So, after a lot of messing around with a spare one I have, I decided to have a stab at getting it apart with minimal damage. I came to the conclusion that a 2.5mm hole drilled through the body of the drive itself would allow access for a small punch to tap out the seal. If all goes to plan, a small dab of silicone in the hole will seal it up once finished. Job jobbed.
Just turning the tabbed washer will tell you how bad it is. If it stops, gets stuck and generally feels tight, it needs to be stripped, cleaned and greased. Leave it and the tabbed washer will fail, maybe even the worm drive and gear itself. The drive in my ESD has had the tabs brazed back on at some stage. And this spare drive has quite a bit of wear on the inner tabs where it fits into the gear.
Whatever happens, it would be good to take a look inside and see what can be fixed. With a centre pop first, and then very slowly with the drill I had enough room for a small punch to tap on the back of the seal.
The seal is steel-backed so no damage when hit, and it came out easily.
Dirty, dry grease wasn't really helping matters. After a thorough degreasing it was easier to see what damage/wear there was. The dried out grease looks full of dirt and rust. Not ideal conditions.
In the end, I tapped it with a small screwdriver on the bottom of the gear itself. Sorted, although what was left of the thrust washer took a bruising.
These parts are never going to be perfect; with a good clean we can at least see what we're working with. Note the inner tabs of the drive plate are quite worn (but still working). The thrust washer on the bottom of the worm drive is worn away to nothing, so I've found a thicker one that will do the job.
I've smoothed down the worm drive holder to give a little more clearance for the thicker washer. The end was heavily grooved/worn from years of the worm gear turning against it.
I also took a little off the outer of the housing so it wasn't soo tight going into the main body of the speedo drive, making it easier to reassemble and test it.
Grease first and then the first thrust washer.
Worm gear goes in next, with both thrust washers in place, and then more grease. The drive gear goes in after and, again, more grease.
Seal refitted. Gentle taps around the outer edge with a small punch and it seated easily.
Then the drive plate, washer and circlip
Turn it by hand a few times to get the grease in and out of where it needs to be and you should find it much easier and smoother. It'll never be like a new one again, after years of running dry, but might keep it going for a few more years. And that's what it's all about.
All that's left to do is apply a small dab of silicone to the hole we drilled to get the seal out and put it back on the bike.