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2strokeYardSale

YZ250 squish clearance

514 posts in this topic

What is the ideal squish clearance for a latest-generation engine YZ250 used off-road?

I've done some extensive web searching and this forum is the only place with people who seem to know anything about it. I asked the same question of my CR250 on another forum and only heard crickets chirping. It seems like RCannon may have come across the magic number here.

I just picked up a 2006 YZ250. It rained that night so of course I had to ride it the next day, sans spark arrestor. The only mods were tires/tubes and some more grease in the lower linkage bearing. I burned less than a tank of VP C12 (with a more generous ratio of Motul 800 for break-in) and didn't change anything but the suspension compression.

Tonight I took the head off and measured the squish clearance at .071", 1.8mm. Wow, that's a lot! With a Cometic high-compression gasket kit, my CR250 measured .057"/1.45mm, which still might be too much.

I plan to have a machine shop take some material from the head to get to my ideal squish clearance. I am aware this will lower the volume of the combustion chamber, but that, too, is desirable. I ride at 4,500-7,000 feet and I use pure race fuel or a mix of race/pump gas. (There's actually a VP gas station nearby, how cool is that?!)

I'm learning a lot about two-stroke tuning because I bought a TZ250 roadracer this summer. That requires careful attention to deck height, chamber volume, squish clearance, ignition timing, etc.

I had a hard time getting clean, consistent carburetion from my CR250 with its Mikuni , but now I'm wondering if excessive squish clearance was the major contributor to those difficulties. (I did manage to get it perfect with a Keihin carb.)

The first ride on my YZ showed no major carburetion issues. But before I go swapping brass, I want to get the squish clearance right.

So what's the magic number for a YZ250?

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I hear that 0.50" is a good safe number. You can go 0.45"-0.40" if you keep a real close eye on things (bearings, etc.)

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How did you measure the squish clearance if you took your head off, or is that just a typo? Moto814 (reputable 2 stroke engine builder) recommends 1.3-1.5mm on a 250cc engine. Mine is at .055.

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Generally the tighter the squish the better. Taking everything into account a good number to shoot for would be 0.050" on a 250 with a fresh bottom end.

The tighter the clearance the more you'll have to pay attention to the condition of the motor. IMHO this would be anything less than 0.045", and anything less than 0.040" will be like playing with fire.

Anything more than 0.055" and I don't think you'll be getting your money's worth, AND shortly after this the effectiveness of the squish band really goes down hill.

Final compression numbers should be based on fuel, elevation, desired powerband, etc. Using higher octane fuel and/or riding at altitude allows for more compression and tends to make the bottom/mid portion of the power healthier. For altitude correction use approximately 2% for every 1000ft, so at your minimum of 4500ft that's a 9% increase of the original compression spec. If your UCCR is 12:1 then your final UCCR would be (12 x 1.09 = 13.08). The numbers are just thrown out there for reference.

FWIW I think RCannon's ended up being right at 0.048" which is great for someone that rides often but doesn't constantly tear the motor down.

Regardless it's always a good idea to recheck the squish each time you take the motor apart for rings, piston, etc. Basically anytime you change ANYTHING, including gaskets. There is always slight variations and you'll want to be sure that you don't find yourself in the danger zone. If you're consistently ending up around 0.050" with +/- 0.005" you'll be fine.

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Funny you posted this, I just got done asking the same questions. Mine's off today to be machined to .048 - .050 squish.:thumbsup:

FWIW my OEM squish is .070, measured last night. Sounds like Yamaha has pretty consistent machining.

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Mine is being done now also. Machinist will set it at .045" and machine the dome to get me back to original compression. I measured 210lbs on a warm motor just above sea level (300ft maybe), so I dont have much room to bump compression while still running pump swill.

The squish movement seems to be picking up steam around here.......

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you guys and your damn fractions of inches!

remember, on most modern 250s, the squish area is approx 50%, this means if you reduce the squish gap by 0.5mm, you will need to remove 1.0mm from the bowl for the same comp ratio.

also, FYI the squish gap on the rm250 is 1.2mm, i think this is where the RM gets its great response and snap from.

