I am about to install my third new piston in my 86 stock 250R. OEM piston lasted 13 years. When original gave up the ghost a local shop installed Wiseco (10 over). That piston experienced a problem 10 minutes into first ride. Motor bound up in a condition that has been described to me as "cold siezure". That piston hung in there until cylinder scoring reduced compression to about zero. Total time on that piston was about 30-40 hours.

Next piston (installed it myself, Wiseco 30 over) ran great for about 5 hours. Third riding trip out with barely warm motor this piston experienced the exact same failure as the 10-over piston. Rings fused into ring-slots, cylinder scored, no compression. Damage is completely localized to the exhaust side of the piston/cylinder on both failed pistons.

Condition is almost like a soft-sieze (motor continued to turn but compression shot). No vibration is evident in motor as failure event occurs - it just feels like motor is binding up and I'm dragging a boat anchor. Damage is done before wheels come to a stop.

Before I put another piston in I need some advice. Anyone have any theories on what could be the cause here? Bent piston rod maybe? Worn crank bearings? Insufficient break-in? Bad mix? (I'm using 32:1 Yami-R). I know I'm grasping here but this one has me stumped.

 

I had a friend who was having trouble with his Blaster piston blowing out. He had a new connecting rod put in because the large rod bearing was worn.
I own the machine now and it is still on that same piston 4 years later. Although, my wife rides the thing and does not beat it to death. Sometimes I abuse it a little, it is still going strong. I think the lower rod bearing was the culprit.

You had to increase the jetting after the bore job right?
Is the shop properly chamfering the ports after the bore?

later
phil

 

mberry, did you look at how I described how to break a motor in in the 400EX break-in post, if not I suggest looking at it. Thats exactly how a 250R should be broke in. Also you mentioned "cold seizure" and it happening with a barely warm engine, thats exactly what a cold seizure is. You have to get the engine to operating temp by warming it up before you even put it in gear, otherwise no matter what piston you use, your going to keep killing it. Even after it's bfoke in you have to warm it up, in otherwords ALWAYS WARM IT UP. A liquid cooled motor is more susceptible to a cold seizure than an air cooled one, reason being is the when first started the exhaust side of the cylinder in much hotter than the intake side, so if load is applied (riding) before the entire cylider is the same temp, the piston will rub against the the hotter side (exhaust side) of the cylinder, thus a cold seizure.

If you are wondering, Wisecos are very good pistons, I have always run them with no trouble, but again if it is not properly broke-in and then warmed up to operating temp before every ride, no piston will last. For example, my 330 cylider is much harder on pistons than a stock cylinder because its much harder metal, but I am still on the same piston after a year of racing and riding as much as 200 miles a weekend at times with no trouble, only 2 sets of rings have been installed. With a Wiesco piston by-the-way.

I know when you want to ride its hard to resist taking off right away, but you have to be patient for about 5 minutes and let that thing warm up. What I do is unload my bike, then I start it and let it idle as I put my gear on, then I rev it up a bit to be sure it's warm (You feel the cylinder, if its hot, then you can ride it), and then I ride, no problem.

I noticed somebody mentioned warn rod bearings being a possible cause, yes that would do it, but any mechanic should be able to tell what the cause was by looking at the piston, because different things cause different types of damage to the piston. Besides if a bike mechanic rebuilt it the first time, they would have checked the rod, all you do is check for any slop if you pull on the rod, it there is, then the bearings should be replaced. If they said it was a cold seizure, then that probably what it was, if your pistons and cylinder are scuffed mainly on the exhaust side, then it is definitely a cold seizure.

Hope this helps,

Sickman

[This message has been edited by Sickman (edited 11-08-1999).]

 

Did whoever did it take some extra material off the exhaust bridge in the cylinder when bored out, like Wiseco recommends? I think it is .006".. my guy instead takes that much off the piston itself, the width of the bridge, which he says works better with Wiseco pistons.. Besides that you shouldn't get cold seizure if you warm it up properly. Wiseco pistons will have tighter than stock tolerances so make sure it is warm before you rev it up, and have proper jetting and cooling. I break mine in with 20:1 oil for 2 tanks of gas.

