A friend of mine has explained it to me and done it twice before. He is probably one of the best machinists/motor builders in the country for old bikes and especially Vincents. His advice to me was to get a lathe and a mill, buy some bearings and just start making things fit together. Get ready for a long road though. Or you could turn it over to someone who is already a qalified motor builder and machinist, but this could get pricey. But I say do it anyway!
I've got all the specs and drawings at the shop, my engine builder is Bob Donnabedian, Bob worked with Danny Macias in the 60's and 70's as a flat track team owner and builder, they built several of those unit 840 c/c Triumph motors with Norton cranks in them as he called them "cheater motors" because there was never any tech inspections for stroke in the national flat track circuit, I got to ride one, and man, did those things haul ass.
according to Bob the biggest pain in the as part of this was making a tapered Triumph crank gear, (he has one left over) because it needs to fit tightly and line up with the clutch basket, he says the rest of the machine work stuff was relativity simple.
Geoff at offsetcrank.com just built an offset Norton crank for me. Using 800cc barrel & 91mm stroke, yields about 874cc. I am using the MAP forged 9:1 pistons to keep the compression lower. It's set up to use a superblend Norton bearing, Norton drive pulley & Britech seal holder.
The cams are similar to the 'Q' cam profile with a bit more lift. Designed to be a torque motor rather than top end performance.
Original pre unit cases are not very strong. I reinforced my original cases, or you can buy aftermarket cases that look similar to original.
Definitely go with the 90/270° crank instead of the stock 360°. There are people in the UK and here who can do the crank and cams for you.
From what I've read it was Phil Irving who first suggested this. He looked at the stock Triumph crank and said, "It will break here." (meaning the timing side journal) And guess what, as soon as you get a stock crank engine putting out more than 70HP, that's where they break.
90/270 staggered crank is the best reason for doing this.There's plenty of info at Britbike.com in the Competition forum and British Modified Specials forum.
Use the Norton bearings and grind the O.D. of the outer races for 0.0025"-0.003" shrink fit in the cases.
It can also be good for economic/reliability reasons.With all else equal,increased stroke will give the same power at the same piston speed.That means less rpm,less stress,less breakage.
On the other hand,you could increase cam duration and intake/exhaust sizes to slightly increase piston speed.
You'd get 4% power increase,with the same stress as a standard engine.
If you use more cam duration,maximum torque and peak power will occur at a higher piston speed. EDIT :If you don't plan to use maximum torque or peak power,you've just fitted the wrong cam and lost low-rpm torque for nothing.
To keep the engine happy,you'll need bigger carb and more intake/exhaust flow at that higher engine speed.
Cam timing and intake/exhaust size has a lot to do with the piston speed where you'll be making good torque and power..