920 top end on 750 cases

baz · Jul 8, 2013

i have a complete 920 top end and i plan to fit it to a 750 combat crankcase i plan to tig weld the weak point at the top of the crankcase at the back is there any reason why this wont work? i realize i am asking a lot of these cases but as they are the ribbed combat ones they are a little stronger? alternitvely i will have to try and find some 850 cases!...............baz

Nortiboy · Jul 8, 2013

A good 750 will outperform a poor 920.

Which brand of 920 kit do you have ?. Most need a fair amount of machining to get it to work. The throats of the crank cases will need to be opened up to start with.

I would not use anything but a 1973 750 crankcase or 850 for this one. (or aftermarket if you can afford it).

I have an RGM 920

Reggie · Jul 8, 2013

I have posted about this before, so here goes again.

I bought a 920 kit in the 1980s, and put it onto a 750 crankcase, probably Combat but I'm not absolutely sure (breather was at the back of the crankcase). The crankcase mouths were opened up at a machine shop to the smallest required clearance and no strengthening added to the now very thin area at the left rear of the crankcases. The 750 head was not enlarged at the squish band IIRC although the holes in the head were altered to an 850 pattern. The valves were left as standard. This gave the engine more torque, but it was breathless at high revs.

So regarding the crankcases "weak point," Roger at RGM said they wouldn't last, and maybe he knows of failures when built in this configuration, but they did last and they are still intact to this day having covered probably more than 15,000 miles, although the engine hasn't been run now for about 8 years as it is now owned by a friend and is laid up in his garage. But I and especially he, used to work the engine hard without problems. The engine put out more torque but probably not a lot more bhp?. I did crack a crank across the l/h big end journal though :shock:

Maybe I was just lucky, but mine worked OK.

baz · Jul 8, 2013

Nortiboy said:
A good 750 will outperform a poor 920.

Which brand of 920 kit do you have ?. Most need a fair amount of machining to get it to work. The throats of the crank cases will need to be opened up to start with.

I would not use anything but a 1973 750 crankcase or 850 for this one. (or aftermarket if you can afford it).

I have an RGM 920

Its an RGM kit,i wish i could afford a set of maney cases

baz · Jul 8, 2013

Reggie said:
I have posted about this before, so here goes again.

I bought a 920 kit in the 1980s, and put it onto a 750 crankcase, probably Combat but I'm not absolutely sure (breather was at the back of the crankcase). The crankcase mouths were opened up at a machine shop to the smallest required clearance and no strengthening added to the now very thin area at the left rear of the crankcases. The 750 head was not enlarged at the squish band IIRC although the holes in the head were altered to an 850 pattern. The valves were left as standard. This gave the engine more torque, but it was breathless at high revs.

So regarding the crankcases "weak point," Roger at RGM said they wouldn't last, and maybe he knows of failures when built in this configuration, but they did last and they are still intact to this day having covered probably more than 15,000 miles, although the engine hasn't been run now for about 8 years as it is now owned by a friend and is laid up in his garage. But I and especially he, used to work the engine hard without problems. The engine put out more torque but probably not a lot more bhp?. I did crack a crank across the l/h big end journal though :shock:

Maybe I was just lucky, but mine worked OK.

thats good to know i like the idea of the extra torque the cylinder head i have is an 850 rh4 from a wasp grasstracker