All 2st How many hours before your first new set of rings?

Iv done only about 35 hrs on my wr300 since new. Most of my riding is in heavy sand and I avoid red lining it as our rides last for around 2hrs at a time in pretty hot conditions.

To date it's running as strong as ever.

Funny you should ask that. I have a 2010 300 with about 40 hours on it and was thinking the same thing. Still plenty of power, the compression with race head is still 185 psi, I dont often ride with the throttle pinned. It is mainly ridden in the woods with very little dust and appears to be running great. I'll probably take the barrel off soon and have a look, guess while I am in there change the ring.

Pull the pipe and look how they are sealing and wearing.

What are the OEM piston and rings in the 300's?

LD is correct, inspect the piston and see how it's wearing or even pull the head off and read the crown but compression measurement is easier.

On the 360, my last topend was at 100 odd hrs with a comp at 158ish...this is after l was running with a leaking base gasket for over 20hrs since a full engine rebuild (hot seizure due to leaning out). Before l did a full engine rebuild, the last top end was done over 200hrs ago when the comp was down to 140psi.

Anyway, if you are at WOT 50% of the time when you are riding, then 80-100hrs is around right but you must do a compression test as this and indicator whether you need to redo the topend...hrs are a greater marker for the owner to compare something to.
Example:
Windsurf said that his 300 is 185psi@40hrs, using an hr meter, if he measured the comp at 100hrs and read 175psi, he rides tight single track using WOT maybe 10% of the time with no issues mechanically, then this indicates that the motor is still running at a very good operating range and the top end should be down in another +/-80hrs as he is losing .166psi per hr.
Now, Sandgroper OTOH, measured his 300 at 185psi@35hrs then at 100hrs the comp read 140psi but Sandgroper rides at 50% WOT due to the deserts he rides, then this would indicate that the motor is at above average condition but is due for a top end rebuild within 10hrs of operations as he is losing 0.535psi per hr.

I'm sure there is an engineer on the forum that will completely flame me on my calculation but that's how l roughly determine the length of time between rebuilds but this all centres around you starting comp and the end comp (you can even do the same with a leakdown test but you need a starting point like a fresh, new or rebuilt motor).

Thanks for the indepth replies fellas. All taken on board and I will look inot each of these and see what prevails.
To be honest tho, apart from scoring in the cyclinder barrel I dont know what to look for if I pull the head off

Cheers
S

I changed rings and piston at 70hrs.
Compression was still good and rings were holding well, but the intake window wore into the piston and was making me nervous.


This is a woods bike and does not see very much WOT.

If rings are worn and have worn the ring groves on the piston the rings will have up and down movement and there will be flame travel stains below the top ring and if really worn the second ring. When things are scored you have gone too long. PC.s piston pictured above still has excellent ring seal. If PC.s wear markings on the skirt are deep enough to feel or catch a fingernail on the piston should be changed however if the markings are only polished wear marks I would have left that piston in longer. (no way to tell from a picture)

+1 on doing a compression and leak down test but it must be done in the beginning AFTER the rings have seated and it is optimum and then compare to that number from then on. This is because new rings that have not seated will have poor leak down numbers. Also a quick way to tell if rings are bad is compare your dry compression readings to a reading taken after a small squirt of oil in the spark plug hole. If the reading jumps more than 15% it is a sign of a tired or not yet seated set of rings.

This is my last piston and ring change.
http://www.cafehusky.com/threads/piston-diagnosis.17196/#post-156823

My next change will be a complete replate of the cyclinder wall and bored to the 'B' type spec Wossner piston.

Diagnosis and doing a piston and ring change on a 2t is a piece of piss, majority of the time you can just replace the rings before doing an entire piston and ring.