Piston to Cylinder Clearance

[Anthony Herpolsheimer]

<p style=”text-align: left;”>Hello All,</p>
I am in the middle of sorting out a blown TM engine (Stuck piston, K9B, water hose came off during a race), supposedly originally build by DDR.  As I had gotten into trying to figuring out piston size for a replacement, I had read from some other 2 stroke forums that .003″ clearance is desirable.  This info was found at a 2 stroke crotch rocket forum as I could not find anything specifically kart related.  Before doing too much, I got the aluminum from the piston out of the bore and took several measurements with a telescoping gauge and then measured that with a micrometer.  After taking measurements from several locations, several times, and taking an average of measurements. I have a bore diameter of 2.12576″.  I then measured the only area of the piston that hadn’t rubbed and deformed itself and came up with a measurement of 2.1157, meaning there is a difference of just over .010″.

Does that seem reasonable?  That seems to me like a huge gap, but like I said, I haven’t been able to find any solid info about proper piston to cylinder gap.  Any help would be appreciated.

Thank you

2008 GP 8 Honda 125 / TM K9B
Building F125 Enduro

[Greg Wright]

.010 is HUGE!!! .003 should be fine. I wouldn’t take your measurements to the bank however. It needs to be checked with a proper dial bore gauge.

Greg Wright
Rapid Racing Inc.
Vintage B-Stock Pilot
"When in doubt Gas it, It won't help but it ends the suspense."

[Christian Fox]

What Greg said. 01″ is double what you would want for an iron bore cylinder, let alone the Nikasil cylinder you have on your TM. The recommended clearance is .07 to .075mm for that motor, or about .0028″. Either way, if your motor stuck, you’ll want to have the cylinder honed before you fit a new piston, so don’t even bother measuring now. To do it right you need a good dial bore gauge and a nice micrometer, or preferably a calibration ring.

[Anthony Herpolsheimer]

Yes, a bore gauge is optimal for measuring the ID of my cylinder, however at the moment I don’t have one..  For now I’m ok using my telescoping gauge and mic to get an idea.  All of the measurements I took were all within .0005″ of each other.  Close enough to get an idea of what I’m starting with.

Also yes, I know the cylinder will need a hone, and the size will change, but my goal with this measurement was to get an idea of what the original piston clearance was, so when the cylinder is cracked checked, honed, and measured, I’ll know what piston size I need.  Once I saw there was a .010″ gap, that threw up a red flag and my reason for posting.  And it’s nice to know that what I found about proper piston gap is correct.  Just seems odd that this supposedly being a DDR engine that hadn’t been raced sense the last rebuild that they would have used a piston that small.

Thanks for all your replies thus far!

2008 GP 8 Honda 125 / TM K9B
Building F125 Enduro

[Walt Gifford]

It’s amazing how some kart shops just throw in a small piston to make a quick buck.

FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience

[Brian Wilhelm]

I believe when you get the new piston, the clearance will be much less than .010. More likely it’ll be around .0025-.0028. Seized pistons generally collapse some. Once you have the aluminum off the nikasil using acid, (do NOT over do this! Remember, nikasil is porous and the acid will go through it and deteriorate the aluminum underneath if you leave it on it very long) the cylinder doesn’t need any size taken out of it. It just needs a standard hone ran thru it to make sure there isn’t any high points. Don’t try to take any scratches or scoring out. It’ll run just fine with them there. My fastest Pavesi cylinder looks terrible, but runs very well.

Where are you going to run your F-125 laydown?

[Anthony Herpolsheimer]

The idea of the piston collapsing did cross my mind.  Wish I had more areas I could measure on it.  It was pretty beat up.  I wouldn’t doubt that with as hot as things must have got that it could deform that piston.

As for the F-125, I’m going back and forth if I should continue that rout, or stay simple and go with a KT100 to get my feet wet in the enduro group.  Either way, I’ll be racing mostly in Michigan and tracks near by.  I think starting next season we’ll have better provisions to be able to travel further for a race, so I’m hoping to hit VIR, Road America, Watkin’s Glen, Daytona, Mid Ohio, and many of the other awesome tracks across this country, probably not all in the same year though.

Where do you run Brian?  Are you an enduro racer?

2008 GP 8 Honda 125 / TM K9B
Building F125 Enduro

[Brian Wilhelm]

I am an enduro racer. I race in F-125 and Unlimited with a Pavesi on a laydown. You are in a good area for the type of racing. Recently I’ve been racing at MIS, (twice a year , (just there a couple of weeks ago), Mid-Ohio, AMP, Daytona, etc. Unfortunately, there aren’t any events at Road America or Watkins Glen, but there are enough other events to keep you plenty busy. Your TKM is a good selection for a F-125.

[Anthony Herpolsheimer]

Well that was the reason for my purchasing the TM.  I have a chassis that’s in the process of completing the transformation to a lay down.  Thought I would get more inspired to finish it if I had the power plant.  Well, last season I put it in the GP to have some fun with and learn it a bit, and then it blew.  Maybe this winter I can make some headway on it.  I jumped into karting with that GP and running with the CIK guys just isn’t quite my speed.  Plus, my friends that I race with are F125 guys so I would like to go run with them.

Well, I don’t want to muddy the tech forum with this kind of talk.  I’m just happy I was able to find some insight on my engine abnormalities.  If you want to talk further about racing, please feel free to send me a pm.

Thanks guys!

2008 GP 8 Honda 125 / TM K9B
Building F125 Enduro

[Greg Wright]

Your statement about a chassis being converted to a laydown is bothersome. Laydown karts are not modified sprinters but instead purpose build machines.

Greg Wright
Rapid Racing Inc.
Vintage B-Stock Pilot
"When in doubt Gas it, It won't help but it ends the suspense."

[Anthony Herpolsheimer]

Greg,  you do have a point.  I had contracted a good friend to start this build.  He has been racing his whole life and built several karts, and assured me this would be a good way to go, without having to spend a ton on a brand new chassis.

I need to get myself back into the build and see what I have and see if it’ll be worth continuing on as is.  I am not super competitive.  I just like going out and putting some laps down.  If I can work this chassis and have fun, I will.  If it seems to be too tough to get what I want out of it, I’ll start looking at other options.  If you have any tips or insight that would help me out, it would be much appreciated.

2008 GP 8 Honda 125 / TM K9B
Building F125 Enduro

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