KX125 piston & cylinder question

[Oblivious Maximus]

  Hi all.

 First off, I hope your summer's treating you well.

 I have a 99 KX125 which we've put around 15 hours on since I bought it cheep last fall. Last ride it had a knock down low so we towed it home, tore it down and found the big end bearing to be bad. Piston (B size) and rod looks oem and the plating is gray and thin with a small area above the exhaust port bridge worn through but no gouges. It's going to Power Seal for replate and hopefully they can fit a B size Wossner piston which I'll send along. I'll also use Wossner rod kit, ProX mains & seals and Cometic gaskets. Just a weekend warrior for visitors and Grand Kids.

 My question has to do with the scuffing at the exhaust port bridge. I just got a friends 2002 125 cylinder back from Power Seal. This one is bored 2mm over and had the same scuff marks, with some plating chipped off the top end of the exhaust bridge and the ring was seized into the Wiseco piston at the same area. I've read that this is a hot spot but I'm wondering if it could be fuel, intake, filter related or more likely left unmaintained too long? I have no idea how many hours are on these bikes since last rebuilds.

 On the 02 cylinder which I just got back the bridge has been undercut a bit for expansion which I didn't notice before.

 And finally, my first dealing with PowerSeal USA has been all good.

 

 

[treedodgingfool]

I can't speak for the 125 engine specifically, but I've rebuilt way too many 250's. I have  KX250 cylinders with the original factory plating still on them (and not worn through) and around 6-10k miles, but that's because of frequent air filter cleanings & regular piston/ring changing intervals.  If you keep the airbox & filter spotless (I always run my hand down the boot to make sure I don't feel any blow-by) and the piston/rings are changed before they need it (before a noticeable loss in power, etc), the coatings will hold-up well. The only issue as you drop displacement is those service intervals grow closer together due to the nature of how those bikes are ridden.  Also, big-bore kits change the wear patterns too, generally increasing it. On my KX250 as a 300, on every other piston/ring change I need a replate since at that displacement I notice more plating wear. PowerSeal is my go-to for cylinder repair & plating too, excellent guys & good work! They have done the plating on our last 9 cylinders.

[KXDINO]

The exhaust bridge is highily acceptable to wear kx125 and yz125 , on our kx125 08 i found by drilling the lube holes on exhaust side by a extra 1mm helps with life , but you need to change the single ring every 3 hrs if you are pro speed level and piston 6 to 9 even with the best oils and filter maintenance , it 125 for you , i found 250f are cheaper to run if they dont grenade , keep a eye on the  power valves as well as to much wear can lead to breakage , i had one that broke but stayed in the open position , the rider said it was a bit harder to get out of the gate with , hear in australia you need to run big bore kits in the 125 or you never keep up to the ktm 150 at state level , they run a separate class for the small bore 2 stroke which is good , just be prepared to do a lot of top ends unless you want to design a cylinder that does away with the bridge port and use a 2 ring design piston.

[Oblivious Maximus]

Thank you both for the replies. What you've said reinforces my thoughts. These are just woods bikes but, smaller displacement motors need more frequent servicing. I have no idea of the time on my 99 motor as it was passed down through a family a few times then stored for a 6 or 7 years before I bought it. The 02 was a young mans race bike. My adult friend bought it 3 years ago and has done nothing but change tires, trans oil and put fuel in it since. The enlarged lubrication holes make good sense to me and pistons & rings are cheaper than cylinders. Thanks for the info on the power valve too. I'll add a compression check column to my maintenance log and take a peek into the exhaust port from time to time. Oh and DINO, I'll get to work on that cylinder design. Problem is I only work with wood.

[KXDINO]

One other thing get a hour meter , you be surprised on how quickly the hours clock up .

[Oblivious Maximus]

I keep a close watch on my old 87, but with multiple rides a meter is a good idea. They say, "Time flies when your having fun."