Fixing the squish?

[Not sponsored]

So I've read through countless threads on correcting the squish band on the 250x, my question is would just bolting a regular yz head on take care of the problem? Or do they have the same issue? I don't mind upping the compression and power a bit if thats the result. Don't really want to be down for a couple weeks while getting the head shaved.

[mlatour]

The regular motocross version YZ250 has the same issue. (larger than optimal squish height)

If not too excessive, an hour of careful figure-8 sanding the head on a plate glass with 400 grit wet/dry sandpaper is all that may be needed.

 

That's how I reduced mine by 0.20mm (2010 YZ125),

going from 1.37mm down to 1.17mm (left room for future experimentation, 125's can run as little as 0.8mm)

Despite the combustion chamber's volume un-corrected it only gained 5psi of compression but,

made an already clean running engine notably crisper on the bottom.

Edited June 21, 2017 by mlatour

[Charles De Mar]

Pardon my ignorance. Yamaha has been building this motor for so long and milling the head reducing, squish, and "fixing" it seems pretty simple. Why haven't Yamaha engineers "fixed" this at the factory? Why not just call it reducing the squish? Does the reduced squish and higher compression effect the life of the engine parts? There are plenty of people recommending this mod. I would be more willing to do the mod if someone knows enough to answer these questions. I have only messed with the needle, pilot and main jet on my '17 250x. I don't like to throw mods at things "I"(me) don't need fixed. I like a well tuned bike that starts easy and has enough power to get out of trouble. I'm just a trail rider and not a racer. I do ride with older racers frequently. I can ride just about anywhere until I think the risk is surpassing the reward. That has been very few places. 

[zeke383]

The stock YZ head needs the squish corrected for pump gas. If this is done, you can run 91 pump in any condition out there. The 250X head doesn't NEED this done, it does not have the same detonation issues. Some, however, have played around with different head mods on the X. Typically decreased squish but same head volume or decreased squish and decreased volume. Personally, I don't think the X needs a head mod but that's just me.

If you really want to experiment, buy a used YZ head and ship it to RB Designs to be worked. If you like it, sell your X head and get your money back.

[biglake]

From what I read the X head has a larger squish clearance than the yz head to lower the compression.

The yz head still has a kinda large squish clearance and pretty high compression. Bolting it on will give you more compression and slightly better squish but wont be quite as good as a head done by rb designs or someone similar

If you want more compression and a tighter squish you could have a local machine shop deck the head and not touch the dome, this would be pretty quick and cheap, you'd have to check your squish first to see how much to remove. I wouldn't just guess and take 1mm off, 0.5mm would be safe and close to what a stock yz head will be but if your machining it you might as well get it as close to 1.2mm of squish as you can.

I have no issues with the stock high compression on my 06 detonating with pump gas. I looked into getting the squish done and lowering the compression a bit to make it smoother trail riding but after some more time on the bike I like the snap it has and wont be messing with it.

[baddos]

I have a 16 YZ250X and correcting the squish made a world of difference for me. It was the first bike I've ever done it for so I was skeptical at first but it was easy enough to correct if I didn't like the result.

[dirtscoot26]

From what I read the X head has a larger squish clearance than the yz head to lower the compression.
The yz head still has a kinda large squish clearance and pretty high compression. Bolting it on will give you more compression and slightly better squish but wont be quite as good as a head done by rb designs or someone similar
If you want more compression and a tighter squish you could have a local machine shop deck the head and not touch the dome, this would be pretty quick and cheap, you'd have to check your squish first to see how much to remove. I wouldn't just guess and take 1mm off, 0.5mm would be safe and close to what a stock yz head will be but if your machining it you might as well get it as close to 1.2mm of squish as you can.
I have no issues with the stock high compression on my 06 detonating with pump gas. I looked into getting the squish done and lowering the compression a bit to make it smoother trail riding but after some more time on the bike I like the snap it has and wont be messing with it.

Had a new 06 as well- definitely still chasing the dragon to make my '16 as responsive as it's older sister was. I heard in part that they switched to a different needle in 07 on, likely associated with the silencer changes? I actually have the shorter '06 on my '16 since its a 1/3 the size. Wonder if that slightly different older needle they used in 06 is the I'm missing.

[biglake]

AFAIK the motor differences between a 06 and a 16 yz 250 are the head, pipe and needle. The pipes easy to spot but the head is slightly higher compression and the needle is one step leaner on the straight section on the 06.

[Wessmen]

10 hours ago, biglake said:

AFAIK the motor differences between a 06 and a 16 yz 250 are the head, pipe and needle. The pipes easy to spot but the head is slightly higher compression and the needle is one step leaner on the straight section on the 06.

The airbox is different from 2015 too