Cylinder--Damaged?

[Mudboy]

I'm doing the 2nd top-end in my 99 rm250. After pulling the piston, it has some pretty significant grooving on the intake side. Inspection of the cylinder reveals that the intake ports are pretty sharp--kind of like a knife edge--where they enter the cylinder.

Can this just be carefully smoothed back down or do I have a real problem. The rest of the cylinder is perfect except right where the two ports enter (on the bottom side).

Also, what do you think caused this? Not letting the beast warm up enough before getting on the pipe?

Thanks for any advice.

 

[smarttoys]

All of your ports should have a chamfer or slight radius of the edges, otherwise you will get some wear on your piston or worse. Remove the top end and slightly chamfer the ports that have sharp edges. None of the ports should have sharp edges in the bore. A sanding roll on a die grinder will do the trick. Be carefull that you do not remove too much material. If you feel uncomfortable doing this, take the cylinder to a competent mechanic. You should be able to get this done very reasonably. If it were in my shop, I would inspect it and do it for free. But that's just me.

 

[Mudboy]

Thanks for the advice. I think I'll give it a try on my own (very carefully!).

 

[EricGorr]

Every late model RM250 with the two intake bridges will scuff the back of the piston because of a poor cylinder design. No amount of oil holes in the piston, or relieving of the bridges or polishing will make any difference. The only way to fix it is to heli-arc weld the top inside corner to a larger radius (from .5mm to 10mm). Of course this requires replating. I originally worked on this problem when I ran Max Power and did warranty work for Wiseco. We tried all sorts of piston shapes and port mods all to no avail. We also made Suzuki aware of the problem but they just blew it off. You see, because of this problem they sell lots of $100 pistons and $500 cylinders. Thats the economics of dirt bikes, sell the bikes cheap, design them to wear out parts, and charge a fortune for the replacement parts.
Since 1999 I've fixed lots of RM250 cylinders in this fashion and haven't had 1 come back with the intake bridge scuffing problem. The fact is the old style oval intake was a reliable design and they never should've changed it.
Kawasaki had a minor problem with they're cylinders and quickly changed the casting core, width and radii of the intake bridges on the KX250.

 

[Mudboy]

Eric: Thanks for the reply! I was hoping this would get "the master's" attention, but I also know you are a busy man and didn't want to directly solicit your advice. How much $$ are we talking for the mod you mentioned?

 

[smarttoys]

I guess it helps to have over 30 years experience working on all the brands. ;)
Eric is a busy man but he always seems to have time to answer my questions. I usually try to keep it short and sweet on the phone though.