how tight is too tight?

Talking about squish here fellas. :bonk:

I had my bike set up perfectly, ran great, yadda yadda yadda. Well I had a race in the cold, wet, muddy sand this past weekend and was jetted a little too lean so I fried the top end. Nothing too damaging, but needed to be replaced after the beating I put on it. well today I got my replacement single ring piston from wiseco and threw it in there. Being cautious I remeasured the squish and it went from my perfect .045 clearance to the new number of .038. Is that too tight? I run straight c12 so fuel isn't an issue, I'm just worried about being too tight. What do you guys think?

Its too tight if spit isnt lube enough... :bonk:

Ooops :bonk:

did you see ?

i think your squish is ok, if your crank and con rod bearings are good.

I've been running .033" and .034" (left and right squish min) for about 40 hrs on my 2001 cr250, mostly MX riding, no problems. I left out the base gasket and sealed with Yamabond. Original bottom end has about 100hrs using 32:1 Klotz R50 and pump premium. Of course it's a Honda and i can't say anything about the min squish in a Yamaha due to it's commonly acknowledged inferior engineering and worksmanship. yuk yuk

There are many posts on various forums of folks running mid 30s squish. It's ok as long piston not hitting head. I use a dental mirror with a built in LED to periodically look thru the exhaust port to look for contact marks on top of piston at the very edge. If piston starts hitting cylinder, it might start deforming piston and reduce ring to piston clearance, possibly wedging the ring. When checking squish, besure solder is tight against cylinder since that is where squish is min. Also be sure to squeeze solder on both sides simultaneously, as measurement will be higher if you only squeeze solder one side at a time. One side at a time in my case resulted in .038 and .039. And this is with a new Wiseco piston that had .002 clearance to cylinder.

In general, smaller squish reduces detonation that usually happens in the squish band, since those gasses are cooler due to close proximatey of head and piston. But, smaller squish increases "maximum squish velocity" which increases turbulence in combustion chamber and supposedly can result in detonation. See Eric Gorr's book.


Edited by 2bitsamile
Its too tight if spit isnt lube enough... :bonk:

Ooops :bonk:


Of course it's a Honda and i can't say anything about the min squish in a Yamaha due to it's commonly acknowledged inferior engineering and worksmanship. yuk yuk

Looks like the marketing dept cut engineering's budget at Honda. Seems they haven't made a two stroke since 2007. :lol::bonk:

You can have superior engineering :bonk: but what about marketing and manufacturing? :bonk::bonk:

Good squish discussion.

When running the squish too tight...a couple things happen that I personally feel are a bad thing. Most notably the MSV get's really high! When an MSV get too high, you often see a strong mid range but poor top and over rev.

Anyways, if you run fresh c12 that hasn't sat a while in a plastic can...and keep an eye on your jetting (so don't do what you did!) your current squish should work great.

I get an MSV of about 34-36m/s which is a little high, but should work fine. I would also suggest frequent rebuilds/inspections at this point, as carbon deposits can get large enough to cause an issue.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Similar Content

    • By Rixter
      A tiny clear rubber tube on bottom of carb.  On bottom sort of on side, by floats, the other end just hanging down.  Was it always like this? Or does the other end go somewhere?  Today after multiple trys with kick, choke on/off ,,,fuel keeps pouring out the tube.  This bike has always been hard to start.   Thanx  guys!
    • By Brock granberg
      I just did my valve adjustments and put my YZ250f back together as far as the valves, shims buckets and cam covers on but I need adhesive for the plastic valve/cam cover and I am stumped as far as what to use and where to get it or even what to call it! Need to get some soon im in need of a ride!
    • By ThatE30GUY
      Hey guys, I bought this YZ85 and came to realise the clutch wasnt working, so I drained the oil (Jet black) and pulled the plates out and cleaned them. I assumed finding a torque wrench would be easier than it has been. Does anyone know how to torque the clutch springs to spec without a wrench? Any insight helps.