On a related topic, is anyone familiar with 810's under the head?
I'm freshening up an 810 engine I bought as a spare for my race bike, so an unknown quantity as a runner. It has the GPM's and the flanged alloy top end. Only thing is the rods appear to be a bit shorter than expected, well the pistons dont come right to the top of the bore, short by about 3mm at the circumference (the crown still being above the the bore).
Its also got a stock 750 head combustion chamber (albeit skimmed to buggery), so the bore to head steps in. Being convex pistons perhaps this was simply a way of getting head clearance without reshaping the heads?
Would be very interested to know if anyone has first hand knowledge of what an 810 piston at tdc should look like and what if any head mods were made.
I recently bought an 810 commando engine which has a stock head. There was carbon buildup around the top of the bore approx 3mm. Pistons are GPM, the pistons don't come to the top of the bore. I have checked the length of the conrods with other commando rods I have and they are the same length. No head mods appear to have been made to the head/combustion chamber compared with my other c/heads.
Hope this helps.
I'm going by memory as I bought one in 73, -the 810 kit was just that, a light weight big bore bolt on for your 750 Norton. You got the cylinder and pistons with pins and rings. No special mods required but that isn't to say you couldn't do other mods along with it.
Try JP Pistons in Adelaide, South Australia. They make new pistons for all types of Commandos, might be able to do one - off specials, they seem very nice to deal with and their factory direct prices were pretty good.
Manufacturers of pistons for vintage, veteran and classic motorcycles. www.jp.com.au
You might want to reconsider trying to bore to an oversize. The liners in the Dunstall 810 cylinders are so thin already that they regularly break and drop the bottom half into the case, with disastrous effect. I used to buy replacement liners from LA Sleeve, here in California. They don't list them in their catalog, but can probably still supply them. Their part number was FL-227. I'm sure you could get other sleeve manufacturers to make a pair for you, if you gave them yours for a sample. If your pistons are still good, that's about the only way to keep your cylinders going in the original 76 mm bore. If you can't find replacement 76 mm pistons, you should be able to get LA Sleeve to make replacement liners in a smaller bore size. I had them make some a couple decades ago to take an 810 back to 73 mm for a 750 vintage racer, and it worked fine for quite a few years. Dunstall also supplied their alloy cylinder kit in 750 form for class racers, but I don't think they sold many of them.
I ended up sleeving mine back to std commando and fitted a set of wiseco's supplied by Mick Hemmings. It meant the pistons came to the top of the bore as expected and put more meat in the sleeves. I had run several sets of JP's in my 750 race motor and had nothing but problems with them , I think JP's motto is 'seize the day !' This was in a race application and clearances were very liberal so this couldnt happen. One Piston in the second set I put in was found to be cracked after it nipped up on that side after its first ever brief light run, the other piston was fine. I'm building up another british twin at present and it won't be running JP's! Perhaps they might be ok in a road application :?: if run in long enough, but I wouldnt touch them myself.