Upgrading a 750cc Commando engine to an 850cc

htown16 said:

To me the biggest factor on how a bike performs is how it is geared. I had two 750's, one with a 19 tooth sprocket and one with a 21. They were like two different bikes. The 19 was much more punchy off the line but seemed like it was wound tight at 70. The 21 was loafing along at 70 but was just adequate off the line. Maybe a 20 would be the ideal on a 750, but I never tried it. My 850 has a 21 and I think the higher torque pulls that sprocket size better off the line and still cruises at 70 with plenty of reserve.

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I tried to race with the Commando gearbox. But the bike was too slow everywhere. I bought the 4 speed close ratio cluster and the gearing was perfect everywhere except off the start line. All I needed to do was fit the standard Commando first gear. Once the bike is moving, you never go below second gear in the close ratios. But the difference in acceleration as you go up through the top three gears is immense. The other thing is the overall gearing. With the top three gears close, you can pull higher overall gearing.
My bike now has 6 gears close ratio, but I don't think I will ever get to race again. A Commando motor is excellent, even when minimally modified. It is fast enough for anyone. I rode 650 Triumphs with full race motors on public roads for about 10 years. And I have ridden them on race tracks. My Commando motor is much better.
With my bike, two things make it fast - the gearing and the steering geometry. I think it would be just as quick, if I used petrol instead of methanol.

I was racing regularly until about 1973. I wish I had known about Commandos what I know now. The fastest bikes were Kawasaki Z900s. My 850 now eats 1100cc methanol-fuelled CB750 Hondas in historic races. It is easily fast enough - Iin the old days, you would not even consider racing an 850 Commando.

Peter Waller said:

How hard is it to convert my 750cc Commando engine up to an 850cc? I would really like to do this but do not know what is involved and what new parts are needed. Thanks in advance for any help!

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Why.....that 72 yellow 750 of yours is nice as...I'd do some sensible handling and stopping up grades.

KiwiShane said:

Why.....that 72 yellow 750 of yours is nice as...I'd do some sensible handling and stopping up grades.

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When you build a fast bike, you need two things - the gearbox and the front brake, The single disc on the Commando it not enough. My front brake is a Honda CB750 spool hub with two Suzuki Steel discs. I use asbestos pads held in two Lockheed calipers, and my master cylinder is designed for use with one caliper. The brake is single finger operation, and it needs to be like that.

Why make this difficult? Pull the 750 lump and substitute an 850.
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Thx for all the replies and I sold the 750 and now have an 850...sorted!