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Cylinder head and cylinder. The head volume was increased by 2.1cc from 21.5cc to 23.6cc. This added volume reduced the compression ratio from 8.9:1 to 7.9:1. The exhaust port height was raised 0.5mm, and the power valve shape was updated to match the exhaust port height.
And does the profile of the dome make much difference or is it just volume? This is the first 250t I've had since my 97 YZ. Seems like the head on that was a lot smoother whereas the 03 is more of a deep hole in the middle. I've forgotten everything I ever knew about 2 strokes in the last 18 1/2 years years of tractors.
While we're on the subject of heads, I had a flashback of a head with craters in it like a golf ball. Anyone remember that?
As for the volume, I use a piece of transparent lexan, acrylic or plexi-glass with two holes drilled in it. One hole is about 1/8" diameter and allows the air to escape. The other hole is sized to snugly fit on the tip of a plastic 50cc syringe. Smear a little grease around the sealing surface of the head, then press the lexan or plexi-glass on there. I just pump water in until the air is all gone.
Absolutely cut the gasket surface down and set the squish clearance. You really gain a lot on these YZs because the clearance is so wide on the stock bikes. They run so much cleaner when you get the squish down below .050". Jetting is easier, the bike runs crisp and there is less chance for detonation.
That lathe is fine, as long as you have enough swing to clear the head-stay on the casting. Cut the gasket surface as square and smooth as you can, then use some sand-paper to lap it smooth. Start with 220 or 320 and work your way to 600. Lapping is done in a figure-eight pattern. I like to stop, pick up the head and rotate it a little every couple of strokes so I don't burn down one area too fast. You don't want to push down, the weight of the head is enough. A flat piece of glass (mirror or a picture frame) is flat enough to place your sand-paper on. If your wife has nice granite counter-tops, wait until she's gone and use that, they are typically very flat.
Don't worry, it will seal if you take your time.
You want to keep that cone shaped band around the outside perimeter. You can reduce the width of it, but you want that squish band to follow the shape of the piston. Often times the squish clearance tapers from a tight clearance at the piston crown to a slightly wider clearance as you move in toward the center.
These YZ250 heads have a prominent "corner" between the squish band and the dome. If you have a compound cross-slide on your lathe, you can use it to roughly split the difference of the angle between the sqush band and the dome. Effectively you're making the dome a little larger and the squish-band a little narrower.
If you set the squish clearance first, you can CC the head and then determine how much larger it needs to be. Then set up your compound cross-slide and cut off a few mm of material. CC it again and if necessary take off a little more.
I haven't read anything on the 250X, interesting to know those numbers, thanks.
Here's a photo of the stock YZ250 head from a 2003 - 2010 US model (20.5cc) and a head which is cut in the way I described above to increase volume.
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I've seen and heard of several Wiseco rods shattering in these bikes and busting the cases.
2002 YZ250
Motor mods :
-Yz250x cylinder head, power valve spring and cdi
-rekluse core exp 2.0
-jd jet kit
-fmf gnarly pipe and stock 2011+ silencer
-steahly 9 oz flywheel weight
-no toil filter
-wiseco piston and hot rods crank
Suspension:
-stock Forks and shock revalved by Total Control for gp/woods/light moto
-applied top clamp so I can run fat bars
Body/controls:
-cr style front brake line with light speed plastic fork guards
-2016 Yz250 plastic, airbox, seat and pegs
-flexx handlebars (10°) with odi emig grips and cycra handguards
-ufo frame guards
-2004 Yz250 master cylinder to get rid of the separate reservoir
-tmdesigns rear chain guide
-ims 3.2 gallon tank
-18 inch rear wheel
-sunstar o-ring chain
-devol skid plate
Overall I love the bike and really don't have any issues with it. With some clicker adjustments the forks are just as good as the sss stuff on my 2016, you just have to adjust clickers more based on track/trail conditions where on the sss you just get a good setting and ride the piss out of it.....anywhere. I do believe the steel frame gets better traction on the dry rough stuff compared to the newer aluminum stuff.
And can someone move this to bike build section?
Happiness is buying the blown up 450f for 5 bills, selling the parts off it and funding good stuff for my 03, and basicly free sss suspension!
Pit Row
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