2 questions

[Guy53]

As anybody tried those 72mm piston availbe for the TY ts ? It bring the motor to 272cc , are they worth the trouble since it's the last o.s. avalable. Second, transmission gear ratio, I am ok with the first 3 on my TY but when I it the 4th, specialy in the wood, BANG I am in trouble. I run 12 front and 55 rear sprocket. As someone found a way around this problem, maybe change the gear in the transmission ?

Guy53

[JC1]

Guy, 72mm only takes it to 260.6cc

Almost all 5sp trials models have a big jump to 4th & another big jump to 5th. Not a lot you can do about it. 4th gear from one of the trail bikes may fit, but it was typically 1:1 whereas the TY is 1.038:1, so it would be a bigger jump to 4th gear (with a slightly smaller jump to 5th gear). Alternately, if you're just doing trail riding you could swap the whole gearbox for the trail bike one (whichever one fits - I don't recall now), but it has a big jump from 1st to 2nd which is not very suitable for trials

[Guy53]

I don't know why I had in mind 68mm of stroke instead of the 64mm real stroke ( I should have cheked my notes ) . As I wrote in my questions, I realy like the spacing of the 3 first gear and I don't want to mess that combination, but I am sure that it would be possible to find not necessarily the 4th in another transmission, maybe a 3rd that would fit the shaft. I don't realy care about a 55 to 60mph top speed as I don't ride trail, but sometine, I would enjoy a closer 4th.

Guy

[JC1]

Now that I think of it some more, I seem to recall that in some countries including Oz if I recall correctly, the early TYs (A & B?) had a slightly different 4th gear (1:1) to the later ones (1.038:1). If yours has the early 4th gear a change to the later one would close the gap slightly. But I think the US market models all had 1.038. David Lahey will probably know for sure but I think he may be away on hols at present.

[Guy53]

Thank you JC1 I have an A and a B model here I will check the 4th gear in each transmission as soon as I can go in the shed to find the parts ( When the weather change: here at the moment it's minus 20 celcius ).

Guy

[JC1]

Hec that's cold!! Almost inconceivable to us as the coldest I've experienced is minus 5 deg C.

The 1.038 box uses 26T gear on the input shaft & 27T on the output.

[David Lahey]

I've never counted gear teeth in TY250 gearboxes, but the gaps in the upper gears on the TY250A seem wider than on the later model motors. It only really matters if you are in a hurry while trail riding and some terrain has the motor either revving too high or too low, and while they have a wide spread of usable power, they are not the most powerful trials motor at low revs