750 cases thru bolted top end.

I'm looking to build a new motor, I have no 850 experience, can I adapt a through bolted top end to my old 750 (66 n15) cases, and how

Yes, you can. Just remove the four side studs and the through bolts will screw right in. I"d suggest you also do what the factory did, and helicoil those threads first. I don't recall if you also have to open up the case mouth for the larger bore. It's been a while since I did a conversion, and the last one I did was for a 920 cc engine, so I did have to bore out the openings in the case. I don't think you have to for an 850, but you should probably check to be sure. It's pretty easy to check if you have an 850 cylinder handy.

Ken

Oops! I just noticed you are talking about an N15, not a Commando. What I said is true for a Commando, but I don't know if it's also true for the pre-Commando engines. Sorry about that.

Ken

There are 750 barrels available with the four bolt-through 850-style Allen-bolt configuration--however, you can't fit 850 barrels to your 750cc cases without shaving the cases to accommodate the bigger 850 "liners"--and even with 850-style 750 barrels you'll have to helicoil the threads in the cases to withstand the higher torque of the barrel through-bolts. You cannot use 850 barrels and heads on 750 cases without weakening the crankcases and disturbing the balance factor--and the radically higher torque of the 850 barrel and head with pre-Commando cases may well overtax the earlier components--the Atlas never made 56 lb-ft of torque


Tim Kraakevik
kraakevik@voyager.net
'72 Combat, '74 RH10 850

To put 850 barrels on 750 cases you have to bore them. There were a few late 750 Commandos that were built on 850 cases so boring wouldn't be needed on those. You would definitely have to bore N15 cases for 850 barrels but I wouldn't advise doing it. There is no extra meat in those cases.
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There were some through bolted iron 750 barrels built a few years ago. I don't know if they are available anymore. I have only seen one of them.

If it were me I would look for a set of through bolted aluminum 750 barrels. Jim

Thanks guys I was plannin' on using thru bolted 750 barrels anyway so opening up the cases wont be an issue I just was'nt sure if I could use them without 850 cases. I'm looking at doing a combat spec (or a little more) G15csr. thanks again!