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15
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8/16/2016
Location
Pittsburgh, PA
US
Hi everyone! I just picked up a rm125 from a friend of mine. The bottom end was bad, but he gave me a build motor from the owner that he got the bike off.
I started to assemble the motor, it only need the wiring put in, the covers, and the head assembled. The motor has a big bore kit and thats about all that i know. So i put the head on with the bored piston and everything looks fine. Go to put the cap on and the piston is hitting it.
Is it possible the engine is a stroker? If so how do i make the cap fit? Bigger base gasket? Bigger head gasket? Or machine the head down?
Thanks
I started to assemble the motor, it only need the wiring put in, the covers, and the head assembled. The motor has a big bore kit and thats about all that i know. So i put the head on with the bored piston and everything looks fine. Go to put the cap on and the piston is hitting it.
Is it possible the engine is a stroker? If so how do i make the cap fit? Bigger base gasket? Bigger head gasket? Or machine the head down?
Thanks
Chances are the rod is 2mm longer, as in 2004-06 RM125s, (and not a stroker). The rod length should be 108mm and the 2004 RM125 went to 110mm. I learned this when I rebuilt a 2003 RM125 a while back. I had the same issue and replaced the crank with the correct one.
You have a few options:
Call Wiseco or Athena to ask if any other piston has the pin hole 2mm higher (maybe a YZ125 piston for example???)
Use a shim under the cylinder and extend the powervalve rod. Cometic Gasket will make you a shim for cheap.
Or, use a set of 2004 cases. The crankshaft position is 2mm lower in the cases.
Good luck! Let us know what happens.
-Ryan
The Shop
When you get time, maybe you can find numbers on the rod to see if you can reference it.
And if it's a Wiseco crankshaft, you will need to replace it anyway. They last 2 rides.
Something is way off. The piston should be 0.5mm lower than the top of the cylinder at tdc.
You might have a wrong piston, the cylinder base may have been machined off (2-3mm too much) or it might have the wrong rod.
Years ago, people running strokers used a 2mm shorter rod from an older model.
Sorry, but it's a mess and will need some measurements and new parts to put right
Tusk Racing makes an affordable case splitter tool and a crank puller/installer tool. Those would help immensely. pitposse.com or ebay.com
Moose Racing makes complete gasket and seal kits for the entire engine- about $70.
I think you need Yamabond for the cases and maybe some loctite and an air impact wrench. (hopefully you have an air compressor)
And of course your regular hand tools- maybe an impact screw driver to remove the stator plate. harborfreight.com
Good luck!
Am i getting to carried away or might this work? Id hate to waste my time any longer on it but i do still have the 01 engine for backup if this would go completely wrong lol
I would measure cylinder height before assuming the rodds is wrong
I don't think we're missing anything. Maybe someone else can chime in.
If the .0625" shim you have is a pain to cut, keep in mind, Cometic makes the shim for like $25. And I think you would have it within like 3-4 days.
-Ryan
I don't think the head stays would be too much trouble. Might need to file a couple holes. I think lengthening the powervalve rod will be the trickiest part.
Pit Row
The rod is probably close enough to copy. The rod needs to plug in the powervalve arm when in the closed position.. you will need to determine which direction is closed. I don't remember. I suggest using heat to bend the new rod.
You should adjust the fuel screw in and out while blipping the throttle off idle. Find the most responsive. If you go out more than 3 turns, you can go to a leaner pilot. If you go in more than one turn, you need a richer pilot. (Turning in the fuel screw in enrichens the mixer)
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