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MG Midget and Sprite Technical - 1275 with marina crank

can any one tell me if you can fit the marina crank
in the 1275 midget block
so you get a long stroke motor of about 1380
we are looking for more torque
hope to never rev it above 5000

m Smith

yes, you use the 1380 bore and offset grind the Marina crank to take Midget rods, gives around 1460 IIRC. Also have to machine out the centre of the std flywheel as Marina crank flange is a bit bigger. With a big single carb it'd make a stonking trials engine...
David Smith

yeah... we had a really great young engineer here on the site that built one of these engines

his name was nick, sadly he has passed on ...but his legacy still lives in the guessing 5 years now scence his last post.

Prop and the Blackhole Midget

Have done same but with marina crank in marina engine with A+ series rods. Seemed to work fine
T Dafforn

Tim, do you get the benefits of the longer stroke with that spec? Are A+ rods b/ends the same as Midget or Marina?
David Smith

hey david,

I pulled up.some of nicks old postingsast night for nostalgia thoughts

nick mentioned he had the crank turned to accept the midget rods and block bearings

I think his spec'd out at 1460

Prop and the Blackhole Midget

Yes Prop, that's correct. But Tim says he is using A+ rods, which may have the larger b/end size so you don't grind the crank down and don't get the extra throw, hence my question.
David Smith

A+ rods are for the 1.75" journal.
Dave O'Neill2

The Marina crank has not the the cross-drillings of the Midget cranks (the early one at least) so we replicate when we fit one.
Paul Walbran

Also keep in mind the 1098 big mains crank as an option as that'll give a bit more stroke and will fit the 1275 block with IIRC slightly thinned thrust bearings. I've got 60k+ miles on one that was tuftrided and frequently taken to 6.5k-7kish. The stroke length isn't that much greater but will take a 1380 to 1420 IIRC. I used a modified 948 4 bolt flywheel on mine. The Marina crank may be more readily available.
David Billington

Yes David, your memory is correct, the thrusts need to be thinned a bit for the 10CC crank. It has 2.4mm more stroke than the 1275, the most you can get with a perfect Marina crank is 3mm more. In practice it is usually only about 2.8 if you have a crank that's already been ground, or needs it.
The difference between the two cranks is 10cc (Marina on 73.65 bore gives only 1431cc, not 1460)

We have done a couple of engines to 1480cc using 75mm bore and 10CC crank, they went well! But they were street/autotest engines rather than race, I think that the bores were so much thinner between them there would be issues used in a race engine. Having said that one of the people racing here has done a 1600 engine with (IIRC) a 74.65 bore and a self-made crank with a very long stroke. And he races it very successfully in an A40 Farina.

For our 1480's, we did offset the bores away from the centre to retain as much thickness between cylinders as possible; this resulted in breaking into the oilways behind the rear bore - expected so alternative provision made for the oil before we bored it just in case it wasn't do-able.

The engines lasted well, outlasted the cars in fact!
Paul Walbran

Didn't take advantage of the longer stroke. The Marina engine was just more available.
T Dafforn

This thread was discussed between 16/04/2013 and 18/04/2013

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