They are the same basic layout as every other motor of the era but there are features that some people haven't come across before. Here are some things that spring to mind:
The conrod is held centred on the crankpin by spacers on the gudgeon pin.
The primary drive crankhaft gear is mounted on a taper which needs a special puller to remove without damaging the teeth.
The cylinder is held on with 7mm internal hex nuts.
The positioning (and side play) of the crankshaft is set using shims behind the crank seal carriers.
The kickstart travel stop is a weak design and very commonly broken which allows dust and water into the magneto casing.
Take note of how the shift mechanism is set up before you pull it apart.
Yes the side play and positioning of the gearbox shafts is important but if you use the same shims in the same spots and use the same thickness centre gasket 0.5mm it will probably be fine.
The ideal footpeg position depends on a lot of things and you won't be able to know for sure until you have test ridden it so yes bolt-on adjustable position pegs are the go for an experimental bike like yours.
For a starting point, I'd mount the pegs so that the middle of the peg platform is in line with the front end of the tyre tread with the axle at mid slot.
Generally a trials swingarm should be as long as possible yet still achieve the wheelbase that you want.
For a bike with something like your B250 engine (an engine that is long from front to back) you will probably end up with as short a swingarm as will fit an 18" trials tyre and the wheelbase will still be on the long side of ideal.
Thanks for a quick reply but I am looking for help setting up the steering stops and would like to know how many degrees to allow the handlebars to rotate. Thanks.
90 degrees is an awesome design target. If you've got it, flaunt it. I'd love to see a photo.
Maximum fork diameter of 38mm means fiddle forks are allowed, what diameter were the last of the Fantic & spanish twinshock forks?
My TY250 works really well with all the parts majesty yamaha sells for the forks now that I have had the top of the sliders bushed to get rid of the wear, rear suspension is laid forward to majesty specs & running a set of rebuilt Fox-shox trials shocks, swingarm has also been lengthened 1 1/2". Quite an enjoyable bike to ride & still looks period
I'm still working and may not even get any time off due to working at a power station. Plenty of things I'm ready to do in the workshop, mostly involving 1970s trials bikes
I had an aluminium tank that had developed a line of corrosion holes in the bottom of one side which were in conjunction with the gum that forms when modern fuel is allowed to evaporate in-situ. The corrosion holes were under the gum. I suspect that there may have been some water also present during the hole-forming process but by the time I got the tank it was dry and just had the gum and the holes.
If you don't allow things like that to happen to the tank, aluminium is one of the best (longest-lasting and lightweight) materials for a fuel tank. I've seen zero corrosion damage on any other aluminium trials bike fuel tank.
What is the problem with the bush going right to the end?
One thing I do know is that I couldn't source the right size bushes for a M85 sleeve gear last year so I bought bushing material with a smaller ID and bored it to size once it was in place in the sleeve gear. Haven't tried the motor yet.
I've always wondered why some 5 speed motors have needle roller bearings there and some have sleeves.
Has the 90 degree pipe been changed from standard pipe , ie bigger diameter . I can see how its shortened as you have described
Yes the top bend is made from a pre-formed exhaust pipe bend. They come in various diameters and one size is smaller than TY250 and the next size up is slightly bigger than TY250
The diameter is as close to standard as you can get when using pre-formed exhaust pipe bends. The vertical run has been shortened from standard to get the pipe closer to the head to provide clearance for the underside of the fuel tank.
Help needed
in Montesa
Posted
They are the same basic layout as every other motor of the era but there are features that some people haven't come across before. Here are some things that spring to mind:
The conrod is held centred on the crankpin by spacers on the gudgeon pin.
The primary drive crankhaft gear is mounted on a taper which needs a special puller to remove without damaging the teeth.
The cylinder is held on with 7mm internal hex nuts.
The positioning (and side play) of the crankshaft is set using shims behind the crank seal carriers.
The kickstart travel stop is a weak design and very commonly broken which allows dust and water into the magneto casing.
Take note of how the shift mechanism is set up before you pull it apart.
Yes the side play and positioning of the gearbox shafts is important but if you use the same shims in the same spots and use the same thickness centre gasket 0.5mm it will probably be fine.
Pistons are very expensive.