New Fairey Overdrive

A few years ago, for a project I could do overseas and also to have a spare, I bought another Fairey overdrive.  It was bought unseen as a noisy but serviceable unit.  Yeah…

I spent a couple of months of late 2022 stripping and inspecting the unit before deciding what to do with it.  It took some time.  Stripping was made much more difficult by the internal damage it had suffered, and involved a little creativity (well, imaginative destruction) to get various seized and self-welded parts to separate.  Once that was complete, it became quite a task to determine which parts could be sourced for replacement.  This needed a lot more than just the service kits available from a few suppliers (I can heartily recommend GLF Classics).

The hardest parts to source were naturally those which have been out of production for quite some time – the clutch sleeve (splined input coupling) and a gear set to replace the mashed lay shaft and rear main shaft gear.  It took a lot of emails, phone calls and nudging, plus a lot of doubtful searching of engineering shop shelves by their machinists, and those I managed to find appear to be the last unused stock of replacements on the planet.  Apparently, after sitting on this thing for a few years, I decided to get on with it just in the nick of time!

Naturally, when dealing with such rare parts, prices aren’t low.  Gavin at GLF made a very reasonable offer on his only new clutch sleeve that had been sent back as a cancelled order by another customer.  The gears cost me £450 plus postage from an engineer who was in the process of shutting down his workshop for retirement, which is what the rest of the sets had been sold at when batch machined in 2008.  So, nobody decided to play silly games, but with the large number of parts I needed, the orders ran to about £700 plus the shipping.  Add the £300 purchase price of this “serviceable” unit, and it became an unexpectedly high cost project.  Still, it was a challenge that I enjoyed, and the result is a fully reconditioned unit with mostly new gears for less than they typically sell with well used gears.  I’ll take it.

What I found when I started inspecting the unit before stripping was a huge amount of endfloat on the main and output shafts and a lot of grinding.  When I removed the rear casing cap, I was met with a failed circlip, a few torn and mangled shims, a bunch of needle rollers from a disintegrated bearing and a single circlip barely holding the shaft in place.  Removing the top cover resulted in a load more of those rollers falling out, a lot more debris and some very mangled selector fork slipper pads.  Oh, and some fluid that was more glitter than oil which stank like burning tyres, so burned was it.

The strip down went on to reveal that one of the thrust bearings inside the rear main shaft bearing had been assembled with the two races against each other but missing the needle bearing between, as if used as shims, which not only caused a huge amount of excess end float that hammered the main shaft in and out, leading to the circlips and rear bearing breaking up and tearing of the shim pack, but also the radial roller bearing inside the hollow of that gear and adjacent to the suffering races to overheat and weld itself together. Not the inner sleeve to the shaft, you understand (well, not only…), but the rollers to weld themselves to that sleeve race and the bearing surface of the inside of the gear.   So, that was a job to undo…

Thankfully, with a good deal of effort, I was able to separate those parts, main shaft was suitable for reuse after a bit of dressing.  It’s a bit discoloured from the heat, but dimensionally correct and the surfaces are smooth.  The rear main shaft gear, apart from its ruined internal bearing surface, was wrecked.  The helical teeth that mesh with the lay shaft had gone through their case hardening and the synchro dog teeth were half their original thickness.  The lay shaft gear cluster was also wrecked on its rear end, though its front gear looked good.  Surprisingly, the output shaft was like new, with no discernible wear anywhere on the part – I suspect it may have been replaced and that was when the thrust bearings were messed up, the back end failing in short order and the unit being removed before the output shaft even bedded in.

The lever, bracket and rope cover all needed rust removal and repainting.  The clevis pins were replaced, new anti-rattle Mylar strips added and a brand new replica selector lever know installed.  I have to say, I’m pleased with the end result (if not the cost).

I did record almost the whole process on video and got half way through editing to make a rebuild guide.  It is hard to make such a video short enough, though, and as it stands, it’d comprise five or six episodes of 20-30 minutes.  I’m not convinced it’d be that watchable at that length, so I stopped editing, but if there is enough interest, I may finish that off to upload to YouTube.