Machining a New Cover for the Sturmey-Archer Gearbox on a Norton CS1

Andy Marks
Jul 30
2 min read

When I bought my 1928 Norton CS1, it had been fitted with the wrong gearbox. Fortunately, the correct three speed Sturmey-Archer gearbox was included with the bike, although many of the parts were badly damaged. One of the worst was the end cover, so after sourcing a replacement casting from New Zealand I machined it from scratch. This article follows that process.

What was wrong with the original?

A damaged Sturmey-Archer gearbox cover metal mechanical part on a blue background, labeled OIL and ARCHER, with circular openings and worn surface.

As you can see in this picture, the original end cover is badly damaged, the top gear lever mounting ear had been broken off and welded back on (badly) and there were a lot of cracks around the kickstart shaft boss.

Somebody had obviously tried to use it as a pattern to cast some new ends, hence the copious amounts of filler all over it. A couple of replacement castings did come with the project but they were a poor finish and, the final nail in the coffin, this turned out to be the wrong end for a CS1.

Machining the Inside

I started by holding the kickstart shaft boss in the lathe and, after getting it running reasonably true, I skimmed the gasket face. I then drilled through before boring for the kickstart shaft bush and counterboring the recess to clear the kickstart mechanism.

Clutch Worm and Fixing Stud Holes

Now that I had the gasket face and kickstart bore I had a datum to machine everything else from. I mounted the casting on the milling machine to bore and tap the thread for the clutch mechanism. This bore is what sets the centres between the two gearbox shafts so it was important to get it correct. I was able to measure it from the gearbox shell.

Whilst set up on the milling machine, I also drilled and counterbored the fixing stud holes.

Kickstart Shaft and Stops

I turned the casting over in the milling machine and used the boring head again for the recesses to take kickstart stops.

Fortunately I was able to salvage the stops themselves out of one of the old covers that came with the bike, but they are available if you need new ones.

Oil Filler Plug Hole

The final job was to mount the casting on an adjustable angle plate to drill and tap the filler and level plug hole.

This gearbox is filled with semi-fluid grease but I top it up occasionally with some engine oil.

Close-up of a Sturmey-Archer gearbox cover being drilled on a milling machine, with silver shavings and ARCHER cast into the part.

Finished

This is when I would normally blast clean and wire brush the casting to blend everything in and give a nice finish. However, as I was leaving the CS1 in its oily rag condition left this as is and it now blends in very well with the rest of the bike.