Converting from 750 to 850

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Oct 12, 2007
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I've got a 72 Combat that I'm considering converting to an 850. I know I need 850 barrels, pistons and head. Can I bolt the 850 barrels to the 750 crankcase? Anything else I need to know?
Anything can be done with enough money and talent!!!

The mouth of the crankcase needs to be opened, but I can't remember if the bolt spacing is wider. I believe it is. I know the 750 head will not fit on 850 cylinders. Of course you will need to balance the crank with the heavier 850 pistons.

Since you need to split the cases for the balance work, I'd opt for finding a complete 850 motor.
The 750 & 850 base gasket part numbers are different, but I don't have both types to compare the two for any differences between them.

Part numbers from the Norvil list:

750 type = 067869

850 = 063812
I am fairly sure that the bolt spacing on the crankcase mouth is the same on the 750 and 850 crankcases, as years ago I converted my first 750 Commando to 920, and in effect all that I did was exchange my barrel for an 850 type with the larger (920) liners fitted. I also had to remove some meat from the crankcase mouth where the liners sit as well.

I would think that the difference for the part Nos of the base gaskets are to accomodate the slightly larger bore.

I don't remember having my crank re-balanced, and dont know how necessary it is. Another option is to have the 850 pistons lightened to match the weight of your 750 pistons. With regard to the cylinder head, I had the four bolt holes (that are in the different position on the 750 head) alloy welded up, and then re-drilled to the 850 pattern.

So in answer to your question Jim, if you were to buy 850 barrels, pistons and head, all you would need to do to make it fit is open the crankcase mouth a bit, leaving about a 10 thou gap (I think). With regard to the balance factor, I don't know how much difference there is between the 750 & 850, and if it is likely to be a problem.
The NOC Service Notes (again !) say that 750 / 850 cylinder stud spacing is identical but then goes on to say that whilst 750 barrels can be fitted to the 850, it will not work the other way round due to the cylinder spigot OD being too large.

I thought I had heard though that on the last (MkV) 750 crankcases it was possible without removing metal.

Bearing in mind the problems with breathing on un-modified MkIV and MkV cases, I think my preference would be to look for a complete 850 engine (It's always nice to have a spare engine 'just in case' as well).
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