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comression ratio choice


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VOC Member
Hi folks

I need to choose pistons for my bike asap - I'm aiming for a good road bike not a racer.

What advice do you have on choice of compression ratio? Smoothness, overall efficiency, driveability, knock effects, coping with available UK petrol (gasoline), durability of the bike, etc.

Thanks in advance



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VOC Member
Eight to ones is a good compromise between flexibility and performance. Nine to ones and even ten and a half to ones have been used on some road bikes but my personal experience is that the engines are not as flexible as with the lower compression ratios. Also mark two cams have been used, by me as well as many others, on the road but once again the flexibility is compromised.


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I have 7.5:1 pistons in my Rapide..starts with very little effort, cruises all day at 65-70mph ..Remember, these bikes are 60yr old classics..I think they should be treated gently, with the respect they deserve..with the occasional run up to 100 when required to show young punk what a real bike will do..


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My Egli's run on 9:1 for 39 years. Starts easily (from a button) and pulls smoothly from 20(ish) mph in top (electronic ignition). Would I use 9:1 if I was starting from now .... probably not ...... would I use 9:1 if I could go back to 1974 ...... you bet! 7.5 or 8:1 probably the best bet, but don't think 9:1s change the characteristics too much, it's still a big torquey motor, but you'll need a strong leg or a good technique for kickstarting.



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VOC Member
8 to one on this one.The pistons are Kempalloids, chucked out the supplied rings and used Honda Chrome rings instead, standard John Mcdougall /Dan Smith method for doing high mileages. 37,000 miles on this setup to date. just completed a little over 3,000 miles in ten days, several high passes included. The Beartooth in Montana was over 11,000, pulled strong up and over with full luggage onboard. There were five Vincents in the group, no serious problems. My air filter fell off, hence the big red thing on there. One rider had a Craven pannier come off after hitting a big bump, it came home wired and bungeed on. One had an alternator mount come loose and that was about it. Of course my speedo quit as it generally does, I don't even count that as a repair anymore. The GPs Speedo provides a good backup.
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Vincent Brake

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VOC Member
I did about 14000KM on a totally overhould Rap.: 7,3:1 (CP piston) MKI (megacycle , not at all like a mkI uk) Mikuni VM28.

pulls a ton and as smooth as a baby.

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