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Big Bore comets


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VOC Member
Big bore Comet

Due to no one else responding I will put in my small observations.
Mark Goodson's 630 or so Comet is a joy to ride, this from a man who is not enamored of Comets. It goes not a lot worse than my Rapide.
The engine done by Bob Dun, is bored and stroked 90mm by 100mm I think.
Bob tells me that it is the longer stroke that makes them go.
Last edited:

Comet Rider

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VOC Member
Vinparts 90mm Bore Kit

In the absence of a reply from Russ, here is what I had to do.

1/ Bore the throat of the crankcases to take the larger liner
2/ Machine the top of the piston to get the req'd CR (8:1)
3/ Machine the rebate in the head for the top of the liner.

Actually apart from the c/case machining everything else was done for me by John Coates.

Also to make best use of the bigger bore I run a 34mm Mikuni, and a Gary Robinson 105 cam.

The bike will pull 70 easily even with me on it!!:D:D



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Big Bore Comet

I Too Have A Bored And Stroked Comet Converted By Bob Dunn.
If You Cant Afford A Twin This Is Definetley The Way To Go. The Main Difference Is The Installation Of An Amc/norton Gear Box As Bob Said The Burman Gear Box Can Be A Little Too Femmer For The Job. You Would Be Better Off Speaking To Bob About The Technicalities As These Are Way Beyond The Likes Of Me
90mm Kit.

Have been busy catching up from 5 days off due to going to the Dutch.
Neil has just about got it right. The only bit that he left off is that you need to machine a squish band on the piston and in the head.
In my view Bob Dunn is the man to do the job and I am just sorting out a crankcase, head & 11mm shorter rod for him to build the big engine that is going into the Vinparts trike:D



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Many thanks for the replies.I think I will try to find a spare engine before i do this mod as I would like to keep a standard engine as well..John

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