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Junglejeff

TY250A piston to wall clearance?

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I just received a wossner oversize piston for my project bike.It does not have info on recommended clearance.Having no luck searching. If anyone can help me out here I would appreciate it.1974 TY250.

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If you are using a good and reliable engineering/machine shop to rebore your barrel, they should have a pretty good idea on clearances. 

You didn’t look very hard either, TY Trials have all the details on their web site for Woosner oversize pistons.🙂

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6 hours ago, section swept said:

If you are using a good and reliable engineering/machine shop to rebore your barrel, they should have a pretty good idea on clearances. 

You didn’t look very hard either, TY Trials have all the details on their web site for Woosner oversize pistons.🙂

I checked there and no info on fitting I can find.My guess is .0003" but not sure.Will send them a message and see what they say.Unreal such a high quality piston does not come with any fitting specs.

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8 hours ago, tony27 said:

https://www.tytrials.co.uk/trailandtrialsuk/prod_2768676-812-Full-Piston-Kit-TY250-Twinshock.html

If you look towards the bottom of the page you'll see a recommendation of 0.002" or 0.05mm clearance

Daa,I did not look down there.Thanks Tony.Bringing crank and jug in after holidays to have reworked. 

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On 12/25/2018 at 5:36 PM, Junglejeff said:

Daa,I did not look down there.Thanks Tony.Bringing crank and jug in after holidays to have reworked. 

Cant beat studying closely👍

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3 hours ago, section swept said:

Cant beat studying closely👍

Sent crank and jug to stealth performance yesterday.I talked to him about the clearance and he said with the wossner it should be good but be careful on break in.Said if it was a wisco he would add a .001" because they use cheap aluminum. He builds crazy stuff and race motors for the snowmobile and high performance boats.

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Wiseco is often referred to as seizeco for good reason, can't say I had any problems running 1 in my YZ465 though.

General riding on a trials bike would be considered as a careful break in by most people, I rode some tree root sections that didn't require lots of revs for about an hour to run my 250C in earlier this year when I fully rebuilt the engine

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5 hours ago, Junglejeff said:

Sent crank and jug to stealth performance yesterday.I talked to him about the clearance and he said with the wossner it should be good but be careful on break in.Said if it was a wisco he would add a .001" because they use cheap aluminum. He builds crazy stuff and race motors for the snowmobile and high performance boats.

Wiseco pistons have a higher coefficient of thermal expansion than Wossner pistons, so need more piston/bore clearance (when measured at room temperature) than a Wossner. In theory the clearance is the same once the Wiseco is up to operating temperature.

Running in a rebore is getting the bore and rings to bed in against each other so the contact surfaces can form a low-friction gas seal. While the running-in is happening, there is a lot of extra heat being generated by the friction of the rings against the fresh bore which can (amongst other things) cause the piston to get hotter than it should, which can cause the piston to get too big which can cause it to nip up.

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6 hours ago, feetupfun said:

Wiseco pistons have a higher coefficient of thermal expansion than Wossner pistons, so need more piston/bore clearance (when measured at room temperature) than a Wossner. In theory the clearance is the same once the Wiseco is up to operating temperature.

Running in a rebore is getting the bore and rings to bed in against each other so the contact surfaces can form a low-friction gas seal. While the running-in is happening, there is a lot of extra heat being generated by the friction of the rings against the fresh bore which can (amongst other things) cause the piston to get hotter than it should, which can cause the piston to get too big which can cause it to nip up.

We have a lot of issues here in michigan with them mainly because of weather.The main problem is the engines don't get up to temp and the pistons never quit slapping.It is terrible if you use them on jet ski,s and boats.Air cooled motors like this are a huge problem for the wisco in our conditions. I prefer to use better pistons such as vertex or wossner. The wossner was actually cheaper than the wisco for this bike.I will wait till spring to run bike in because not sure I can get a proper break in winter.Most likly will do the 5 minute run in cool down method. 

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