Author Topic: YZ125 56 mm piston?  (Read 221 times)

Offline YZed250

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YZ125 56 mm piston?
« on: October 23, 2017, 10:08:00 AM »
I have a 2005 YZ125 basket case motor that I'm building back to working condition.

Who sells a 56 mm (+2 mm big bore) piston? I know of Vertex, so far.

I'm thinking of using OEM main bearings and seals, ProX rod kit pressed together by Q&E. Cylinder will be an OEM YZ125 cylinder bored+2mm and plated by Millenium (Tom Morgan) or Eric Gorr or Powerseal.

Open to suggestions on this build. Not looking to build a fire breathing pump-gas only LL qualifier; just a YZ125/134/144 with a healthy and reliable engine for occasional vet mx and single track trail ride.

« Last Edit: October 23, 2017, 10:32:03 AM by YZed250 »

Offline SLAPAHO

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« Last Edit: October 23, 2017, 11:01:45 AM by SLAPAHO »

Online moto314

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Re: YZ125 56 mm piston?
« Reply #2 on: October 24, 2017, 06:43:55 AM »
Go 57mm bore (139cc), the 56mm feels more torquey down low but the 57mm make the engine feel so much faster everywhere!  The 144 kit feels super torquey with no top end and vibrates, the 139 feels much faster than the 144, except at very low RPM.  Don't forget you need to machine the power valves and head for the larger bore.  Millennium is good but I'd go with the best:
http://www.maxrpms.com/yz139-big-bore/
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Offline YZed250

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Re: YZ125 56 mm piston?
« Reply #3 on: October 24, 2017, 10:17:13 AM »
Go 57mm bore (139cc), the 56mm feels more torquey down low but the 57mm make the engine feel so much faster everywhere!  The 144 kit feels super torquey with no top end and vibrates, the 139 feels much faster than the 144, except at very low RPM.  Don't forget you need to machine the power valves and head for the larger bore.  Millennium is good but I'd go with the best:
http://www.maxrpms.com/yz139-big-bore/
Thanks, you are correct. YZ125=54 mm, Athena YZ144 is 58 mm. Revvy 2-ring YZ139 sounds awesome! I'm still looking for a Yamaha cylinder, so depending on the condition of the plating and the bore size that I get, I may build it as 54 mm or, possibly punch is out to 56 mm or even max pwr 57 mm twin ring.

Offline YZed250

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Re: YZ125 56 mm piston?
« Reply #4 on: October 24, 2017, 02:51:38 PM »
Pro-x and Wiseco both make a 56

https://www.lasleeve.com/pistons-and-components/pro-x-piston-kit-56mm-7210d

https://www.ebay.com/itm/Wiseco-Piston-56MM-Prolite-For-Yamaha-YZ125-YZ-125-05-11-/191339529094?_trksid=p2385738.m2548.l4275
Seems like piston weight (or lack thereof) and port timing have a lot to do with a 56mm advantage over the 58mm bore. It would be great to have all the pistons side by side for a weight comparison.

Offline YZed250

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Re: YZ125 56 mm piston?
« Reply #5 on: November 14, 2017, 08:32:27 AM »
YZ144 build is going down this week. Q&E pressed a Wossner rod into the webs a couple of weeks ago for me. Eric Gorr ported cylinder, head and 58mm Wiseco piston are in the mail on their way to my house.

Offline Gator

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Re: YZ125 56 mm piston?
« Reply #6 on: November 21, 2017, 08:15:09 PM »
If it vibrates you can have the crank weighted next time you do a piston.  I think it is Crank works in Arizona.  Did my  cr125 and it was smooth as stock afterwards. 

Offline Shamus

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Re: YZ125 56 mm piston?
« Reply #7 on: Today at 06:05:40 PM »
Id Talk to Varner about it.
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