B25 engine noise

[Stephen Ruffle]

My B25 has a loud engine clacking sound. Much louder than your normal engine clattering. It only happens when engine warm and only happens on no or light loads. Clacking goes away immediately when the throttle is opened ie not a big end. I've rebuild the engine with new bearings throughout, new piston, new valves and guides and springs. Valves are not hitting piston. Bike starts and runs well. It had the noise before and after rebuild.

[Janacek]

Had similar symptoms on one of my bikes (not a BSA) the noise you describe was not in my case from the engine,but from worn gears which used to chatter away with the vibration at tickover in neutral,but at higher engine revs or when the engine was pulling, the noise went away.

[JulianS]

I had a 1968 Starfire which when hot made a noise like a tappet set at 50 thou. No tinkering with valve clearances made any difference. I removed the head whilst the engine was hot and could see the liner moving up and down in the alloy cylinder jacket when I kicked it over. The lip of the liner had been hitting the recess in the head making a horrible din.

Maybe this is your problem?

BSA issued service bulletin 176 in July 1968 with the factory solution.

In December the same year parts service bulletin G54 page 5 showed that the early cylinder had been superseded by an improved version to cure the fault.

[Derek996]

Or possibly small end wear?

[Rupert]

Worn slide rattling in the carb? - "noise goes away when the throttle is open".
When ticking over with throttle closed put your finger in the carb inlet and press against the throttle slide...

[Stephen Ruffle]

Thanks guys for quick response. It's not the carby, it's not the small end. Loose liner is interesting. I'll try removing head when hot as suggested.

[Stephen Ruffle]

JulianS is correct. I pulled the head up 3/4'' while hot an saw the liner move up and down. Any suggestions on how to fix it?

[Andy Kay]

Looking at the part numbers in the data posted by JulianS, the shims appear to be available in the USA but not in the UK.

[JulianS]

My solution was to replace the cylinder with the later pattern one.

[DAVE BRADY]

Hi,

There must be an industrial adhesive that could work?

Dave.

[Stephen Ruffle]

Does anyone know the difference between the 40-949 barrel and the 70-9263 barrel? Is it just a tighter interference fit or does it use a lower expansion alloy? All the second hand barrels I can find are 40-949 and may have the same problem. Otherwise I can get a new liner made to fit my 40-949. I'm a bit hesitant to go with the shims.

[Rupert]

Bearing fit loctite under the liner flange should stop it moving.
The 70-9263 barrel and its liner are a different design to cure this problem. The liner has a flange below the top spigot which is recessed into the head joint face of the barrel and is therefore clamped below the head gasket to hold the liner in place.

[Stephen Ruffle]

Thank you for all your help.
I'll get a new liner made and fitted in my original muff.