I think more so, the piston and rings expand when they get hot. The up and down motion and heat wear on the piston and rings (instead of Shrink). Yes, if the cylinder is in good enough shape you can get away with doing a Hone and keeping the stoke bore. You must get it honed, you don't want to just put in rings and piston. Note: there are different types of cylinders that require different procedures, it just depends on what material they are made of.

The ring wears on its outside edge were it is in contact with the cylinder. This will decrease the width of the ring and allow more air to travel between it and the cylinder (less compression). On my bike the rings have a pin that keeps them from spinning on the piston. This is needed becouse the open ends of the ring would catch on the exhaust port. I am not sure how this applies with 4-strokes, anybody?

 

With the newer 4-strokes, the typical rebuild should only need a piston, rings, wrist pin and possibly valve seals. Most of the bikes are also using a nikasil liner in the cylinder, which is harder than a iron sleeve so it doesn't need bored. My recomendation is to get a compression tester and test your bike and check with what the manual says. Loss of power could be more than loss of compression, check your jetting and make sure your airfilter and sparkplug is fresh. You didn't get a bad batch of gas did you? I would hope your motor would last more than a year.

 

4-stroke top end rebuilds? How hard are you guys pushing these things?

Unless you are pro MX racer, rebuilding a top end on a 4-stoke isn't something you do very often. I've seen fine running 4-stokes with thousands of hours on them. My Buddy's got a street legal Honda XR400 with over 150,000 miles on it, and the hours are probably off the wall. He's never done a rebuild on anything in the engine and it still runs strong.

Even multi-valve engines producing significantly more RPMs than an XR400 or similar engine, should last well into the 1000 hour range if they are not abused and are taken care of. Like I said, if you race MX where RPMs are screaming all the time that's another story. If you race woods, or are just riding for fun, grunting around most of the time you can make a 4-cycle engine last a long, long time.

A guy I ride with has a 94' Warrior with an estimated 1500 hours on it, and his brother has a 400ex with an hour meter that reads in the 600 area right now, and it hasn't been on the quad since it was new. Both bikes are well maintained, and neither has even had as much as a valve adjustment. The 400ex has been ridden hard too, it's been raced several times and it's also been out to Moab, and some So. Cal dunes where he lived for a year, the Warrior admitedly has only been ridden in the woods and on fire roads.

If you ride 5 hours a week, in twenty weeks you've hit 100 hours. That's not even 6 months.

 

I agree

I have a 2-stroke and this year is the first time I did a rebuilt on it since 1998.

 

Well i will be running sand drags with whatever i get. The thing i am wondering, is simply this. I know that these new 450 come hot from the factory, they have a dirt bike engine and they are a very high reving 4 stroke. i am just wondering how many hors i can expect to get fromt eh bike before i start losing compression.