumb is to fit the distributor that was fitted to the car by the factory.

[PHUQ]

I remember reading that the MGBs had forged cranks and the Marina cast but I've no idea if there's any truth in it. I'm sure I've been told that not all MGB cams have pump lobes but that might have been referring to aftermarket ones. Engine mounts and oil filter plumbing (sometimes) are different too, all easy swap across stuff since you have the Marina engine to hand.

The other thing to note is that there were quite a few changes over the lifespan of the B series in the MGB, which for the most part the Marina followed. The big one of course is the change (pre Marina) from 3 to 5 main bearings, but there are several variations of cylinder head, some better than others (later doesn't mean better!), HS or HIF carbs, I believe different cam specs etc; and this assuming that your unknown history 50 year old engine hasn't been altered during it's life of course.

[david painter]

I would be very cautious about buying a used engine. I would actually like to see and hear it running, you then know what you have actually got. It is a awful lot of work to fetch and fit a engine that you have bought on the computer only to find it isnt any better than the one you have already got. For my money i would have the machining work done and rebuild it with new parts, converting the cylinder head to unleaded at the same time. Im not sure if its on here or in the club mag some one has written a review of his B series rebuild. The only exception i would make fitting a untested used engine is from a trusted source.
Dave

[balmy]

Early B’s had a steel crank and later ones cast.
Marina’s had two types, both cast but different to the B cause of the removable spigot bush because of the smaller diameter input shaft. One flat sided one and one that had large rounded counter weights according to Peter Burgess.

Out of interest-Where would you call a trusted source?


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[david painter]

Well that.s the million dollar question? My opinion is if you havent seen and heard it running you have a very expensive cast iron paperweight! If it turns out to be anything else you have a bonus. I would not personally buy a used engine sight unseen expecting anything else but needing a rebuild and it would not be for what they are advertised on ebay for.
Dave

[martinclan]

Marina’s had two types, both cast but different to the B cause of the removable spigot bush because of the smaller diameter input shaft.

So is the Marina crankshaft different to that in the MGB? I assumed it just had a different spigot bush?

Cheers Robin

[martinclan]

So is the Marina crankshaft different to that in the MGB? I assumed it just had a different spigot bush?

Cheers Robin

Just bumping this thread as I never did get a definitive answer.
And there is now someone selling reconditioned short MGB engines for not much more than it would cost to do it myself. So....

Is the Marina crankshaft different to that in the MGB? I assumed it just had a different spigot bush?

Cheers, Robin

[lil_red_roadster]

Yes. The MGB spigot bush recess in the crank is larger than that in the Marina...driven by the difference in gearbox input shaft diameters. IIRC some early Marinas (to 1974, according to Peter Burgess) were fitted with a removeable insert in the crank, which, when removed opened it up to MGB-size. Not a major issue if using an MGB engine with Marina 'box (fit a larger bush) but if going the other way the end of the crank may need machining out. Also some differences in material (early B were forged steel, later B & Marina were cast iron; also counterweight design varied).

[martinclan]

Thanks for the answer. I can be confident now should anything turn up
Cheers Robin