Read this KX250 Piston

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8/3/2018 10:53 AM

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Over the winter I had my cylinder related and matched to a standard bore pro x piston on my 1998 KX250. I assembled everything coating the piston, wristpin bearing and cylinder in HP2.I fired the bike up and did a heat cycle on it and everything seemed to go great. I went to start it the other day and it seemed like the motor was locked up. the kicker wouldnt move. It finally ended up breaking free. Instantly before I did anything else I pulled the pipe off to look in the exaust port and the cylinder walls looked fine but the piston not so much. Im 100% sure I had 32:1 fuel in it and my radiator is still full of coolant. Any help or ideas on what may have happened would be appreciated.
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8/3/2018 1:26 PM

Dirty air filter or intake boot? The piston looks pretty scratched for having only one heat cycle. Hard to say why the piston would feel locked up from sitting. Maybe there was a smidge of rust on the ring- even though you ran oil through the cylinder.

I wouldn't worry too much and just try to run it. Pull the air filter and look down the air boot first... Just my 2 cents.

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8/3/2018 4:39 PM

I was thinking it looked like grit got in the cylinder somehow too but I clean the filter after every time I ride it and make sure the air box and intake boot are spotless too. The bike was very throughly cleaned before the motor was put back together. Thanks for the suggestion though.

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8/3/2018 4:43 PM

Dirt is the cause.

Paw Paw

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8/4/2018 6:14 AM

Would that have caused the piston to seize too?

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8/4/2018 6:21 AM

Yes. Dirt will cause extra friction and thus more heat and heat causes seizure when the piston heat expansion is faster than the cylinder. Putting the engine under a load before it is fully warmed up can do the same thing even if no dirt is present. Poor fit of the piston / rings to the cylinder will also cause this.

Paw Paw

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8/4/2018 9:01 AM

dirt did not cause it to seize. it was likely bored too tight.

if that was a new plug and it only has an hour on it or less, you have other problems. might have a bad crank seal. you should pressure test the engine.

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8/4/2018 9:07 AM

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8/4/2018 11:19 AM

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I pulled the bike apart today and heres some more photos. Like I said the piston has roughly 6 minutes on it. Also I should have mentioned the spark plug was not brand new. It had a few hours on it. I really appreciate the input from you guys.
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8/4/2018 8:01 PM

like i stated, get it measured but it is not from sand. looks like there was not enough clearance.

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8/4/2018 8:40 PM

barnett468 wrote:

like i stated, get it measured but it is not from sand. looks like there was not enough clearance.

The piston/cylinder were sent to Mellennium technologies and matched by them.

Nathan, get a measurement of the bore, piston, and post them up here. Also, post a photo of the bike so it gets some love and there is an understanding of its cleanliness and state of being.

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8/5/2018 10:13 AM

4 corner seizure ? Improper warm up? Water pump functioning properly? Piston to cly clearance to tight? Ring end gap too tight ? Premix Oil in the gas? I"m following to learn the problem & solution....

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8/5/2018 4:20 PM

Did you make sure to flush the cylinder in soapy water. I know the cutting dust will do that to your piston like you took 80 grit to it.

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8/6/2018 8:22 AM

barnett468 wrote:

like i stated, get it measured but it is not from sand. looks like there was not enough clearance.

SLOWTIME wrote:

The piston/cylinder were sent to Mellennium technologies and matched by them.

Nathan, get a measurement of the bore, piston, and post them up here. Also, post a photo of the bike so it gets some love and there is an understanding of its cleanliness and state of being.

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Heres a few photos of the bike. Like SLOWTIME said its an extremly clean and well taken care of Pro Circuit Splitfire bike ive been slowly building. Ill get some measurements of the bore and let you guys know. I dont have a bore gauge big enough for the cylinder, just a set of micrometers. If anyone knows a better way to measure the bore let me know.
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8/6/2018 8:43 AM

A.j._Stevens wrote:

Did you make sure to flush the cylinder in soapy water. I know the cutting dust will do that to your piston like you took 80 grit to it.

I didn't use soapy water but I always wipe the cylinders out with a clean rag when I get them back from Millennium. Ive never had any problems before this incident

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8/6/2018 1:11 PM

A.j._Stevens wrote:

Did you make sure to flush the cylinder in soapy water. I know the cutting dust will do that to your piston like you took 80 grit to it.

nkoebes826 wrote:

I didn't use soapy water but I always wipe the cylinders out with a clean rag when I get them back from Millennium. Ive never had any problems before this incident

I always just get a bucket of hot soapy water and submerge it a few times to flush it and then rinse it in hot straight water and wipe it down.

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8/6/2018 4:52 PM

you can certainly measure it with micrometers and snap gauges if you know how to use threm . measure the piston around 1/2" up from measuring front to rear.


