Maximum (safe) overbore, 750 Combat

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RoadScholar

RoadScholar

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I am faced with punching my 1972, 750 Combat barrel out to + .060; pistons are available. A reputable source told me that a +.060 overbore was an iffy size and that he had seen examples of barrel cracking at that bore size. I am seeking opinions and experiences about this surgery; I'd rather bend over for a good used barrel then to be faced with rebuilding this engine again. I have no plans on racing, but I do like to pin the throttle now and then.

Any one have a used 750 barrel that would respond to a +.020 or a +.040 overbore?
 
Exploring whether these barrels could be sleeved could be a better option ?
Much stronger, if done before the point of no return.

Hey, the old pistons might fit again !
 
Yep a resleeve would be the way to go and the cheapest way as well if your barrels are still good.

Ashley
 
as you have been told 060 over is a hit or miss proposition and sleeving is an even worse idea. the 750 barrel is weak and i have seen them break or crack above the base and the spigot below the base . save your self the trouble and find a good used or better IF the pocket will allow buy a new 850 style in 750 bore from andover.
 
bill said:
and sleeving is an even worse idea.
Click to expand...

???
Hepolite etc have been supplying spun cast iron sleeves for Nortons since the WW2 era,
so if there was anything wrong with these someone would have noticed by now.

As long as they are done BEFORE the cylinders are beyond the point of no return,
it will be as strong as it is now...

I have several engines that have been sleeved - not Commando though - and they seem to have
done a considerable mileage and still serviceable...

e.g.
http://www.feked.com/norton-commando-75 ... liner.html
others do them too.
Very common in the diesel world too...
 
It's not the sleeve itself that is the problem. There is not enough material on the 750 barrel to install it. It will actually go through in a few places towards the bottom (so you can see the sleeve from the outside). This means that the section that secures the barrel to the cases is barely attached to the upper part of the barrel.

You can get some serious cracking or damage, even the barrel separating in half.

So while an 850 can be sleeved with no issues, a 750 should not be

Somewhat of the same issue over boring to +60. Not much material left.

Best is to go to a std bore and take it to +20....or +40

Matt / Colorado Norton Works

http://www.coloradonortonworks.com
 
as Matt said, they are very thin, but I think some are thicker than others so you may get the odd cylinder that will take a good boring.
Don
 
FUCK,?????????????? If you cant rebore them out to 60 thou putting sleeves in wont make them any stronger
 
But the std sleeve itself has integral strength, if it didn't then watercooled engines could not be wet sleeved.
 
It's safer to overbore to +60 than to resleeve. I have never had trouble at +60 even when I was younger and had little mechanical sympathy and used to wear bores out faster than chains! Resleeving was never an option back then and used parts were plentiful and cheap. +60 takes out about .75mm of the bore (1.5mm increase in diameter) whereas a resleeve would be more like 3 or 4 mm ( so 6 to 8mm diameter) at a guess and then the new sleeve would add any strength back. I would overbore before scrapping the barrel and look for a long term replacement for the next time!
 
the thing is on a norton the cylinder is a separate part that is bolted to the crankcase and the head bolted to the cylinder. on most modern designs the cylinder block is either part of the crankcase or has through bolts all the way from the head into the crankcase

kommando said:
But the std sleeve itself has integral strength, if it didn't then watercooled engines could not be wet sleeved.
Click to expand...
 
Thanks for your replies.

I am convinced that a +.060 overbore and that installing sleeves are not viable options. I have done some digging and discovered that all the e-bay 750 barrels, which are not plentiful, must be the ones that were part of a limited run of gold/titanium/unobtanium alloy because the sellers are asking more than Andover Norton, some even have owner uninstalled fins...

My plan is to enhance cocktail hour tonight by making an order to Andover Norton, such an effort will give me plenty of time to worry about something else...
 
RoadScholar said:
Thanks for your replies.

I am convinced that a +.060 overbore and that installing sleeves are not viable options. I have done some digging and discovered that all the e-bay 750 barrels, which are not plentiful, must be the ones that were part of a limited run of gold/titanium/unobtanium alloy because the sellers are asking more than Andover Norton, some even have owner uninstalled fins...

My plan is to enhance cocktail hour tonight by making an order to Andover Norton, such an effort will give me plenty of time to worry about something else...
Click to expand...

You could go a little bit rash and buy 850 cylinders and get the head converted over, I didn't think Andovers cylinders were that badly priced ,they wont need a rebore either

kommando said:
But the std sleeve itself has integral strength, if it didn't then watercooled engines could not be wet sleeved.
Click to expand...

Have you ever seen the difference in wall thickness between a wet sleeve and a dry sleeve, it's about 1/4'', even then it wont stop the top pulling off the flange, through bolts would help but probably wouldn't stop it
 
They could certainly be Nikasil coated and honed back to a safe size.
Cal Bore tech. Jim
 
If you are buying the Andover 850 shape barrels for your 750, be careful with the timing side fit. On original 850 barrels there is a raised area at the drive side base - I imagined it to be the sprue from casting, but that's just in my imagination - the Andover block has a similar area on the timing side also. I bought a set a couple of years ago and had to have that area machined back as it fouled the inside of the timing chest, stopping it from sealing. I think I may have posted pictures in a previous thread.

They were expensive and I had to work myself up to spending the money, but I thought it was a better option than buying someones mechanical disaster on eBay.

Regards,
David
 
The idea of repairing the current cylinder based on the comments in this thread discouraged me from initiating any action. Last night I ordered a new cylinder from Andover Norton; the price was actually less than some gold diggers are asking on E-Bay, I will check all seating/sealing surfaces for anomalies and deal with them accordingly (if present).

I may glue the expired pistons in the bottom of the bores and turn the "assembly" into a planter, maybe a lamp...
 
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