70 mm stroke crankshaft

Giday, I have acquired a unused crankshaft, possibly made for a 500 class race engine, that has a 70 mm stroke and 1 3/4 inch big ends. En26 throws, K1045 flywheel, nitrided. So it says. I worked out that if i use 650SS pistons its just within tolerance for a 500 class engine if it was to go racing, or perhaps an interesting road engine with a set of 810 Dunstall barrels I have, for a capacity of about 635cc. Now I'm inclined to do a "fools rush in where fools have been before" so I though I might seek your collective council . . . . . . . If it went racing in could be in a solo or outfit, Thanks.

My short stroke Triton 500 had a 63mm stroke billet crank, and definitely needed a 6 speed close ratio gearbox, which it never received. I used 4 speeds close ratio and I could choose where to lose a race. With high gearing, it was slower around the tight stuff, but fast enough at the ends of the straights. With low gearing it was faster around the slow stuff must too slow towards the ends of the straights. The motor delivered almost no power below 6000 RPM and would rev to 10,000 - until I developed a 2 into 1 exhaust system. I enjoyed racing it, however it was actually a nightmare - bad for the nerves. The motor was built from Triumph 650 parts. It was like having a nasty dog.

acotrel said:

My short stroke Triton 500 had a 63mm stroke billet crank, and definitely needed a 6 speed close ratio gearbox, which it never received. I used 4 speeds close ratio and I could choose where to lose a race. With high gearing, it was slower around the tight stuff, but fast enough at the ends of the straights. With low gearing it was faster around the slow stuff must too slow towards the ends of the straights. The motor delivered almost no power below 6000 RPM and would rev to 10,000 - until I developed a 2 into 1 exhaust system. I enjoyed racing it, however it was actually a nightmare - bad for the nerves. The motor was built from Triumph 650 parts. It was like having a nasty dog.

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Hmmm, has given me things to think about. Engines aren't built in isolation from what your setting out to achieve

A word of advice, following Acotrel's advice could be fatal particularly on frame setup. Read his other posts by all means but treat with caution and note the numerous replies refuting his info.

During my lifetime I have crashed a lot - I know what I am talking about. If you use more lean to get around corners faster, you become more tyre dependent. If it rains while I am racing, I am always in with a better change of winning. The first decent motorcycle tyres arrived in about 1972. Many guys have only ever raced on good rubber. A Manx Norton on shit tyres can often corner better than a TD3 Yamaha on good tyres - the Manx stays more upright and oversteers in corners. A Yamaha which oversteers is dangerous. The rear tyre steers the bike when you gas the motor and the rear suspension compresses. Peaky motors are dangerous. Steering geometry, tyres and rear suspension need to suit engine characteristics.

5146 said:

Hmmm, has given me things to think about. Engines aren't built in isolation from what your setting out to achieve

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I suggest a 70mm stroke motor would be good if you can afford the gearbox which is needed to make it fast.. With 6 speeds close ratio, it might be lovely. I suggest a 500cc Paton twin might be horrendous. Have a look at the bike, it might be what you would be making. I know what pain feels like. Percy Tait beat Agostini and MV, using a 65.5mm stroke 500cc Triumph twin on the Spa Francochamps Circuit in Belgium in the early 1970s. Spa is a big circuit.

I am actually being kind to you. With my 500cc Triton, I am lucky that I am not dead. I loved it dearly, but it was a really nasty piece of shit. I rode it in the first ever historic race in Victoria in 1973. I usually rode it in Allpowers C grade races. I was half way around the circuit before the others even got their feet off the ground -

kommando said:

A word of advice, following Acotrel's advice could be fatal particularly on frame setup. Read his other posts by all means but treat with caution and note the numerous replies refuting his info.

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Giday, thank you for your advice. I have noticed that not everyone agrees with Acotrel , but am interested in what he has to say. He tends to bring out people’s opinions. Thanks.

5146 said:

Giday, thank you for your advice. I have noticed that not everyone agrees with Acotrel , but am interested in what he has to say. He tends to bring out people’s opinions. Thanks.

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Be very careful
There's a big difference between opinions and facts