BSA Bantam bottom end

[redcoat]

Greetings all,

A first post, and I'm begging for advice:

A local bike shop has a Bantam (shocks at the rear, not plungers) that looks like it has had some time and money spent on it. The shop owner says the previous owner had "everything done with the engine- head and barrel off, barrel bored, new piston, new rings yad yada". Put it together, started it, and it went thump thump thump from the bottom end. He claims the new top end produced compression too high for the bottom end to handle.

Now, I'll admit I'm in the colonies, and I don't know much about 2 strokes, or old British bikes. Seems to me that at the least it's toasted bottom end bearinsg and possibly a damaged crank shaft.

Questions: Suggestions as to what caused this in the first place?

Does the story ring true?

If the bottom end is knocking, is it much to pull these engines apart?

How expensive is (and how hard to find) a crank shaft?

Regards,

Justin

[33d6]

Hi Justin,
The most common cause for what you describe is the previous owner forgot to add oil to the petrol or put too little oil in.
The dealers claim that the new top end being too much for the bottom end can be sort of correct if the bottom end was nearly worn out in the first place. A new top half will quickly finish off a very tired bottom end.
Like all British two strokes the Bantam is very easy to work on and not expensive but does require someone who knows what they are doing. Engine spares are fairly easy to find. The Bantam was around for a long time and was continually updated through its long production run so you need to know which particular Bantam you have before you start rushing in..
Have fun.

[L.A.B.]

Bantams use a pressed-up crank, so a new big-end assembly can be fitted.

Spares:

http://pages.eidosnet.co.uk/~canddautos/

Info:

http://www.mistgreen.com/

http://bsabantamracing.co.uk/

[Blue]

crankshafts can be repaired; if you can't simply regrind the shaft and fit oversize bearings, you can have it built-up with 'stellite' and ground to the original specifications.  Unless it's been terribly bent or destroyed, it's unlikely you need a replacement shaft.

[redcoat]

Thanks for your replies folks! The present owner swears he ran it with 20:1 gas/oil. I am not convinced. Anyhow, thanks for your ideas- the stellite sounds good if needed, and a regrind and oversize bearings sounds the easiest.

Last question, start it to hear the banging myself, or pull it apart and look for the source? A running, rattlling engine is easier to put a stethescope up to! But if I cause further damage...

Justin