750 cylinder barrel rebore

hello everybody i'm new in the forum, i'm from roma, italy
i'm restoring a 73 "barn find" 750 commando roadster,
i will post some photo soon
i've a technical question, standard size for a 750 cylinder barrel is 73 mm
my cylinder barrel is 74 mm, is this already +40 or i'm wrong? so next rebore must be +60 if i'm right
can anybody explaine me if i can use this barrel or i need a new one?
many thanks in advance
thomasdunstall

Bad juju to go to .060" over in a 750 as thins the cylinder bores so much there are tales of seeing piston moving through the cracks or barrel moving u/d with piston. Re-sleeving a bit iffy too to get sleeves thick enough not to get same as above. If not barrel shape worn or scored, like my .040" over Trixie Combat last year, I had them touch up re-hone the bore then I put in new pistons and rings and enjoyed a year on her now. On my Peel Combat with barrel shaped wear 040" over jugs, I had them extra honed straight then had pistons knurled to take up slap slack and got a bunch of hooligan hot rod over red line play out of her. In the long term start looking for a bargain barrel,

My 750 commando is bored 60 over a few thousand mikes ago with no issues yet.had to go that far because a wrist pin circlip came loose and scored the cylinder wall.
British Cycle Supply list 80 over size but I don't think id go that far .

My Combat has been 60 over since 1978. Yes, it sat for 22 years before being "restored" (no pun intended, Windy) with the 60 over pistons reinstalled with new rings. 3.5 years and 9500 miles later still strong and trouble free....maybe the few extra thousandths are why I beat up on my fellow 750's??

If your cylinder measures 74 mm, it is +.040" already, as you suspect.

I think a lot of the horror stories about +.060" cylinders breaking are from racers. The original Commando 750 cylinders were a bit flimsy around the cylinder base flange, and boring them to +.060" only makes it worse, leading to the cylinders cracking at the base. Norton only offered pistons up to +.040", but lots of folks have gone +.060" over on street bikes with no problems. I suspect the risk on a street bike has a lot to do with riding style. If you regularly rev your engine past redline on the street, you're probably at much more risk from a +.060" overbore than if you ride it within its limits. Years ago I had a 750 cylinder break right at the base on my race bike with only +.020" overbore. But that's on an engine that was seriously abused on race tracks.

Ken

Isn't the first question, does it really need a rebore?

No swoosh, the question is...
Is 74mm equal to .040" oversize, and, can I safely go to .060" over.
Both of which have been answered.
As I read it, no offence.
G'day Thomasdunstall, welcome aboard.
Are the bores marked badly ? can they be honed and new pistons fitted.
If they're are not rediculously loose, you could fit new .040" pistons, it's been done before.
Also, Pistons can be made to order.
As with lots of things "Norton", there seems to be a lot of guess work involved by us, the owner.
Some times we just have to "roll the dice" and make a decision, because if you ask too many, usually 6 people will say yes and
6 people will say no, all with good reason and experience.
Keep an eye out for a set of Barrels
Good luck'
AC.

If you run it continuously at redline at the Drag Strip Ea weekend , you may have trouble .

If its OFF the bike , make a ' U ' thing , and some spacers , to slide up and measure sleeve thickness above the flange ,
all round . And let us know .

Could be anywhere , ' Core Placement ' at casting could vary . so could be o.k. overall , or thin in places , or not .

Just like a big mike ( micrometer shape ) cross piece say 1/8 short ( & a few others ) and youll get a good idea
whether its wafer thin or hunkey doory .

If your not thrashing it to death , the word death is not likely involved . THOUGH see you retourque the flange nuts periodically ( before any big trips ) , just check theyre Tight & EVEN . Jumping about could cause it to go bang . :shock: