750 or 850

Stephen Hill · Nov 27, 2011

I just bought an "850" motor. The cases are stamped 20M3S131948. When I look this number up, it would have been produced between March and Sept 1969. The engine has 850 barrels. A large sump plug, plus a smaller hex drain at the lower front edge. There is no left crankcase vent. The inner rear timing chest doesn't have a blanking plate like on my 1968 or 1972. Instead, that face is a solid casting, except the rear facing spigot for a vent, like on my 1973 850.
I think this must be an 850 engine restamped as a 750. Are there any distinguishing features that will help me figure out what year of 850?

Stephen Hill
Victoria, Bc

swooshdave · Nov 28, 2011

Stephen Hill said:
I just bought an "850" motor. The cases are stamped 20M3S131948. When I look this number up, it would have been produced between March and Sept 1969. The engine has 850 barrels. A large sump plug, plus a smaller hex drain at the lower front edge. There is no left crankcase vent. The inner rear timing chest doesn't have a blanking plate like on my 1968 or 1972. Instead, that face is a solid casting, except the rear facing spigot for a vent, like on my 1973 850.
I think this must be an 850 engine restamped as a 750. Are there any distinguishing features that will help me figure out what year of 850?

Stephen Hill
Victoria, Bc

Pictures....

RennieK · Nov 28, 2011

Hi Steven, is this a matched set? There will be matching 3 digit numbers stamped here:

If so they are at least 1973 vintage or newer. If not there are other possibilities. Even so it is possible they are 750 cases enlarged for the 850 barrels.

From the outside you can check the casting around the speedo drive. The 750 has a small channel going towards the front that the thicker 850 case has filled in. Also where the speedo casting meets the timing chest it is fanned out slightly on the 850 and on the 750 it buts straight up. see here:

Here's a better photo of tach drive casting:

750 or 850

Stephen Hill

swooshdave

RennieK

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