Why didn't I buy a runner?
I have now sourced a +0.060 piston and had the barrel bored to fit. Went to put the barrel on the cases only to find that the gudgeon pin is far too big at 19mm diameter. The conrod eye is a tight 19mm without the small end bush.
Have others had this issue with replacement pistons and what is the solution?
I'm thinking ream out the conrod but will that simply give issues about finding a new bush to fit?
All help gratefully received, TIA.
That does not sound right.
The B40 small end bush, part 66 492, dimensions are given as 0.75 inch ic x 0.815 inch od x 1.062 inch long. Very thin walled.
Is there a casting number on your conrod?
Sounds like your con-rod already has the bush installed. 3/4 inch is 19.05 mm, so only a sliver needs reaming out to get the pin to fit.
You should only use Imperial measurements, as mixing with metric will be confusing. i.e. 3/4 inch is 750 thou.
Definitely no bush (I took it out). Rod looks a bit scored.
Casting number 41-234.
Is it definitely a B40 piston? I have AMC pistons of the same basic bore sizes and I'm sure they have a much bigger gudgeon pin.
How about showing us a picture of the piston crown and any markings on it?
Paul.
The piston is clearly an after market part but it's the same pattern as the one that came out. What do you think?
Looks like the small end bush was spinning in the rod to cause that scoring?
What did the outside diameter of the bush look like? The bush walls are very thin just 1/32 inch.
You may have a C15 conrod, same 41-234 number. Whats the width of the small end? If it is narrower than the length of the B40 small end bush that would confirm C15. If so you need to check you have B40 flywheels as they are bigger than C15...
Thanks Dave, looks like you may have hit on the problem. The rod is 22.3mm wide at the small end. I've got another crank with a blown big end and the rod is definitely wider.
When I get time I'll split the cases (again) and measure the flywheels. Should be 63/8" rather than 61/4"?
Thanks again everyone, I'll keep you informed.
So, I've split the cases again and yes it's a C15 crank, "what a difference an eighth makes"!
I do have a very worn B40 crank and I'm off to have it split on Wednesday evening with a view to renewing the big end and replacing the 'rod. It should all go together then.
After that I've only got the gearbox to worry about, what could possibly go wrong?
Thanks again for all the help.
I keep my fingers crossed 🤞 it can't be that odds are against BSA maniacs!
Once again many thanks for the help.
The B40 crank is now split. Amazingly a guy close to where I live has a replacement roller big end pin, still in the wax, to suit.
Last remaining issue is a replacement rod. The old one has a very worn small end bush (not an issue really) but the drilling for the gudgeon pin is very definitely off centre (maybe a problem?)
Does anyone have a decent 41-55 rod at a good (aka cheap) price?
Thanks