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So I know that throughout the years or the yz’s they have changed he heads multiple times. So I’m just curious if there is a stock yz 250 head that will fit on my 13, that will give me a little more bottom end?
You are better off sending a head to a good engine builder to have it machined for tighter clearance.
If you run race fuel, you can bump the compression a bit beyond the 2003 - 2010 USA spec.
I start with the 1999 - 2002 heads and machine them to my own spec.
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I also open the dome up just a bit on the 2003 - 2010 head so the compression doesn't go too high.
Even if you run race fuel, you may not want to go below 20cc of total head volume as it begins to be very barky on part throttle which can mess up your corners on harder packed tracks. Retarding the timing can tame some of the bark.
With pump fuel, just get the squish cut and stick with stock head volume of around 21.5cc (which means you have to enlarge the dome after removing material to set the squish).
I'm a machinist, so I just cut my own heads and have had excellent results.
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Can you please explain real quick the difference between squish and dome volume? For some reason I thought it was one in the same. Do I have to do the solder test and send in to a engine builder? Or can I just say, .045" at 21.5 cc?
Ideally, you want to always take a measurement with solder to know for sure where you sit stock but like CW said, most I've seen seem to be .074"-.078" so some builders may have just a general cut of .030" if there is no measurement to go from. I have seen as high as .081" so the solder test would tell the tuner they could go a little more than normal.
Resurrecting this thread, sounds like there's smart guys in here😆
Ive got an 06 yz250, and need a new head. Stock for that year is the 5up, but I've found one from a 2000 that's the 5cu.
From what I can tell that will bolt on just fine, but does anyone know if there's a meaningful difference between them?
At this point the bike is just trails and cruisy mx as I'm old and fat now. Not super worried about max performance, more so worried if it would cause reliability issues.
Thanks!
Pit Row
Strictly motocross model (not the X model)
1999 - 2002 had larger chamber volumes.
2003 - 2010 USA heads have a smaller chamber volume.
2011 USA models were updated to the world wide spec with the larger chamber volume, neutral sensor, I think the jetting changed, etc.
The 2000 head is a good starting point. I'm not staring at my notebook, but I believe they are 21.5cc volume.
The larger volume spreads out the power and mellows the hit when compared to your USA model 2006.
That spec I posted previously is pretty damn good if you can do the work, or have a local shop that can do it for you.
I believe the 5CU head has a combustion chamber volume of 22.0cc
The 5UP is 21.0cc, and the 5WM 21.5cc. Note: the 5WM head off a 250x is 23.5cc
I use a 5UP head on my 24' 250YZx. I mix a little of 110 race fuel, 1-gal to 4 gal of 93, just for insurance against detonation,
That's great, thanks for the info - exactly what I needed.
Awesome thanks! I might go the race fuel route as well, I definitely had some pinging. Check out the bottom edge.
I’m a super basic machinist.
Would be interested in cutting some head inserts. RZ500 x 4. Potentially volume to suit unleaded fuel.
Is it normally done on a lathe? Any tips on getting the curved shape?
Probably the mill for setting the final squish?
Thanks for any info
And thanks for not saying “if you have to ask….”
Most heads have a centered spark plug hole, so you can do them on a lathe with zero problems.
Basic machinist... You need to know how to "dial in" on the gasket surface or risk making a crooked cut.
You also need to have good judgement on how to securely hold a part and create a reasonably good surface finish for the gasket.
Take small cuts and frequent measurements... It's hard to put material back on the part after you cut off too much.
A threaded "mandrel" can be a decent way to hold a head with a centered spark plug hole.
Cutting a flat gasket surface is pretty easy with a face tool and the cross-slide.
You can cut a squish band by setting the "compound" on your lathe to the angle you want.
Cutting a nice dome is a bit more complicated.
If it was a big deal to get an exact, repeatable dome, I would put them in the CNC machine.
Since I just do my own heads and occasionally help friends or local kids, I do them in a 1960s Clausing manual machine.
I have a bunch of different pieces of high-speed steel tooling that have various curves ground into them and I just do the domes by eye.
After cutting, I lap the gasket surface up to 600 or 800 paper, then check the volume using a cheap syringe and plexi-glass with two small holes drilled in it.
Squish band width matters. I'm not the expert on two stroke porting and head design, so I'm not going to try to explain that side of things.
There are some resources (books, videos, software) out there that can help inform squish band width and calculate squish velocity should you decide to play with that stuff.
The YZs are perfect bikes to learn on because the parts aren't super rare and expensive.
Be prepared to run quality fuel and tweak your jetting.
Get a notebook and write down each thing you try, what you change and the riding conditions that day.
You can take a cast of a chamber using wax, epoxy (use mold release), or foam to get a better visual of the shape.
Search the internet for the bold keywords above if you need to.
DEP for the win
I have a couple of DEP pipes for the YZ250 and they're great!
The Bill's makes a little more torque and I seem to get better starts.
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