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#868208 01/05/22 12:50 pm
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Hi Folks,
I'm rebuilding what was once a Hagon grasstrack bike for sprinting. It's basically a C15 engine, but with what I think is a shortened B50 crank (80mm stroke, roller big end, end fed ) A cam of which I'm uncertain, but it doesn't match any of the BSA ones, A65 valves and running on methanol with a Gardner carb and magneto ignition.
The bore was worn, so I had to fit a Triumph piston, taking the capacity up to 299cc.
The compression ratio comes out to 16:1 if I don't alter anything so I'm planning to pack the barrel up a bit further (it's already raised for the long stroke) to bring it down a bit.
I was thinking that 14:1 looks like a good figure for methanol.
Has anybody run a C15 at this sort of figure and whether the stock head studs will take it?
many thanks


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If the stock studs won't take it then you can make new studs, the C15 ones are waisted with the larger end screwed into the crankcase and all smaller above. Make the new studs the same diameter as the crankcase end and open out the barrel and head holes to suit, plus you need new head nuts but the crankcase will need no modification. I had this issue on a hot B40 with high compression and this cured the head gasket leaks and the B44 factory studs are the same larger diameter from end to end.

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Thanks Kommando, sounds like a plan.


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Are you going to use the bike for what it was built for or something else.

Mr Mike

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Hi shedrider , This my first post , just a note on packing the barrel , this will lower the compression ratio , but really bad because you lose the squish band ,(tumble and swirl) which is really important for a more homogenous mixture which helps against detonation . Cheers .

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80mm stroke sounds interesting. If the crank is/was B44 or B50, then it's a 5 piece crank pressed together, actually not unlike B40 but B40 was only 70mm. If so it would have to have custom flywheels or offset crankpin or evidence of new holes. B25 crankshaft is 70mm as well but one piece forged with bolt on flywheels. I have one that has been welded and ground off-center for 75mm stroke, so may be possible to do the same for 80mm stroke.

Check that 16/1 compression ratio, as that wouldn't be easy to achieve. I have a B50 bored to 92mm with JE piston, about .035" squish band, 90 mm stroke, and I have measured compression ratio to be 14/1. Of course you are more under square at 69 x 80 than my 92 x 90, so may be possible to get 16/1.

You show your location as Peterborough, but what country?

Tom


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