Those of you with long memories may remember that while the gearbox seemed ok before opening it up and having a proper looksee both 6th gear pinions were well mangled. I have no idea how they came to be like that and it was weird because all the other components including the bearings looked fine. We looked at buying new pinions, several places stock them but they come out at about £130 the pair. Super sleuth Ian got on the job and managed to haggle for a full gearbox that he purchased at an obscenely low price. It arrived at the Boston Bike Bits combined workshop / torture chamber in fine fettle and was checked against the old parts to see which were best. I have transplanted in most of the new parts but used our original bearings, which just felt a wee bit better. At the moment I have just put it together loosely to check all the gears select and feel smooth, all of the dogs have been checked for squareness.
For those not familiar with bike gear boxes most of the gears have dogs on them – square bits cast in to the gear that can engage with other gears on the same shaft. There are usually two shafts of gears that engage with each other. Usually 3 of these gears have grooves in them that the selector forks engage with. The selector drum has a complex series of grooves that when rotated cause the selector forks to move left and right along the two shafts – usually referred to as input and output. As this happens the different sized gears engage with each other to give the different ratios. It’s all terribly clever when it works but as it’s very finely engineered very slight wear in crucial places can cause all sorts of issues. Here is a photo of a selector drum with a significant chip out of the track for one of the selector forks – this wouldn’t stay in gear as the dogs moved far enough to disengage. It’s not out of the RD but it illustrates one of the things you should be looking out for when rebuilding a gearbox.
Selector forks can get bent or worn making gear selection difficult and dogs can wear so they are rounded causing the bike to jump out of gear. A worn selector drum can also make gear changing difficult, it’s not uncommon to see wear in the carefully machined tracks that determine the position of the selector forks.
Anyway I picked all the best looking bits and then put it together to make sure it was all good – it was. I will now have to wait for the crank etc to come back before I can rebuild the rest of the engine.
In the mean time I decided to Take it all apart to give everything another clean. Bad move, space cadet, another fuck up happened. Way back in part 2 or 3 I had a bolt I couldn’t get out – it was one of the JIS headed screws that holds on the input bearing retainer plate. I wasn’t really too worried as I thought I would deal with it later. Sadly when I had the crank case upside down I rather clumsily managed to break it. Quite how I have no idea – I thought it was mild steel but it was hardened and I think must have had a fault. Anyway whatever it was it resulted in some of the best swearing you have ever heard.
Of course nobody seems to have one, its not the sort of thing that wears out and the new box didn’t come with one so I will have to make a new one, being a generous sort of person I will take you through the process.
Life didn’t get any better for me when I decided to tackle the rust on the rear of the frame. Although this is an oily rag resto I wasn’t comfortable about leaving rust to do the Devil’s work in areas of critical strength so I set about doing a bit of cleaning up. You may recall from way back when that when I went through the electrics I found a few issues so as I was working at the back of the bike it seemed like a good time to have a prod around. Somebody had done some pretty dodgy repairs in the past and had re-soldered some wires then wrapped the joint with insulating tape. Now there is a very good reason why NASA don’t use similar techniques on their space vehicles – it doesn’t last long as a fix. Indeed the indicator was shorting to earth through this temporary fix that looked like it had been applied about 20 years ago. I took the whole thing off so I could get at it properly and decided to repaint while I was at it. Wish I hadn’t bothered because the sodding paint reacted. I have given myself a whole new job to do now rubbing it all back, applying isolator and trying again.
Things were not good in this area, the wire was creating an intermittent short and the rust and muck needed dealing with.
For now though it’s back too the engine and the broken bearing retainer, I had to make a new one. I found a piece of steel the right thickness and super glued the broken pieces to it. I then drew around the two pieces with a marker pen and marked the hole positions. The two holes were drilled and the part roughed out on the bandsaw before being given a final fit with a couple of hand files – one flat and one half round. It will get cleaned up and then will be case hardened. Probably doesn’t need it but as the original was hardened so will the replacement be. I will make absolutely sure it fits first as adjusting it after hardening won’t be so easy. For those of you that don’t know case hardening is done by heating the part up until it glows yellow then quickly quenching it in used engine oil. The carbon in the oil does some chemical stuff and the rapid cooling does some other chemical stuff and it ends up being hard. Test it with a file – if the file skids off the surface it’s hard, if not do it again – properly this time.
Never rely on just the superglue if using an old part as a template, if it gets hot while working it the bond will break and you end up starting again. If you draw round it and it separates during the process at least you still have a guide to work to.
Here’s the old and the new.
The good news is that all the newly machined engine parts are back!! Both cylinders are now the same size at 66.00 mm giving us a new capacity of 369.something CC. It all looks good, we have new pistons, rings, gudgeon pins and the all important new circlips so we don’t get a repeat of what happened to the last unfortunate owner. I have also got a complete gasket set and a complete seal set – everything will be replaced with new. There is no point going this far and reusing anything that may have wear or be aged, it’s a massively false economy.
So now, I guess, it’s time to start putting it all back together again. Tea brake first though, I need biscuits. Seeya next time – Dave.
