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dannyboi

Ty 250 R mono cylinder bore

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Hello, I just bought a standard piston kit for my ty 250 88, but after trying to fit it I realised it was too big

the piston is 70mm, but the bore on my cylinder is 67mm

is there anywhere that sells a 67mm piston? I can’t seem to find it anywhere 

 

thanks, Dan

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Sure you don`t need a 67.25 or a 67.5. Chances are a thirty year old bike could need a bore. By the way a YTZ250 piston will work.

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6 hours ago, dannyboi said:

Hello, I just bought a standard piston kit for my ty 250 88, but after trying to fit it I realised it was too big

the piston is 70mm, but the bore on my cylinder is 67mm

is there anywhere that sells a 67mm piston? I can’t seem to find it anywhere 

 

thanks, Dan

Try Wiseco or Wosserman  Give them your original part number for your original piston.  I went thru that sourcing issue with my TY  250A and finally got one that works from Wiseco.  I can get you the part number if you need it.

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7 hours ago, Tillerman6 said:

Try Wiseco or Wosserman  Give them your original part number for your original piston.  I went thru that sourcing issue with my TY  250A and finally got one that works from Wiseco.  I can get you the part number if you need it.

I would really appreciate that 👍🏻

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7 hours ago, lineaway said:

Sure you don`t need a 67.25 or a 67.5. Chances are a thirty year old bike could need a bore. By the way a YTZ250 piston will work.

I was looking to get it bored out to 70mm, but they say they will need a good couple of goes to take the 3mm off so it’s looking around £120, I bought the piston kit off of wemoto for £35 hoping that would be it

a YTZ standard piston will fit my 67mm bore?

 

thanks

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I had a Shirt 270cc conversion done to mine, it transformed what was already a nice motor.

Can you still get that done ?

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On 4/16/2019 at 2:32 AM, dannyboi said:

I was looking to get it bored out to 70mm, but they say they will need a good couple of goes to take the 3mm off so it’s looking around £120, I bought the piston kit off of wemoto for £35 hoping that would be it

a YTZ standard piston will fit my 67mm bore?

 

thanks

std bore on the ytz250 is 68. 68.25  would be much better than going to 70mm. Leaving it at 68mm is probably not the best for performance or longevity.

Edited by lineaway

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You should have the piston before it is bored. This seems the easiest solution unless you can find a mono 250 piston 1st or second over. I have seen pistons, but no rings.

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I looked for a long time for an oversize TY mono piston in Canada and the US, I found a 3rd os with ring that if i remember cost me $215.00 US I still have it on the shelve even if I sold the bike. I think it's a relic.

Guy

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16 hours ago, dannyboi said:

I would really appreciate that 👍🏻

dannyboi, 

This took me about 6 weeks to finally find a new piston that would fit.  In my case the bore itself probably did not change that much, but most shops will want to have the cylinder to assess the damage if there is any and see how much oversize the piston will have to be to make it fit.  This could end up with a re-bore to slightly oversize or even a new sleeve, but in any case the shop you choose will have to make those decisions.  But anyway I got you the Wiseco part number  for the piston I am using now.  234M0700  the s/n is I-158656 if that helps. The original bore size was 2.7539" but there is a taper that they machine into the cylinder as a matter of course.   I would strongly recommend that you weigh all your old parts to the gram  to compare with the new ones before you start assembly again. Incidentallly, this same part number is the same one they use for the YZ250 76-79 and the IT 250 88-90   The skirt has a semicircle cut out of the carb side lower edge instead of having two holes in it like the original.  Also the rings are narrower than the original.  There will be detailed instructions that come with the new parts from Wiseco. Follow these to the letter. You will probably have to size the new rings into the empty cylinder when you get it back. Believe it or not, but the upper ring and the lower ring have different end gaps that they specify in the instructions.

     If you end up with the same part number would you do me a favor and weigh the new piston, rings, pin and circlips and let me know what you get for numbers?  I may want to eventually make the old weights match the new ones just for peace of mind. My original piston was somewhat scored up from a partial seizure, but it was the original diameter.  I figure that since the bike was a 74, no one could fault Yamaha for not building in lots of quality to make it last that long. I am still running the same con rod lower end bearing and con rod, flywheel and even the carb is holding up well.

 

Also you will need a new base gasket and probably you will want to anneal the old copper head gasket to make it re-usseable. The arrow edge goes forwards.

 

The studs for the head have long and short threaded portions. The long portion goes down when you put it back together.   I had to use the full torque on all the fasteners on the cylinder to keep it from leaking (No lock tite needed, but a little anti-seize compound on the threads is a help. You may also want to lap your head sealing surface again to get rid of any scratches on the sealing surface where it touches the copper gasket.

I got all my work done at Millenium Technologies mostly because that's their specialty and they do about 2000 re-bores a month on all types of engines.

They are not cheap, but they have excellent customer service and you will get a written estimate before they start work.

 

I ended up calling Wiseco to source the piston and top end bearing, wrist pin and circlips 1800 321 1364.  They should have all the oversizes in stock.  But before you order another piston and it turns out to be the wrong size, you might want to get your cylinder inspected and then ask them for their recommendation about what oversize you might need.  It saves a lot of headaches in the long run.

 

Good luck and let me know how it turns out!

Tillerman6

 

 

 

16 hours ago, dannyboi said:

I would really appreciate that 👍🏻

 

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16 hours ago, dannyboi said:

I was looking to get it bored out to 70mm, but they say they will need a good couple of goes to take the 3mm off so it’s looking around £120, I bought the piston kit off of wemoto for £35 hoping that would be it

a YTZ standard piston will fit my 67mm bore?

 

thanks

dannyboi, I think there is a typo in that message.  I never heard of a "YTZ" 250.   I HAVE heard of a YZ 250, and in the years 76-79 you might find a piston that will work for you.

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