750 or 850 ?

There is a bike on Craigslist that is supposed to be a 71 and has camshaft end breather which is consistent with 71 but the barrel appears to be an 850 since unlike the 750's it isn't bolted to the crankcase at its base.
Is it possible to attach an 850 barrel to 750 case?

http://capecod.craigslist.org/mcy/3233082737.html

rx7171 said:

There is a bike on Craigslist that is supposed to be a 71 and has camshaft end breather which is consistent with 71 but the barrel appears to be an 850 since unlike the 750's it isn't bolted to the crankcase at its base.
Is it possible to attach an 850 barrel to 750 case?

http://capecod.craigslist.org/mcy/3233082737.html

Click to expand...

Yes, there is one in my garage at the moment that has 850 bolt through barrels on 750 cases, but actually the barrels are sleeved to 73mm for 750cc....the bike is not mine and I did not build the motor so I have not checked and I am not sure of the origin on the head, but the 850 bolt holes on the barrel/head are wider set so you would expect an 850 head as well?

In the write up on the site he references he says he bought new cylinder barrels from britcycle, I see they supply alloy bolt throughs with varying sleeve sizes!

Probably 850 barrels, although the latest 750 barrels supplied by Andover Norton are also "850" style.

Thanks for info.
I learn something new on these bikes every day.

Bob

I have just re-assembled a 71 750 Commando Roadster with non-alloy 850 barrels that are bored only to 750. Not sleeved. Bought as new stock from British Spares here in Noo Zealand. The story I was given was they (Norvil?/Andover?) had a bunch of 850 barrels left over and bored them to 750... and a few left? They are bolt through, original markings, and though not original with regard crankcase studs I don't mind given better reliability?

I was very happy to locate this. The bike I bought off ebay from San Fran at Christmas turned out to not have a blown gasket, but a cracked barrel. All good now, did new pistons, rings etc and I am a very very happy commando camper...

In trying to discover how the original barrel became cracked I noticed a rather strange feature - in the ventilation (?) holes between cooling fins of teh barrel (noticeably fore-and-aft) there appears to be two pieces of loose metal wire rattling around which would have been impossible to insert after casting. And impossible to extract. My only thought is they used something in a sacrificial manner to create the internal cooling voids in the casting process? Quite interesting because I suspect that one of the wires was hitting the bottom of the mid-fore head bolt (the ventilation void is connected to the bottom of that bolt-hole) and possibly causing an unusual stress across the barrel-head combination... Others may want to check for this. If you take a light and shine through you may be able to see similar?

Out of interst alone, Anyone got any experience or thoughts on this?

cheers

ANDOVER did 750 replacement barrels , or ALL barrels with 850 casting , & machined those for 750s to 750 specs . ACORRDING to someone here somewhere .
Andover Perhaps / Norvil .

So appears O.E.M. post production replacement . Is that Clear .

Is it considered a worthwhile improvement to run the through stud fixing 850 type barrels on all Commandos, when doing a fresh build?

Only if youre useing NITRO , and get the ignition timing wrong .

More Overbored cyliders getting overthin above the flange , or running with base nuts loose shock loading sleeves .
dunstall barrels on hogslayer are base flange type . Say no More . Mk III cases were alledgedly Helicoiled from new .

I would n`t be getting carried away using drag racing as an exemplar, flash power readings & short load/heat/reciprocating cycles dont always translate into durability... if T.C.`s set up had a successful Bonneville record to it credit, - well, that would be meaningful...

the 71 referred to in the CL ad is in fact mine. And yes, I bought the cylinders from BritCycle supply in Nova Scotia. They were marketed as 'heavy duty' cylinders....

The long and short of it was that I had a change of heart on a 50 mile ride and rescinded the ad. This bike is my baby and I have it sorted out really nicely. I don't need the doe, and it's such a joy to ride.


Might be time to treat myself to a corbin gunfighter and a belt drive for this winter's upgrade project.

Karl