Tech W/ Kyle: Making (And Repairing) A BSA Gear Shift Quadrant

Tech W/ Kyle: Making (And Repairing) A BSA Gear Shift Quadrant

One of the most common parts that is often damaged from use and poor maintenance on a BSA is the gear change quadrant splines 

When the gear change quadrant splines develop wear or loose it’s “edge”, the gear shift lever could fail to rotate, (which means no gear changing)

Today I wanted to share with you this 2-part series in which Kyle will cover the following..

  • Making a new shaft 
  • Cutting splines
  • Repairing and modifying an original gear change quadrant 
  • Finishing 

Let’s dive in 

The modification 

Gear Change Spline In Lever

The modification we are covering today is pretty straight forward and can be done on all A50 / A65 unit twins including all BSA unit singles as well as Triumph cub models

The theory is to take your existing gear change quadrant, cut it, bore it, and install the spline shaft followed by welding (or brazing) and you will have yourself a practically new gear change quadrant 

Finding an original gear change quadrant is getting slim so this modification is practical and does work if executed properly 


I will be making my own splined shaft, however we do sell them new on the site so you do not have to make your own - since I had the material on hand I decided to make my own 

The material used must be weldable (4140)

Below are the supplies needed to be able to make your splined shaft 

  • Lathe
  • Mill
  • Rotary table

If you purchased a new shaft from us, or another vendor you can skip the following steps below and jump to series 2 which will be posted next week..

Machining a shaft - #1 (the lathe) 

Machining A Shaft On Lathe

Using a 1/2” nominal size cold roll steel (4140) I first ensure that both sides are flat and square 

After both sides are faced off, I then follow up by drilling a center on only one side (this will aid with truing the gear change when the 2 pieces are joined prior to welding)

Breaking the edge with a chamfer tool I lastly use a 1/2” radius button insert to add the needed radius for the gear change locking bolt to slide through (see photo above)

Cutting the splines - #2 (the mill) 

Cutting Splines Using A Rotary Table

Now that the 1/2” shafts have been machined on the lathe, it is now ready for cutting splines

I first have to set up the rotary table by ensuring that the rotary table is perpendicular to the table 

For the splines I’ll be cutting, we need to cut a total 36 splines, which means each spline will be 10° apart 

Now that the splines are tight in the 3-jaw chuck we are ready to cut the splines 

Goal is to make 1 pass per each cut to the proper depth…

Finished Cut Splines

After 36 rotations and passes using my cutter, we now have a successfully made splined repair shaft 

Quadrant Repair

BSA Original Gearshift Quadrant - Damaged


Now that we have our new splined shaft made, it’s time to dive into repairing an old gear change quadrant

This specific quadrant on hand is for my 1962 BSA C15S project that I plan on building soon..

Although the quadrant is in decent shape the splines for the gear lever are worn down (see photo above)

To execute this repair job, I highly recommend using a lathe as concentricity is very important..


Measuring Depth of Shaft


Before we do any cutting or machine work on this quadrant, we must record the shaft protrusion (length) from the shoulder of the casting to the end of the shaft 

Record and document the correct measurement on a note pad

Each gear change quadrant will be slightly different, however, not ALL quadrants are the same spec - for example, a B44 quadrant will have a different shaft protrusion than a A65 quadrant..


Cutting Shaft


Now that we have the length documented, we will have to cut the shaft off using an angle grinder

Try to get as close as you can to the casting flange - make sure you keep the cutting wheel straight and true

I do NOT recommend parting this shaft off on a lathe

I do recommend leaving a bit of materiel near the casting flange to allow a “face pass” on the lathe




The best way that I have found to mount the gear change quadrant in a lathe is to use a 3/8” 5C collect

Once mounted we will face off the shaft until it cleans up all the way down the casting flange

Now that the face is cleaned up, we will use a spot drill to start the drilling process

Starting with a 1/4” drill bit, we bore the quadrant to the proper depth followed by a metric drill bit just shy of 1/2"

Once we get near 1/2", we will lastly use a boring bar to size the hole no more than .001” larger than the 1/2” shaft (.501" approx)

Using our spline shaft we will test fit the shaft into the quadrant

If you are happy with the fit, you are ready for welding

To aide with welding and penetration, I do recommend adding an undercut on the splined shaft to allow the weld to build up


Brazing Gear Shift Quadrant


Now that our shaft is made and the quadrant machined it is now time for welding

You can choose to weld or braze

I’m not a welder, but I can braze confidently 

For this repair I opted in for brazing because the convenience for me, and the fact that the braze can "fill in"

Once the shaft and quadrant are brazed together we will have to mount the quadrant back into the collet to allow a quick face off the weld build-up

Only clean up what is necessary 

Job complete

BSA Gear Shift Quadrant Complete

If you have followed the steps above you should have a nice gear shift quadrant ready for service 

I suggest checking the finished shaft for any noticeable run-out  


Thanks for reading todays post 

Click here for more blog posts like this one 

Should you have any questions, please drop us a comment using the comment section below 

 Ride safe! 


  • Scott Dundas

    A very common problem with the A65. In the early stages of shaft wear it’s amazing how good you get moving the gear lever into position to shift down ! I used to make a point of buying good second hand shafts at swap meets but finding them today is rare. Back in the 70s I saw a shaft that had been fixed by drilling through the lever and shaft and bolting the whole assembly together. Don’t know how well that worked. Best to always check the tightness of the pinch bolt and keep the threads greased !

  • Tom

    Drill a hole through the shifter and shaft , tap it and thread an1/8 in hardened bolt with a hex head. Worked on my b50

  • TommyMcFarland

    What is the angle in degrees on the cutter for cutting the splines. Thanks.

  • Richard

    What should the total length of the 1/2" shaft after facing off both ends? Cheers Rich

  • Classic British Spares

    @Josh – sorry, 36 splines – not 30

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