wr engine problem please help

[nimiz1]

yesterday after a 2 hour ride the engine did not start and there was no compression , i opened the engine today and saw that the rings were stuck to the piston , there was no heat problem or lack of oil or water ,the cylinder looks very good with no damage to the piston also so did everithing alse.

why did this happened to the rings

i like to hear your suggestions please.

it is a wr-400 1998.

[Matty05]

It is an old high performance bike. Maybe it is just time for a new piston and rings?

I bet you haven't changed them since new!

[BIGBLUEKNIGHT]

matty what happened? the bikes no longer say mine and my girlys

[black_n_blue_thumper]

She left him last week for being awnry on TT. ??? Just a joke, thats all, a harmless joke.

[TeamScream]

yesterday after a 2 hour ride the engine did not start and there was no compression , i opened the engine today and saw that the rings were stuck to the piston , there was no heat problem or lack of oil or water ,the cylinder looks very good with no damage to the piston also so did everithing alse.

why did this happened to the rings

i like to hear your suggestions please.

it is a wr-400 1998.

If it is a high hour engine, then what may have happened is "hammering"

The rings fit in the ring grooves with very little clearance, however there is clearance.

So as the piston moves up the rings via friction do not move until the bottom of the ring groove in the piston push it up, and vise versa as the piston moves down the ring sticks to the cylinder until the top of the ring groove moves the ring downward.

After many many hours of this movement the ring actually starts to hammer on the groove since the ring surface is harder than the groove surface it starts to distort the groove ever so slightly which (theoretically) could cause a ring to stick.

Conversely if the piston makes any contact with the cylinder wall (sans the oil barrier) it is possible (but highly unlikely) that the ring groove is distorted via the drag induced by the piston itself.

The last theory is that the ring itself gets distorted as a result of the ring groove/ring tolerance anomaly and that the outer surface of the ring develops a burr around the circumference of the ring and then like a fish hook sticks in place once it is forced into the groove during an exhaust stroke cycle.

The reason it most likely did not happen during a power stroke cycle is that the rings are actually forced outward toward the cylinder wall due to the gasses getting behind the ring itself forcing it outbound.

Any of these scenarios usually happen after a piston shrinks (again due to many many cycles, and generally are a result of "hammering" or piston slap). The OEM Yamaha pistons are relatively immune to this shrinkage but it can happen on high mileage/cycle engines, it is extremely common on after market piston like JE and Wiseco, which is why many builders run much tighter piston to wall clearance tolerances with these aftermarket pistons than are used with OEM builds.

As an example my current WR engine is being built with a piston to wall clearance of .030mm or .0012 in. which is about half of what you would use if you were using a standard bore and an OEM Yamaha piston. The theory (proven) is that once the after market piston does it's shrinking it will settle in near the usual tolerance and generally live there for the rest of it's life.

This is just me thinking out loud, you asked so I let my imagination loose and here is the result.

If you can get the ring out of the groove and look at it with a magnifying glass (or better yet an optical comparator) you may find a clue, I would also compare the bad ring groove(s) to the grooves on the new piston and see how much if any the groove(s) enlarged or distorted.

No im not stoned.

Regardless of what you do DO NOT assume the bore (cylinder) is ok, you may have run-out that exceeds acceptable tolerances and your (re) build may go bad sooner than you think.

Now also would be the time to consider getting a 426 cylinder and OEM piston and turning that 400 into a 426.

I would seriously consider new lower end bearings and a new rod while you have it that far apart.

[nimiz1]

thank you all

i used an OEM piston and the engine was rebuild only 1.5 years ago with the lower bearings and a new rod.

i am going to put now the wiseco piston kit and gaskets and hope averithing will be ok.