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How to tell if my piston only needs rings changed

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Just checked compression right now during some routine maintenance and it's at 130 psi with 13.2 hrs so that's not as bad as I thought it would be.  Looks like I'm going to do better than my last top end that didn't even make it to 13 hours and was down to 83 psi.  But then again I discovered my airbox wasn't sealed on that round and I found sand in the carb so that test is out.  All my previous top ends lasted at least 20 hours but no compression tests were done on those.

I don't have a test on a newly rebuilt top end yet because I was having issues with my gauge.  But I just figured out what the problem was and can get readings from now on.  So I'll keep tracking, see if we can get some useful data here.

 

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What are you guys doing to clean up the burrs around the holes?

Bigger question is would you still use this piston?  I royally screwed it up (don’t ask, bone headed mistake not paying attention).  Trash it or try to work with it?

 

 

 

507FAD23-5C54-4DDD-87D0-D0D6830FBD2C.jpeg

Edited by Mike_80

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12 hours ago, yz250f 78 said:

Jeez. Next time punch mark it. Maybe a new drill bit with that also. 

 

HA!  Yeah that wasn’t the problem, like I said, don’t ask, I did something even dumber.  I was even brave enough to post a pic of my art work for everyone to see LOL!

I had forgotten about this post from 4 months ago.  But if it’s of any value to a future reader I was able to clean up the burrs and still use it without issue.

Unfortunatly the saga with this cylinder continues and I’ve been down for a while because I can’t throw any more money in to rebuilding the top end every 12 hours.  Next step is a new cylinder and yet another new top end.  And hopefully I don’t burn it down this time (details in previous post).

 

 

 

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1 hour ago, Mike_80 said:

Unfortunatly the saga with this cylinder continues and I’ve been down for a while because I can’t throw any more money in to rebuilding the top end every 12 hours.  Next step is a new cylinder and yet another new top end.  And hopefully I don’t burn it down this time (details in previous post).

Ongoing top end failures can sometimes be traced to bottom end issues: air leaks

or, simply slapping new pistons in a worn out / out of spec cylinder

 

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15 hours ago, Mike_80 said:

 

HA!  Yeah that wasn’t the problem, like I said, don’t ask, I did something even dumber.  I was even brave enough to post a pic of my art work for everyone to see LOL!

I had forgotten about this post from 4 months ago.  But if it’s of any value to a future reader I was able to clean up the burrs and still use it without issue.

Unfortunatly the saga with this cylinder continues and I’ve been down for a while because I can’t throw any more money in to rebuilding the top end every 12 hours.  Next step is a new cylinder and yet another new top end.  And hopefully I don’t burn it down this time (details in previous post).

 

 

 

12 hours on a top end is too quick from my experience. Mind you I’m a 175 lb Vet (MX only) rider. If you’re really on the gas all the time it’s going to accelerate the wear. I’ve owned 3 last gen CR250’s and the air box/air boot seals have all a been weak point. The 07 didn’t rear it’s ugly head until later but because of age the foam broke down. I use a MX Bonz seal kit and it’s much better at keeping dirt out of the engine. If you are still on the OEM seal I suspect it’s letting dirt in past the filter. Shine a light into the air boot next time you remove the filter and check for residue. 

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On 10/2/2018 at 10:22 PM, Mike_80 said:

 

What are you guys doing to clean up the burrs around the holes?

Bigger question is would you still use this piston?  I royally screwed it up (don’t ask, bone headed mistake not paying attention).  Trash it or try to work with it?

 

 

 

507FAD23-5C54-4DDD-87D0-D0D6830FBD2C.jpeg

you can deburr it by using a bigger drill bit. Try a 1/4" dia drill bit and just spin it lightly over the surface just to remove the burrs. that should give that hole a nice clean chamfer.

as for whether to reuse, if you were just trail riding, I'd say, yea, clean it up and reuse it.

but if you're rippin' it up and WFO on the pipe on a track, I wouldn't chance it. just go w/ a new piston setup. It's alot cheaper than to deal w/ a piston that could potentially shattter in pieces while you're WFO.

 

Edited by jeddclampette

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On 2/19/2019 at 12:23 PM, jeddclampette said:

 

you can deburr it by using a bigger drill bit. Try a 1/4" dia drill bit and just spin it lightly over the surface just to remove the burrs. that should give that hole a nice clean chamfer.

as for whether to reuse, if you were just trail riding, I'd say, yea, clean it up and reuse it.

but if you're rippin' it up and WFO on the pipe on a track, I wouldn't chance it. just go w/ a new piston setup. It's alot cheaper than to deal w/ a piston that could potentially shattter in pieces while you're WFO.

 

I cleaned up this piston exactly the way you’re describing, and ran it hard without any issues.  All my issues have been related to another problem yet to be determined.

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