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8/7/2018 7:05 AM

Wiping the cylinder out is not enough. Those transfers are always filled with blasting media and dirt after getting replated. New tooth brush and warm soapy water. When you got the cylinder back it should have came with instructions on how you needed to clean it before assembly.

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8/7/2018 8:05 AM

Hondas4Life3 wrote:

Wiping the cylinder out is not enough. Those transfers are always filled with blasting media and dirt after getting replated. New tooth brush and warm soapy water. When you got the cylinder back it should have came with instructions on how you needed to clean it before assembly.

When ever I get my cylinders done by them I always get the power valve service done also so when I receive them back the power valves are completely assembled and greased. I assumed they would clean all the media out before assembling the power valves so there isn't grit in there ruining those too but who knows, I may be wrong.

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8/7/2018 9:00 AM
Edited Date/Time: 8/7/2018 9:01 AM

Hot soapy water wash down, then wipe down with clean white t-shirt with wd40, if white tee shirt is not spotless, it's still has media on the cly walls....nice looking bike.

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8/9/2018 7:49 AM

The piston measured 2.697". I measured the bore at the top since thats all I could really get too and got 2.710" . Millennium told me they wanted me to send the cylinder and piston back so that they can evaluate it. Ill post what they say when I hear from them.

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8/9/2018 8:45 AM

that is ,013" and it definitely doesn't have .013" clearance.

take a piece of notebook paper 2" long and 1/4" wide then fold it over the bottom of the cylinder so 1" of it is in the cylinder then install the piston from the bottom and try to push it past the paper. tell us if it takes light force or moderate force or if it is too tight to push past it with moderate force.

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8/9/2018 9:09 AM

nkoebes826 wrote:

The piston measured 2.697". I measured the bore at the top since thats all I could really get too and got 2.710" . Millennium told me they wanted me to send the cylinder and piston back so that they can evaluate it. Ill post what they say when I hear from them.

I did a quick Google search and found some 2-stroke piston to cylinder wall clearance info. It seems your application should have .0025-.0030 inches of clearance. At your numbers, that's .013 inch! That's 10 thou too much! I hope that's not the case. I would imagine you heard piston rocking or somewhat of a clatter at those numbers, but who knows.

I'm assuming you have a 2.1mm oversize piston. I thought they did .04 inches or 1mm increments but I guess there's more options than that. (I've been researching as I type this. Lol.) Did you measure the piston on the lower front and back of the skirt? A 68.5mm piston should measure 2.694ish inches. That would be .0025-.0030 inches under the advertised size to make up for cylinder wall clearance. Your numbers are exactly 68.5mm. Unless, your plater made the cylinder bigger to add the clearance. That doesn't sound right to me though. And again based on your numbers, that's too much clearance. shocked

This is all interesting to me and want to learn as well. Let us know what you find out.

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8/9/2018 9:11 AM

barnett468 wrote:

that is ,013" and it definitely doesn't have .013" clearance.

take a piece of notebook paper 2" long and 1/4" wide then fold it over the bottom of the cylinder so 1" of it is in the cylinder then install the piston from the bottom and try to push it past the paper. tell us if it takes light force or moderate force or if it is too tight to push past it with moderate force.

Great idea!

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8/10/2018 10:40 AM

barnett468 wrote:

that is ,013" and it definitely doesn't have .013" clearance.

take a piece of notebook paper 2" long and 1/4" wide then fold it over the bottom of the cylinder so 1" of it is in the cylinder then install the piston from the bottom and try to push it past the paper. tell us if it takes light force or moderate force or if it is too tight to push past it with moderate force.

Thats a good idea, thanks for that suggestion. I sent the cylinder and piston back to Millennium 2 days ago since they wanted it back to look at it. So sadly I cant try that trick. I looked up the Pro x piston and it says its 66.35mm which is roughly 2.612 inches. I used harbor freight micrometers when I measured the piston and cylinder since its all I have in my box at home. I should probably just throw those things in the trash. They never seem to be accurate

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8/11/2018 7:07 PM

barnett468 wrote:

that is ,013" and it definitely doesn't have .013" clearance.

take a piece of notebook paper 2" long and 1/4" wide then fold it over the bottom of the cylinder so 1" of it is in the cylinder then install the piston from the bottom and try to push it past the paper. tell us if it takes light force or moderate force or if it is too tight to push past it with moderate force.

Sir,
Your clearance thoughts are right on. I am not trying to hack this thread or sell anything yet plan to add a positive experience to restorers. I am 59 and have owned 48 bikes since 12.

My local ATL shops require $125 to bore a cylinder. Outrageous. I have located an EBAY machinist that I have had bore 2 vintage cylinders for 49.99 plus shipping.

Suzuki_star_racer
Steves Cycle
641 Centennial Rd
Rutherfordton, NC 28139

I talked to him before shipping and I have the shop tools to confirm the returned borings are within 0.0002".
My experience is good........

Just a viable thought guys.

