Folks, One for the stroker Gurus out there... Attached are image of the head of my daughter's KX 65 that I just removed. I have found the damage under all the carbon I cleared off. Q - can this be saved? It ran all right prior to removal, but not the best to be honest, I had put that down to a crappy plug, now replaced, and fuelling issues but I am also about to rebuild the carb. I am going to put a new cylinder kit in but I don't want to button it al up if this is a potential failure point. I'd prefer not to have to buy a new head as I have spent enough on it allready but if it is needed its needed. If it makes a difference we dont' race it, its just for her to potter around on. Cheers in advance BTD
Looks like broken piston ring damage. I wouldn't worry if it is to be used for pottering about, as it was running happily enough beforehand. Although it would be simple enough for a local engineering works to skim the damage out, given the use described, it may not be worth the small outlay. If the engine was to be used in real anger, the high points of the damage marks might create hot spots, but even then, personally, I wouldn't worry..
I'd get somebody with a TIG to fill the damage and grind it back, but it will need a skim of the mating surface afterwards. It looks like a pin has been rattling around in there. What does the top of the piston look like? I'd imagine it's been re-used before. I'd also be tempted to just use it as is, the only danger in the real world being the holes filling with carbon and causing hot spots for pre-ignition.
Only my opinion but nothing wrong with running that as is, whatsoever from what I can see from the pictures. As Borgo Panigale said, the majority of that is previous piston ring breaks and i'll stick my neck out and say it's the trauma from two or more previous incidents. This gives you a big clue in what to be wary of when next run as some more prone to this than others, maybe on the weak side and/or timing advanced up ridiculously but could also be down to lack of maintenance at the right intervals. I'd be more worried about the grade of paper used to flat the face and how flat the mating faces are, as being water-cooled it might be the first thing to give trouble apart from mixture/timing issues.
I've seen worse on various engines (including my Son's car!) and running fine. As it's a kid's bike, I doubt ultimate performance being potentially affected by 'hotspots' is of great concern? You could get it welded or even a new head but I doubt it'd make any noticeable difference, as long as it seals OK to the cylinder (is there a gasket or 'O' ring?) I'd leave it as it is.
Definitely not detonation marks, as others wrote piston ring or other debris. Skimming that out, will drastically change the head cc volume...and possibly the squish band. I would leave as is, but would check the head cc and measure the squish.
Dont worry on the broken piston ring damage on teh head You need to pull the dowel pin and use A sheet of Glass and Emery Paper (400 then 800 Grit) to clean up the mating surface for the gasket. New Piston kit - then reassemble using a new gasket set.
Cheers folks, appreciated. It had obviously been repaired on the cheap after something gave way. Whoever did it must have hired a real Gorilla to torques the head and barrel down, it took some pounding on my spanners just to free them up, both sets of bolts were rusty and I had to hammer and pry the barrel off it had been squashed down so tight, it even pulled the studs out; perhaps it was over torqued to stop a coolant leak. Funnily enough I had expected the nicasil liner to be toast but apart from some very light scoring around the ports it seems fines, although as its all apart I may as well get it re-honed as all the cross hatching has gone and there appears to be a degree of glazing. Sadly the bike has just been neglected, constantly ridden hard and put away wet without even the simplest of regular maintenance, but then I got it fairly cheap when I picked it up last May. Saying that when I started a minor to medium maintenance schedule before Crimbo it quickly became apparent that it required a the full bhuna. I've now had the frame and swing arm powder coated, (nice job for £60!) and the engine is the last job so it should look nice, especially with all the apico bling I have procured, and go like stink when I get round to re-assembly; eventually. It has been a steep learning curve as I know hehaw about 2 strokes and have never done a strip and rebuild like this. I have found, much to my annoyance, that some of the components are so small I have real difficulty in seeing them, even in a bright garage and with my glasses on! Old age certainly don't come itsel'...to solve that probelm my next project strip and re-build will be a Cat D9, no effing glasses required!
Update - So took the top end to my local bike spot and talked to their man. He reckons th head should be fine. get it skimmed if you want but not really necessary. He suggested a light sand to remove the worst of the damage, so that's what i'll do, However, he suggested that my Nikasil liner was toast after all with some quite noticeable scoring round the ports. Seems my idea of damage and a mechanics with stroker training is wayyyyyyy off! He said I could rebuild it as is but it might damage the new piston and rings requiring another re-build, and more expensive parts. Suggested price was c£125, as long as you remove the studs because if you leave them in thy may charge you £25 a stud to remove them! Now all I need to do is find a decent outfit to do the work for me!
Langcourts in Weston Super Mare the best place in the country for re-plating chrome or Nicasil bores IME. Steve R
Langcourts or Poetons will both do it right but, you might save some cash if you can find a good, used barrel? I had a KTM 500 (2 stroke, single) on which the rollers fell out of the little end bearing and got sucked up the ports with the fuel and jammed between the piston and cylinder wall, engine seized, dead stop - I had to hammer the piston out of the barrel, it had even bent the con rod. I was considering re-plating but the shop I bought the new piston from sold me a good used cylinder for £60.
Looked for a 2nd hand unit, nae joy. Its now boxed and ready for a trip to Langcourts, £166-200ish. A new one is twice that. 3 week turnaround.
