Big bore BSA b25

baz

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May 26, 2010
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For trident Sam
My bike was actually a T25 trailblazer but that's just a bit of badge engineering to me it's a b25
I used an xs1100 piston and liner
I pushed the BSA liner out of the barrel and bored the barrel on my lathe to take the Yamaha liner
I shrunk the barrel onto the liner with a 4 thou interference
I bored the piston to take the BSA gudgeon pin
And bored the head to take the Yamaha liner flame ring
I rode it like this for a bit but the compression was just too low to make it go any better
Maybe like a fleetstar performance but a bit more torque
So I took it apart again, shortened the barrel slightly and took some meat off the head to get the compression up
This lit a fire under the bike so I wanted a bit more
So I took the head off again and welded the combustion chamber re shaping it and giving it a very tight squish
I opened up the inlet port and fitted a 30mm carb
It now had 11-1 compression
I had to cut the back out of the black box silencer and remove some of the baffling but fitting a reverse cone was the answer
It would rev to infinity
I had to shorten the push rods a fair bit
And the tappet clearance set to zero
And I fitted Boyer ignition because it was pinking at low rpm with the points,and revving on a little
But it was only after I'd bought the Boyer I realized one of the springs was broken on the advance retard unit so it probably didn't need the Boyer
The conversion was well worth while for me
It made more power throughout the range with the added bonus of the bike becoming smooth
Before the conversion I was always blowing tail light bulbs and everything would shake loose
I should have weighed the difference between the yam piston and BSA one
Sadly I have to sell the bike and my b44 to pay my mortgage endowment shortfall!!
I am planning another one when I have time ,this one will be a shortened b44 barrel and a b44 head but with a b25 crank and rod
I've already welded the stud holes in the crankcase and moved them to the b44 centers
Cheers
 
Very interesting, thanks for posting,
Far too much work for me to ever take on.
However, I'm with you on the endowment shortfall :)

ta
sam
 
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