The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: TassieChris on 11.09. 2017 07:04

Title: A7 star twin big end bearings
Post by: TassieChris on 11.09. 2017 07:04
Hi Guys
Have just pulled my 1949 A7 star twin engine apart and found it needs some work in the bottom end.
The big end bearings are the main issue - They are already .040 oversize (or should that be undersize) and show a fair bit of wear - I can get new .040 bearing shells but can't find anything larger and the crankshaft measures 5-6 thousandths too small to just fit a new set.
So I either need some .050  bearings or some way of building the crankshaft up!
Anyone have any solutions?
Maybe there are bearing shells from something else that will fit.

Everything else I need seems to be available - The bores are pretty good at +.030 but I will fit new pistons and rings etc as the gudgeon pins are worn.

thanks in advance

Chris.


Title: Re: A7 star twin big end bearings
Post by: KiwiGF on 11.09. 2017 07:13
I've been told a Triumph big end shells will fit and take you to below -040 on a small journal crank. I'm not sure if Triumph rods also need to be fitted.

I've got this to figure out if I ever need to regrind my crankshaft, Im sorry I don't know which model Triumph  *dunno*
Title: Re: A7 star twin big end bearings
Post by: JulianS on 11.09. 2017 09:25
Solutions would be either source a better shaft or get a competent engineer to build up then grind the journal back to standard.

Using a big end shell from a different make can be difficult, it needs to match the thickess and length (measured across the shell at right angle to axis is the bolts)  and the big end eye in the rod also needs to match.

The A7 shells were the ame from engine XA7 600.

Below is from the 1981 AE catalogue and gives dimensions of common bikes shells.
Title: Re: A7 star twin big end bearings
Post by: Black Sheep on 11.09. 2017 10:03
You can get the crank metal sprayed and reground to standard. Varying degrees of success have been reported with this. My own experience wasn't good.
Title: Re: A7 star twin big end bearings
Post by: Butch (cb) on 12.09. 2017 09:22
People who know will chime in about the importance of the radii at grind I'm sure.
Title: Re: A7 star twin big end bearings
Post by: bikerboy on 25.09. 2017 01:18
I would not go with a metspray far to many bad reports. I think the modern day method is for an engineer to weld it to build it up then to regrind it.

Thats what I had done to one of my bikes anyway and surprisingly it was not that expensive