Fitting a Magneto Replacement to a BSA B31 Single

Author: Tri-Spark   Date Posted:10 August 2021 

One of our customers, John, wanted to fit a Magneto Replacement kit on his BSA B31 single. Ordinarily we do not recommend modifying our ignition systems, but John assured us he had the proper equipment and skills to make the changes to get the fit right.

John turned down the bolt mounts to just under the bolt holes.  He made a 4 mm spacer to even up the mount, and tapped 3 x 1/4” unf holes at 120 degree off the  R/E bullet bolt pattern.

Inside the timing case he drilled the mounting holes a little oversize to allow for adjustment, then used a flat rotary burr to remove parts of the seal mount to allow for this.

The trial assembly is shown in the pictures below. 

He used “Sikaflex” to seal and help secure the unit.




John is very happy with the Tri-Spark as now starting is a one kick job!

Here are some pictures of the finished bike.





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