BSA B25 1969

[vanman]

hi GUYS i am at my wits end with my b25,i have now re built it for the second time,starts fine runs sweet and then as it warms up Awfull big end clatter.

Theres good oil return to the tank.

If i let it cool for a while it starts sweet again and then this orrible noise.

Any ideas or should i set light to it.

[alan_nc]

I know you said you have been through it twice but it sure sounds like a clearance problem. It seems like a silly question but did you check the dims on both ends of the crank and the receiving points on the cases? Also, is the crank straight? Did you place it in V blocks, rotate it while running a dial indicator on different sections?

Was this crank in this engine originally and did it run well? You may have a crank that wasn't heat treated properly and expands to much when hot.

Just a couple of thoughts.

Alan

[charlie prescott]

Hi Guy's

Hi Vanman.

Sounds to me as if it is the conrod. Did you replace it?

I would strip the motor back out quickly before it goes bang. these rods have a habit of braking without notice. did you replace the shells? and did you check the rod for length and straightness and check for cracks? I would seriously replace it, and have the crank checked.

Regards Charlie.

PS this should be in the Classic bike section as the bike is not PR65. Or perhaps we could have a "British Twin Shock",(BTS) section Andy.

Edited December 4, 2010 by charlie prescott

[stump magnet]

had exactly the same problem with my brother in laws b25 turned out the liner was moving up and down with the piston when the engine was warmed up as the fit was not too good so i machined the head a fraction so it held the liner down and still sealed the head gasket, the comp ratio was probably raised a tad but it was on a pre 65 metise scramble bike and it went like hell for a 250 after that with no probs just an idea if you cant find anything else wrong.