IMO 1.0mm is safe on the 250, i run 0.9mm on my 144.

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How are you measuring your squish clearance? Solder through the plug hole? What diameter solder do you need? Or do you need to measure it?

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I took off the head and placed a bore-length piece of rosin core solder in the center, same orientation as the wrist pin. Replaced the head, sans o-rings, just barely tightened the nuts, and turned it over with the rotor. Removed the solder and measured each side with a micrometer.

I don't recall the diameter of the solder, thick enough to squash down to the expected .060-080" thickness. On my TZ250 (two 56mm 125cc cylinders) I use a smaller diameter solder for its smaller squish clearance. On that bike I also do this two or three times with different pieces of solder and average the measurements. I can adjust it with the base gaskets.

I'm going to call a machine shop and see if they can take .020" off the head.

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One small thing. Dont use the solid solder. Its too hard and could damage something. I miss the good old days. the lead solder was very nice to work with......

Make sure the stuff you get is "rosin core". I suspect the acid core would work but it was difficult to find.

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Couple questions, when I check the squish should I do it a couple times and then average the results or just once? And do I need to install the cylinder to head O-ring or does it matter?

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Leave the o ring out. No need for it while checking clearance. I went with my largest measurement rather than an average. I did not want to ge tit too close. The measurements were virtually identical using the RB measuring method. I had less luck with the wire through the spark plug hole.

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This really shouldn't even be considered a mod. It is just common knowledge of what a 250cc 2 strokes squish should be from the factory. Its basically part of blue printing your engine back into the ideal specification that it was designed to be. I've always thought of it as one of the first things you should do to your 2 stroke to make sure it is right. Theres really no excuse for Yamaha to not have the squish right from the factory. With the CNC machines of today they could easily build these within .0005. Why would they want to open the clearances up enough to hurt engine performance and cause detonation. Or maybe its part of their scheme of helping out the race fuel manufacture.

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Going through the spark plug hole is silly and difficult. Just pull the head; it doesn't even cost you a gasket. Drain the coolant and turn a wrench.

I find it hard to believe Yamaha (and others) build this much clearance just to avoid problems from worn cranks. This is a purpose-built motocross racer, not a TTR play bike.

FWIW, I had an Aprilia RS250 (Suzuki-made twin 125 cylinders) that had outrageous squish clearance, too.

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I find it hard to believe Yamaha (and others) build this much clearance just to avoid problems from worn cranks. This is a purpose-built motocross racer, not a TTR play bike.

While I do agree, I bet the bean counters at Yamaha would not. They've got to consider Joe Average who only does a top end every 150hrs and wouldn't know a worn rod bearing from a handlebar.

Talking to a machinist who machines many heads stated that Honda & Yamaha have the tightest tolerances (notice how many of us measured .070" +/-.003) while Kaw, Suz and all euro manufactures are all over the place. Yamaha could very well recalibrate their die's for tighter tolerances, but they'd surely be dealing with more issues caused by user error. Sucks, doesn't it?

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They've got to consider Joe Average who only does a top end every 150hrs and wouldn't know a worn rod bearing from a handlebar.

I guess I'm one of those guys (except I do change my top end after 40 hours). How do I determine if the rod bearing is worn?

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What can I measure the solder with, besides a digital micrometer(because I don't have one)?

A standard micrometer?

You pretty much need a micrometer or calipers to get the measurement.

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What can I measure the solder with, besides a digital micrometer(because I don't have one)?

One thought I have is this. Take your measurement and bring the solder with you. Unless your going to mill your head by dragging it on the driveway your going to a professional. They will have the micrometer.

Bruce 372 brought up a brilliant point that was somewhat glossed over. Make sure you and the machine shop are speaking the same language. Inches vs Millimeters. This is something that could ruin the head if your talking MM and the shop only speaks inches.

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