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MG Midget and Sprite Technical - Marina Engine

Does anyone know, when fitting a rib-case gearbox to a Marina engine, does the first-motion shaft fit the Marina crankshaft spigot bush, do you need a Midget bush, or does it need an adapter?

Asking for a friend.
Dave O'Neill 2

From the archives. Marina Engine/ribcase gearbox mating.


T Dafforn, Warwickshire, United Kingdom

Posted 22 May 2013 at 12:26:29 UK

Hi Guys,

Thanks for the sudden flutter of activity!
have been out and measured it all again.
The OD spigot (measured from two gearboxes) on the gearbox is 12.6 (near as damn it 0.5 inch)
The inner diameter of the spigot bearing on my marina engine is between 12.7 and 13 mm
the outer diameter of the spigot bearing (roller) is 19 mm (0.75 inch).
This seems to match the OD of the bronze spigot bush in the end of crank (assessed by placing the replacement roller bearing behind the bronze bush in situ and eyballing it..
So looks like the dimensions are OK. The in situ bronze bush is probably a little worn so should be replaced.
I'll get a replacement bronze bush from AH spares and then make a decision on which solution to use?
Cheers
Tim




Edit.
And isn't there some issue about needing the Marina flywheel machining?
anamnesis

Daniels book is a good source of reference.

Rear backplate, front plate and timing cover + flywheel change over to Midget ones.
Gearbox location dowel sizes different?


richard b

Thanks, I’ll have a look.

I have one of Daniel’s books. Hopefully it will be in that one.
Dave O'Neill 2

There is a difference in the diameter of the flyweel mounting on the crank. Cant remember details offhand, about 3mm I think.
I have a couple here, I'll try to remember to dig one out and measure.
Paul Walbran

I’ve not done the swap but as I read Daniels book the Marina crank fits but the Ital needs modifications.

R.
richard b

AFAIK the Marina crank is also bigger than the Midget.
Dave O'Neill 2

Marina crank has 1.75" bigends, and is not cross-drilled. Worth doing if you are going to push it.
The 1.75 bigends also make for stroking the crank by offset grinding the bigends to 1.625. Hello another 45cc.
Checked the flywheel end: the flywheel flange is 3.125" diameter, and there is only one locating dowel. I didnt have a Midget crank to compare, but memory is that I found a variation in diameter swapping between the two. Or is it my memory getting feeble again....
Paul Walbran

I measured a Midget crank flange tonight at 3.027"
Dave O'Neill 2

This thread was discussed between 15/08/2021 and 16/08/2021

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