Trials Australia Forums

I think our rattly ol' Sammy Miller ('76 TY175B) deserves a new piston and rebore.
Anyone have any recommendations? A 'Yambits' one is about 30 quid, but it has to come from England.

Anyone know anything about 'Forseti' pistons?

Next...
Q#1 Whats the max OS bore size for a TY175 B.
Q#2 Has anyone got a spare barrel with a couple of bores left in it!?

I can remember a lot of people were recommending the Wosner piston for the TYs a couple of years ago. I know Wiseco goes up to a 2mm oversize for Bultacos. Contact David Lahey as he is the TY Guru on this site. Graham.

Oooh the pistons via Serco are a bit pricey. Forsetis are less than half the price.
Actually I think the barrel is just on first oversize, with wear, so we probably won't need another barrel. I'm encouraging my grandson to do this job, but advising etc via email isn't the easiest re getting accurate bore wear measurements. (He's locked down so I can't visit). Hoping we'll get away with +0.5. (2nd OS).

The Wossner is the pick for the TY175 and you should be able to get a Wossner piston kit from an Australian supplier for about $AU200.
Wiseco is good quality too but made for being worked hard rather than for trials use.
If you do go for a cheapy piston kit, please let me know how it went because I've not known of anyone using one yet.

Has anyone used one of the Forseti brand pistons in their TY?

We now have a new Forseti piston kit. The first thing that's obvious is that the transfer ports in the piston are larger than the original Yamaha piston, -Assuming of course that our original piston is the original Yamaha piston.
Old piston markings are: ART 1/3 45502 6625
Old piston transfer ports are 14.7mm high, lower edge 9.5 from piston base
Forseti piston ports are 22.3 high, lower edge 11.5mm from piston base -rings are a couple of mm lower positioned on the Forseti.

Possibly for a DT? any ideas?

Jools wrote:We now have a new Forseti piston kit. The first thing that's obvious is that the transfer ports in the piston are larger than the original Yamaha piston, -Assuming of course that our original piston is the original Yamaha piston.
Old piston markings are: ART 1/3 45502 6625
Old piston transfer ports are 14.7mm high, lower edge 9.5 from piston base
Forseti piston ports are 22.3 high, lower edge 11.5mm from piston base -rings are a couple of mm lower positioned on the Forseti.

Possibly for a DT? any ideas?

Your old piston is a Yamaha DT175 (which is also called MX175 in some countries) piston. The 455 prefix on the part number is the clue there. The 6625 marking is the diameter (66.25mm). Standard TY175 bore is 66mm.
The shape, size and location of the window holes in the rear of the piston are not of any great significance, but if you want to make sure you are getting the most bang for the buck, you can lower the bottom edges of the windows to match that DT175 piston.
Do the windows line up with the rear transfer ports?
The rings being a bit lower is not important.
The height of the rear skirt relative to the gudgeon pin (on a TY175 piston) is not important because the inlet timing is free (managed by the reeds).
The locations of the ring pins are important because the ring ends should not pass over a port once assembled.
The height of the edge of the crown above the gudgeon pin is important because that controls the exhaust timing and the clearance to the head.
The shape of the crown is important because the distance between the crown and the head is important for the combustion process.