Time for a new piston?

[MuppetMule]

My bike is a 2003 RM250 and is still on the original piston and rings I believe.

I just bought it and have the original spares kit AND another full new piston and gasket set.

Do you think it would be a good idea to give her a change, Im guseeing it has done anout 25 hours work max. It starts very well and appears to have loads of power, but it is new to me.

Also the pistons have a yellow paint mark on the top, does this indicate what size piston from factory tollerance it is?

Final question, one piston is flat and the other is slightly domed, why, which is factory, the flat one?

Thanks

 

[nickyd]

only one way to tell - pull it and measure.

 

[MuppetMule]

Do you mean a compression test?

 

[nickyd]

a compression test will help you gauge the top engine's condition - buts its not an exact science. I would do a test and write the number down - then test every couple weeks/months etc dependant upon your riding schedule and keep an eye on its deterioration....If you have excess carbon buildup however, it can skew the reading....typically I'll do a top end and take a compression reading as a baseline measurement...then from there on cycle as I mentioned and keep notes...when it reaches my wear limit I replace....MUCH cheaper to replace sooner rather than later. I meant pull the cylinder and measure the piston and cylinder to make sure its within spec.....for instance...my RM had great compression - ran insane....never a problem..BUT the piston was in there prior to me buying it so on a whim I pulled the top end and found a hairline crack up the skirt - measurement wise I was not yet out of spec but I was about 2 rides away from a major rebuild. I'm not trying to scare you - just trying to make you aware....do a compression test and then pull the reeds and pipe and take a peek through the ports at the piston if you want to see how she's doing. you can see some evidence of blowby etc through this method...is it accurate? Not really - but it may put you at ease. hey - if you pull the top end and its all good - a new top end gasket kit is small insurance against disaster.

 

[MuppetMule]

Thanks, I am just getting used to all the service requirements one these bikes, like you say, better to be safe than sorry.

I just need to get to know my bike, and start to buy some of the special tools.

 

[RM_guy]

I wouldn't go longer thatn 25-30 hours on a OEM piston. They are cast and may be in spec (as the above post indicated) but can shatter if you leave them in to long. I always put in a Weisco piston. They are forged and much stronger and will last easily twice as long as the stock piston.

I'm glad to hear that you want to learn and get the right tools. That's the right attitude.