73 850 engine cases

hello everyone . im new to working on nortons and ran across a 73 850 with a blown motor it broke the rod and and busted the cases but was wondering if and 750 bottom end parts interchange and if the cases where the same thanks

No, the 850 has a heavier crank and the barrels need a bigger hole in the cases. It's been many years but 750 rods are ok I believe. You need a 73-74 set of 850 cases if you can't repair yours. Are both sides fried?

the cam cover side is pretty bad there is prolly a 3/4 inch hole where the rod bolt shot out and cracking around it . thanks for the info

The 850 cylinders also attach to the cases differently than 750s. I see 850 cases on eBay sometimes, but they get expensive.

The 850 cylinders also attach to the cases differently than 750s.

Not really, all 850 cases will take 750 Commando cylinders. Only one type 750 cases will take an 850 cylinder: Combat cases. You need to enlarge the bore of the cylinderbase of the 750 cases. Only the Combat cases have enough material around the base area to make this possible.

nortonspeed said:

Not really, all 850 cases will take 750 Commando cylinders. Only one type 750 cases will take an 850 cylinder: Combat cases. You need to enlarge the bore of the cylinderbase of the 750 cases. Only the Combat cases have enough material around the base area to make this possible.

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This sounds surprising to me. The only modification to the combat that I recall was the head planning. The other thing the combat engine inspired was the change over to super blend main bearings. My combat purchased new in 72 had the new bearings fortunately for me. I don't ever recall that the cases were beefed up for the combat. What makes more sense to me is that after the into of the 850 in 73, new beefier cases were cast to support the heavier crank and barrels of the 850. It would make sense then that even the 750 cases produced in 73 were the beefier cases. These would be easily id'd by the cast over distributor sprocket drive in the timing side case. I'm just speculating here, I'd never thought of this before but it makes sense to me.

It would make sense then that even the 750 cases produced in 73 were the beefier cases.

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Hi Renniek, the last batch of 750 Commando's in 1973 (engine no. 220000-230935) probably also had the beefier cases but i am not sure about that. The Combat cases were defenitely beefer than all the 750 cases before and also the only Commando cases without the large oilsump filter (big mistake!).

from what i can find in my manuals allot is the same stroke main bearing size cam bushing are all the same just a difference in bore i figured that would be the differnece in the 2 cases and maybe the bolt pattern . thanks for the info from everyone.

nortonspeed said:

The Combat cases were defenitely beefer than all the 750 cases before

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I think this applies to all '72 (200000+) cases, and not specifically to the Combat models, as not all '72 Commandos were Combats.

The 920 that I built 20 or so years ago was put onto 750 Combat crankcases. This left one area due to the necessary machining where the crankcase mouth was very thin, and this was the left rear portion of the crancases (6, 7, 8 & 9 0'clock) but as I stated in my other post for teesrider, there were never any issues that arose from this. The 850 cases have more meat designed into this area than the 750 cases, and it is easy to see this when they've been bored out to accept to accept 81mm liners I/D that are 4mm wider in diameter than the 850! I believe that Pete Lovel will have machined the cases to the minimum acceptable clearance to accept the larger liners. Incidentally Roger at RGM always warned me that it wouldn't work and that it would break but despite many a good thrashing it has held.

So I would think that you could put 850 barrels on to the 200,000 + crankcases, but they would want machining to the minimal acceptable clearance. With regards to earlier crankcases, I have no experience or knowledge.

Also just though that I would add, that you would probably be better getting 850 cases if possible, and if you did get some 750, 200,000+ cases, they would need the crankcase mouth machining out to accept 850 barrels.