YZ125 1979 Race build

I'm going to try to upload the pics in order for ya. It was a pretty easy mod.

Before Top

Before Chamber

After welding. Stud holes drilled and plug hole tapped

Machined Chamber (see porosity)

Top Machined

Welded up the porosity and then cleaned up the welds. No pic of that though....
Also cut about an inch off each side so it wouldn't stick out like wings.

Thanks for the compliments! I'm only into it for about $150. Comparing the astronomical prices of DG, FMF and Webco heads, this was a no-brainer!

See the two flats I milled on the under side of the fins? That's how I got a surface to reference. Just went from there.... Lotta math went into it as well. I can tell you I targeted about 20 M/S MSV at 10500 RPM and 12:1 UCCR IIRC. Runs bitchen on 93 pump gas and 32:1 Blendzall.

Gotta start somewhere... It's a casting after all!

I never measured the cranked compression. I assume it would be about 160psi. The math says it should be 183, but that's bogus. My UCCR is actually 13.5:1.

After milling the bottom lands, I flipped the head over and milled the Spark plug hole and stud holes and spot faced them. I've got a 1" Mandrel which screws into the spark plug thread so I could create the combustion chamber on the lathe. I'm using a tool room lathe with no CNC, so getting the form was a trick and a half.

To do your own, there's a lot of math and thought that goes into it. PM me on here and maybe i can help out.

A little teaser Pic from last summer. I fit a 2001 YZ250 ignition, complete. Same taper, same Backing Plate size. I think I had to create new bolt hole (slots) though... Much more mid and top end, plus the low end is easier to handle in the New England rocks roots and mud with the external flywheel.

Two reasons: 1st, it took a bunch of research to figure out which tapers and backing plates matched up. The 250 did, but the 125 didn't in most cases. I wanted a modern ignition so I could use a HPI programmable ignition box. I haven't gotten that far yet though...
2nd reason is that the internal flywheel was hell in the New England Woods! A little more weight made the bike much easier to handle. Even on an MX track it comes out of corners nicely instead of wildly stepping out.

Hi Andrew, I don't have the measurements on hand. The shock is way inside the frame, so I'm not sure I could measure it without taking it apart, but I'll take a look next time I'm in the shop.

Yep, definitely missing. Compare to the pic on page 1.