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8/11/2018 7:13 PM

barnett468 wrote:

that is ,013" and it definitely doesn't have .013" clearance.

take a piece of notebook paper 2" long and 1/4" wide then fold it over the bottom of the cylinder so 1" of it is in the cylinder then install the piston from the bottom and try to push it past the paper. tell us if it takes light force or moderate force or if it is too tight to push past it with moderate force.

ktmfun219 wrote:

Sir,
Your clearance thoughts are right on. I am not trying to hack this thread or sell anything yet plan to add a positive experience to restorers. I am 59 and have owned 48 bikes since 12.

My local ATL shops require $125 to bore a cylinder. Outrageous. I have located an EBAY machinist that I have had bore 2 vintage cylinders for 49.99 plus shipping.

Suzuki_star_racer
Steves Cycle
641 Centennial Rd
Rutherfordton, NC 28139

I talked to him before shipping and I have the shop tools to confirm the returned borings are within 0.0002".
My experience is good........

Just a viable thought guys.

Let edit at 49.99 each cylinder. Sorry for my bad.

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8/11/2018 7:15 PM

barnett468 wrote:

that is ,013" and it definitely doesn't have .013" clearance.

take a piece of notebook paper 2" long and 1/4" wide then fold it over the bottom of the cylinder so 1" of it is in the cylinder then install the piston from the bottom and try to push it past the paper. tell us if it takes light force or moderate force or if it is too tight to push past it with moderate force.

nkoebes826 wrote:

Thats a good idea, thanks for that suggestion. I sent the cylinder and piston back to Millennium 2 days ago since they wanted it back to look at it. So sadly I cant try that trick. I looked up the Pro x piston and it says its 66.35mm which is roughly 2.612 inches. I used harbor freight micrometers when I measured the piston and cylinder since its all I have in my box at home. I should probably just throw those things in the trash. They never seem to be accurate

No prob, I learned that trick from a master cylinder borer named bill quals of q and e cycle in 1975. The paper is around .00225 - .00250 thick so it is a decent quick reference. If you can't push a 2 stroke 250 piston past it then it is definitely too tight and if it falls thru with no effort it is definitely too loose.

You can also use narrow feeler gauges but they are getting hard to find.

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8/11/2018 7:18 PM
Edited Date/Time: 8/11/2018 7:19 PM

barnett468 wrote:

that is ,013" and it definitely doesn't have .013" clearance.

take a piece of notebook paper 2" long and 1/4" wide then fold it over the bottom of the cylinder so 1" of it is in the cylinder then install the piston from the bottom and try to push it past the paper. tell us if it takes light force or moderate force or if it is too tight to push past it with moderate force.

ktmfun219 wrote:

Sir,
Your clearance thoughts are right on. I am not trying to hack this thread or sell anything yet plan to add a positive experience to restorers. I am 59 and have owned 48 bikes since 12.

My local ATL shops require $125 to bore a cylinder. Outrageous. I have located an EBAY machinist that I have had bore 2 vintage cylinders for 49.99 plus shipping.

Suzuki_star_racer
Steves Cycle
641 Centennial Rd
Rutherfordton, NC 28139

I talked to him before shipping and I have the shop tools to confirm the returned borings are within 0.0002".
My experience is good........

Just a viable thought guys.

ktmfun219 wrote:

Let edit at 49.99 each cylinder. Sorry for my bad.

some shops bolt the cylinder to a fixture much like bolting a plate (torque plate) to the deck of an auto engine before they bore it so it is more concentric after the head is bolted on because the torque from the head bolts can distort some cylinder bores slightly. the shops that do this naturally charge more than others that don't.

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8/11/2018 7:40 PM

barnett468 wrote:

that is ,013" and it definitely doesn't have .013" clearance.

take a piece of notebook paper 2" long and 1/4" wide then fold it over the bottom of the cylinder so 1" of it is in the cylinder then install the piston from the bottom and try to push it past the paper. tell us if it takes light force or moderate force or if it is too tight to push past it with moderate force.

nkoebes826 wrote:

Thats a good idea, thanks for that suggestion. I sent the cylinder and piston back to Millennium 2 days ago since they wanted it back to look at it. So sadly I cant try that trick. I looked up the Pro x piston and it says its 66.35mm which is roughly 2.612 inches. I used harbor freight micrometers when I measured the piston and cylinder since its all I have in my box at home. I should probably just throw those things in the trash. They never seem to be accurate

barnett468 wrote:

No prob, I learned that trick from a master cylinder borer named bill quals of q and e cycle in 1975. The paper is around .00225 - .00250 thick so it is a decent quick reference. If you can't push a 2 stroke 250 piston past it then it is definitely too tight and if it falls thru with no effort it is definitely too loose.

You can also use narrow feeler gauges but they are getting hard to find.

Yup I use 1970 Starrett taper gauges at 0.002 on cylinder bores

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