feetupfun

There should be a circlip on the shifter shaft on the left side of the motor. The circlip bears against the shifter shaft oil seal and prevents the shifter shaft from moving towards the right. It may not be immediately visible because it may be concealed inside a plastic roller/wheel that protects the shifter shaft from the chain rubbing.

Maybe one of the gudgeon pin clips came loose and put a big groove in the bore and it needed a 2.0 mm oversize re-bore to clean up the bore so they could keep riding it?

0.015" is good. 0.15" is way too much.

Nice to see. I have one for the same reason you mentioned.

The OSET 24 is smaller than an adult size trials bike so is not as comfortable to ride for an adult-sized person as an adult-sized electric trials bike is. Electric Motion trials bikes and free-ride bikes are adult sized and are popular. Google "electric motion trials" or "EM trials"

When a frame becomes pushed out of shape by riding, like in those photos, I take the motor out to make it easy to push it back into shape.

I'm always interested in the way Bultaco changed things on some models without changing the model number. The model numbers that had the most changes are the most interesting to me. I have a fair handle on the changes through model 49 and through model 91/92 but not the others. I'm rebuilding a model 85 at the moment so am particularly interested in knowing at what production numbers the model 85 changes happened? I'm most interested in when the footpeg mounts became welded on, the swingarm pivot nuts were recessed into the frame, the steering security lock was fitted, the chain tensioner became rearward-facing and the chain oiler was incorporated. I only have two model 85s to refer to, one in the 2000s and one in the 5000s, so I'm hoping others can reveal what is on their model 85s. I've seen the sales brochure for model 85 Alpina and that bike has bolt-on footpeg mounts so I assume that bike was early in model 85. My model 85 in the 2000s has welded-on footpeg mounts and recessed swingarm pivot nuts, forward facing chain tensioner, no steering security lock and no chain oiler. My model 85 in the 5000s has welded-on footpeg mounts, rearward chain tensioner, steering security lock and chain oiler I figure that the footpeg mounts became welded-on type at the same time that the swingarm pivot nuts were recessed.

The tank/seat is from the first series of Alpinas (models 85/97/98/99). Being a model 97 (Alpina) frame it will have an identical frame and front end and wheels to the series 2 model 80 Sherpa T. The model 85 internal Alpina gear ratios will be a bit of a pain for trials use because you'll need a very large rear sprocket to get first gear slow enough.

Two stroke trials bikes ridden in trials type riding go well with a 5 or 6 heat range plug (NGK heat range numbering). If the two stroke trials bike is used for sustained/continuous riding, like along a road or at the beach, or in desert sand, or trail riding that involves long climbs, a 7 or 8 heat range plug will work better.

What point are you getting stuck at? Are you having trouble getting a good connection between the drift and the bearing inner? Is one bearing any easier to connect with than the other? If heating didn't help, then whatever they used is probably not loctite so may be an epoxy. If it is epoxy then the aluminium will probably melt before the epoxy burns. In the worst case you can remove bearings with grinding tools. Maybe post some photos up

While the one in you photo may well be a one-piece forging, if you were to make one starting with a standard lever, the bush at the top end could be welded on.

Yes you have the dish the correct way. Yes 13/55 is a good choice for competition riding.

The type with the sliding bar shown by MikeS300 could be easily made by modifying a standard kickstart lever, but it doesn't move the top end of the lever out of the way of your shin any better than the standard lever. I feel that a lever that swivels at the bottom (like a Beta lever) would provide better leg clearance.

27 mm x 1 mm left hand

I usually keep my old trials boots on hand to lend to prospective trials riders but if they can't borrow something their size, I suggest they wear ankle-supporting industrial leather work boots. Work boots are way better than any non-trials motorbike boots for trials. Proper trials boots are better though, mainly because the heel on work boots sometimes gets in the way You probably should say where you live if you want to borrow boots

When you say there is a lot of clutch arm travel before it starts to work, that is normal for motors with a rotating cam clutch mechanism like yours. The important thing about pushrod length is that the angle between the clutch arm and the slave cylinder is about 90 degrees at the point when the pushrod starts pushing against the pressure plate. The SY250 does have a falrly easily-bent clutch arm. The original design on that motor when it was in the TY250Z was for a cable clutch. The hydraulic clutch setup was designed for when Yamaha started selling the motors to Scorpa for the SY250. The location of the exhaust chamber on the SY250 dictated a compromise shape for the clutch lever. To answer your pushrod question, most likely the pushrod is the same as what's in an early 1990s YZ250, because that's what the TY250Z motor was developed from. You could check this using part number cross-referencing.

Aftermarket reed blocks are/were not a common mod on the twinshock TY250 motor. Many people fit aftermarket reeds to the standard reed block. Standard TY250 reed blocks are readily available second-hand. Sounds a bit like it might have a reed block spacer. That would move the carby rearwards

It depends on how hard the motor and brakes are being pushed on the Sherco. Cruising on flat land should be OK to ride continuously but if you are racing around then it may overheat the brakes and/or the motor. If the fan cycles on and off while riding then it's not being overheated.

Yes they came with the usual set of Yamaha branded tools in a plastic pouch. Japanese Road/Trail bikes also came with a similar basic tool set

Yes rider weight is carried by the frame rail contact zones and the steel mounts are only to hold the seat in position

You might be able to swap with someone who wants an A model flywheel. It's a popular mod for the later model TY250s for people who compete on them

The pre-Honda front wheel has the rim centred on the hub spoke flanges

The steel ring on the BCDE flywheel is 27.65 mm wide and 8.6 mm thick. Other dimensions are the same as the A model flywheel

I've seen them advertised on US eBay at times

I came across suitable photos tonight so here they are. The flywheel on the left is TY250A. The flywheel on the right is TY250 BCDE