Rod - 86 250R

 

Another thing; when I was putting mine back together, I almost didn't notice it, but the base cyl gasket didn't have the holes cut out of it for cooling. I would imagine putting it together this way would have very grave consequences.

Rod

 

I know what your problem is...Wiseco. 80% of the people I have talked to that have used one will never again. Good 4-stroke pistons though. You shouldn't have to file anything off your cylinder..get a stock cast piston.

[This message has been edited by roostmonkey (edited 11-08-1999).]

 

I have a question for you. Is the same shop doing all the work here? It could be that this shop is setting up the clearances too tight. For a suggestion, jet up a couple of sizes before breaking it in next time. The jetting could be so lean that you're runing a lot hotter than you realize. Although wiseco pistons are fine, you may consider one from Niks Pro-X. THey cost about the same so why not?

 

First of all - thanks for the many knowledgable replies and suggestions. My confidence is returning that I'll get my beloved 'R' back in action fast. In a moment of frustration I was wondering how much I could get for it to put towards a new 400EX - but I've regained my senses.

Some more background on the chewed-up piston saga: The first of the two Wiseco pistons (the 10-over) was installed by a local shop. The second of the two (the 30-over) was installed by a friend (much thanks Kenny) and myself. On the second piston, the 30-over, the machinist who did the cylinder explained that he had seen this type of failure before; all wear/damage on exhaust side of piston - he was first to identify it as cold-siezure and suggested that perhaps the first bore was just a bit tight. This machinist then explained that the 'spec' numbers sometimes create a tight fit. He bored the cylinder "just a bit" over the spec number to be sure to give the new Wiseco piston room to expand.

In niether case did I have the exhaust bridge shaved or ports chamfered - embarrassing as it is I didn't know this was required.

Also - niether the shop nor my buddy and I installed a bigger jet. Again - with such a modest increase in displacement (just 30-thousandths on the cylinder) I didn't know that upping the jetting was required either. I'm still learning - costs me about $165 in parts for each lesson.

SO - Any recommendations on how much to increase the jetting for a "soon-to-be" 40-over bore on an otherwise totally stock 86-250R? What about that exhaust bridge shaving and port chamfering - any second opinions there?

Thanks again to all who have already replied. It is much appreciated.

- mberry

 

As far as jetting, what was it like before, jetted a little rich, spot on or not sure. You shouldn't really have to increase the jetting if it was a little on the rich side, reason being you don't want it too rich that you load up the engine, which is what you want to avoid as much as over-reving during break-in. If you do richen it up, just dropping a bigger main won't help, because you don't hit it until about 3/4 throttle. A common misconception when somebody blows up a motor due to a lean condition is usually caused by the main jet being lean. Even though that may be, more often the failure as actually caused by the needle or pilot jet being lean, so thats why keeping an eye on jetting and checking every throttle position is important.

What pre-mix oil are you running. For break-in I'd recommend using either Yamalube or Motul. I actually run Redline, but it is so slick it won't let the rings seat, so we break-in motors with either Yamalube or Motul then switch to Redline in warm weather. In the winter I've heard that the best oil is Maxima Super M because it doesn't sepparate in cold temps.

As far as gaskets are concerned, you should not have to cut them unless you run a big bore and have to trim excess gasket due to machined cases. Killefitt mentioned holes for the base gasket not cut, well stock ones aren't cut for a reason, so they shouldn't be cut out. You may actually create a coolant leak by doing so, and if it gets into the crank case, well you can imagine. I've seen what happens when too much coolent makes its way into the engine, it makes a loud boom, and blows the pipe off (bending it up), which is pretty cool to see when it's not your bike.

Hope you get the problem solved this time

[This message has been edited by Sickman (edited 11-10-1999).]

 

Sickman, the stock base gasket I bought had no holes at all in it, only the ones for the studs to poke through.. so it definitely needed some holes. I made them all the same size as the cylinder holes. My head gasket (cr250) had some smaller holes in the back intake side and larger ones on the front exhaust side (even though the head itself has full size holes all around) I left this one the way it was.. better cooling on the exhaust side I guess.

Rod