rm 125 87 cylinder on a 86 ?

I was told that the 86-88 motors all have interchangeable parts. I am also under the impression that you need to sleeve the cylinder if you need to bore it over .050" as the skirt is too thin.

Good luck!

Negative.
The 86' engine was the old piston-port/case reed style setup, and the 87 was the first year of a full-reed cylinder.
The cases are different where it mates up to the cylinder.
All you have to do is look at the two base gaskets from each model to see the difference.

so i am doomed ?

Negative mxrose3. 89 was the first year of the case reed motor.

Your fine 86-88 cylinder works

so i can use the 87-88s ?
no ports / power valve issues ?

Crap I'm wrong. 87-88 same. 85-86 same

What's wrong with your 86 cylinder

the 86 have a crappy top end

and the worst power valve assembly ever

so i am searching for original solutions

I just rebuilt a 1986 rm 125 and it's not the same as a 87/88. I think the whole motor would bolt up in the frame though.

1985 no power valve
1986 is different power valve setup than 87-88.

1986 is a stand alone design only year.

MXVet261 wrote:

1985 no power valve
1986 is different power valve setup than 87-88.

1986 is a stand alone design only year.

Pit racer is right. However, there was a family that lived in Zanesville, Oh named Weaver (Mark was a wirery and pretty damn fast local, his dad Charlie was pretty good at working on motors too), and they blocked off the power valve actuating lever access and bolted a 1985 cylinder and head onto the '86 bottom end. Don't know how they did it, but it ran pretty good on top after that.

Are the 85 and 86 cylinder base gaskets the same?

SmokinJoe439 wrote:

Are the 85 and 86 cylinder base gaskets the same?

Not sure, but the cost of a gasket to compare.