RM250 Top End Failure Pics

customfab

Member

Aug 7, 2005

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• Nov 18, 2010

• #1

Long story short. Had Eric Gorr do a all-around tune for woods riding. Received new piston kit (265cc bore). Installed everything & put back together. Bike immediately had a strange ticking noise that I can only describe as "valve noise".

Anyway, sat the bike last 8 months. Finally pulled the top end. Found a cracked and scored piston. All damage adjacent to the reed valve.

Cylinder is scored and divoted in adjacent areas. Pretty good trauma.

What the hell happened? I'm afraid to get the repairs only to have the same issue. Pics attached. Like to diagnose it.

BTW, not trashing Eric. I ran the bike for about 30 min total. PITA to start but once running....awesome power. I can only imagine the gain with a proper piston. Yes, dumb of me to run for even 30 minutes. I've rebuilt several engines in my life....never screwed up any chevy strokers so I'm clueless here. :bang:

 

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• 01 RM250 Bore2 (Medium).jpg

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Last edited: Nov 18, 2010

customfab

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Aug 7, 2005

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• Nov 18, 2010

• Thread starter
• #2

Piston....

 

J

_JOE_

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May 10, 2007

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• Nov 18, 2010

• #3

I'd say the intake runner cracked. My old RM had the same thing happen after the over bore. After talking with several people it seems to be pretty common.

 

customfab

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Aug 7, 2005

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• Nov 18, 2010

• Thread starter
• #4

Joe,

I've inspected the cylinder. Couldn't visible see any cracks. There is some mushroomed of the intake runners and two large divots. Thing is, I'm pretty sure this problem was present immediately upon assembly. I'm wondering if I could have installed it incorrectly creating the problem. Ticking was present from 1st second of firing up. Or maybe a lean condition caused the damage?

Here's a close-up of the intake runners:

 

Last edited: Nov 18, 2010

customfab

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Aug 7, 2005

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• Nov 18, 2010

• Thread starter
• #5

Piston:

 

IndyMX

Crash Test Dummy

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Jul 18, 2006

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Amo, IN

• Nov 18, 2010

• #6

Stupid question.. But.. This happened back in March?? 8 months ago?

Could it have been a cold seizure?

 

A

adam728

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Aug 16, 2004

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• Nov 18, 2010

• #7

My personal guess: the end-gap of the rings wasn't set.

Too little gap and the ends of the rings meet each other and leave no room for movement. Then they snag ports, put far too much drag on cylinder walls, etc etc.

Either that or the piston-to-bore clearance wasn't enough. But that wouldn't expalain the large divits at the ports.

 

whenfoxforks-ruled

Old MX Racer

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Oct 19, 2006

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Merrillville,Indiana

• Nov 18, 2010

• #8

Do you have a pic of the other top ring centering pin? Vintage Bob

 

customfab

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Aug 7, 2005

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• Nov 18, 2010

• Thread starter
• #9

Might not matter but I checked the rings today. Within spec but I did not check them during the original install.

 

IndyMX

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Jul 18, 2006

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Amo, IN

• Nov 18, 2010

• #10

doesn't really matter to check parts now..

 

customfab

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Aug 7, 2005

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• Nov 18, 2010

• Thread starter
• #11

They were fine.....sending back to Eric for another plating. He'll give me the final word anyway. His kits are clearanced anyway.....so not the rings.

 

2-Strokes 4-ever

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Feb 9, 2005

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Missouri

• Nov 19, 2010

• #12

Reed petals?

 

whenfoxforks-ruled

Old MX Racer

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Oct 19, 2006

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Merrillville,Indiana

• Nov 19, 2010

• #13

What year is this a '00? I am pretty sure, unless he puts the entire engine together, you still need to set the end gap.

 

R

rmc_olderthandirt

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Apr 18, 2006

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• Nov 24, 2010

• #14

Are you sure you had the rings placed over the centering pins properly?

I wouldn't expect that you would get the piston into the cylinder with it off but if you somehow managed then the end of the ring could end up really gouging the cylinder wall.