beng said:
Dunstall ended up with the Domiracer stuff as everyone knows. I have a copy of a letter written from Dunstall to a couple of racers in the USA, dated December 1972, ten years after he had received the stash of works twin parts, stating that Dunstall had one works-type Domiracer engine left that he was willing to sell, that had a short alloy cylinder, eccentric rocker adjusters and Commando-sized rod journals and matching rods and a larger than stock drive-side crank bearing He wanted 200 pounds for it.
When Dunstall says the crank measurements are "as for a 650", he is talking about the rod journal size, they used 650 rods, which are the same dimensions as 750 or 850 rods. In the above letter I quote, since the letter is from the early seventies, he makes the reference to Commando rods instead of the 650, because by then the 650 was history.
As far as the engines having larger main bearings, in what way were they larger? 35mm is quoted for the drive side bearing in the above letter, and nothing is said about the timing side at all, and this is certainly for the shaft not the O.D., there would have been a lot of options for machining the drive side for bearings when accommodations for the alternator and production machining did not have to be made.
The 650ss and 88ss head are the same and interchangeable between the two bikes for 1962-onwards, the head worked with both 66mm & 68mm bore engines.
A racer that lives close to me took a 750 crank, whacked off the shafts and bored holes the correct distance from the rod journals for the 500 stroke, then he pressed in new shafts and tig welded them on the inside to hold them in place. Now he has a 500 crank using the 1 3/4 rod journals. He shortened his cylinder to match the rod length, and that is how he made his "Domiracer".
Replicating the top-shelf development twin engine with it's alloy-nikasil cylinder, 2mtt magneto and eccentric rocker gear is going to be a lot of trouble and expense for anyone, and I would applaud their effort to do so.
I just pointed out the things I did to put out there that to have a good Domiracer replica all that trouble is not necessary. That since some Domiracers had engines that were built up using mostly tweaked and re-machined production parts it is possible to do the same now and be historically on the money and in their power range.
Also when racers blew their works twins up, especially after the works closed after 1962, they raced them with whatever production parts would let them get back on the track.
About the 650 Domiracer that I believe Derek Minter used on short circuits, Doug Hele described it as a "tuned roadster engine". Heinz Kegler had a standard appearing 650ss to ride around while he was working for the experimental department at Norton which had a light flywheel, raised compression, big carbs and a special cam with radius lifters he said was tested at just under 130mph. Another tuned roadster engine......
So lowboy frame, Manx frame, or slimline, iron or alloy cylinder, eccentric or standard tappet adjusters, 2mtt or k2f mag, etc.. Choose what you like and what you can get your hands on and whatever you put together the chances are excellent that the factory did it too and also raced it.