Determining Cylinder Bore?

["Dangerous" Dan K]

I just pulled the cylinder head off my 02 KX125. It needs a top end, and I wanted to make sure it's stock bore before ordering anything. The only # on the piston is 0259C, and a google search comes up with nothing useful. I think I've been told that the cylinder will be stamped with a size if it's over-bore, but I'm not sure.

I have a set of digital calipers at work I can bring home tomorrow to measure the cap (or I could just bring the cap to work) but I'm not sure how accurate that will be, if it's been bored over just a touch. Any advice?

Thanks,

Dan

[obsolete]

I'd borrow or buy a bore gauge. It's gonna be the most accurate.

["Dangerous" Dan K]

Thanks for the tip. A buddy of mine just inherited a shitload of tools from a mechanic friend, many motorcycle specific. I'll have to see if there is a bore gauge in there somewhere.

["Dangerous" Dan K]

No bore gauge in the collection. Oh well. I think the new plan is to bring the piston and head to my local shop, have them check it and order me a new top end kit. The piston is beat, hoping the cylinder is OK. One thing that strikes me as strange is that the top of the piston is really, really scored up. The sides don't look bad.

[Paul_E_D]

Did a circlip break? How's the head look. What does the scoring look like?

["Dangerous" Dan K]

I don't think a circlip broke, but I'm not 100% sure yet. head is off, piston is still on. I'm going to go pull the piston, I'll post pics of everything when I get it inside. The head looked ok from what I could tell, but it's yet to be inspected really close under a very bright light. More info to come in however long it takes me to pop a circlip, get the piston off, get the piston and head inside, clean them up and get some pics posted.

The scoring looks like someone was trying to bang the cylinder into the head with a screwdriver and a mallet, just a bunch of flathead-ish marks on the top.

I'll have my netbook in the shop with me as I work, so any last-minute advice or input is always appreciated.

["Dangerous" Dan K]

Update: Cylinder and sides of piston look fine. One small scratch on piston side I can catch with a fingernail, but overall the side is fine. Cylinder appears to be mint.

The top of the piston is way worse than I thought, and there are matching marks on the cap (don't know why I didn't think to look there when I saw the top of the piston the first time).

Pics to follow as soon as I can get some decent ones, which is proving to be less than easy.

[obsolete]

Sounds like it needs to go to a machine shop, no need to fuck around when you have mechanical damage.

["Dangerous" Dan K]

Here are the pics. I'm no mechanic, nor do I play one on TV, but I'm pretty sure I can file this shit in the "not good" category.

["Dangerous" Dan K]

Sounds like it needs to go to a machine shop, no need to fuck around when you have mechanical damage.

Thanks for the reply. If it comes to that, I can probably order a big bore kit for what the machine shop and new overbore top end (plus probably new cap, we'll wait to hear what the guys in the know say) would cost. Hop her up to 145cc and bolt on replacements to everything you see in the pics. I'd like to get out of this for less $, but I'm not optimistic after looking at that cap and the top of that piston. It'll be interesting to hear what people say now that I've got the pics up.

[Paul_E_D]

That looks mostly like detonation. The bike was run lean for quite a while. If there are no broken parts, like the plug wire, ring, or circlip, than it's definitely purely dets.

The head cam be fixed by a machine shop. I would have them measure the cylinder for roundness and/or bellmouthing. Might be time for a replate.

["Dangerous" Dan K]

That looks mostly like detonation. The bike was run lean for quite a while. If there are no broken parts, like the plug wire, ring, or circlip, than it's definitely purely dets.

The head cam be fixed by a machine shop. I would have them measure the cylinder for roundness and/or bellmouthing. Might be time for a replate.

Thanks for the reply. I guess the question is will the machine shop + top end kit be as much or more than a big bore kit?

No broken parts at all. Plug wire looks good, circlips were fine and both in there, etc. Aside from the top of the piston and the cylinder cap everything looks good. Even the cylinder itself and the sides of the piston look good.

[SSearchVT]

Do a check online to see if oversized piston and rings are available for your bike. If the cylinder has a coating there's a good chance it might not be borable.

The damage in the head and on the top of the piston is from detonation. Take the carb apart and make sure it's clean (always a safe bet on a 2 stroke), make sure the right plug was in the bike, and that the timing hasn't been messed with (might not even be adjustable). You might get lucky and be able to get the head machined so it can be reused, but most of the time it needs to be replaced. A word of warning about the big bore kit - there's already something amiss in the engine - it won't go away by itself.

[Paul_E_D]

Don't bore it. Fix it. Replate cylinder (if needed), regrind head, new top end, check jetting and timing. Ride and enjoy. Send the head to someone who works on lots of kawi 2strokes. I'm sure that can be cleaned to an acceptable level. You're looking at about 300 bucks +- to get things sano.

["Dangerous" Dan K]

Do a check online to see if oversized piston and rings are available for your bike. If the cylinder has a coating there's a good chance it might not be borable.

The damage in the head and on the top of the piston is from detonation. Take the carb apart and make sure it's clean (always a safe bet on a 2 stroke), make sure the right plug was in the bike, and that the timing hasn't been messed with (might not even be adjustable). You might get lucky and be able to get the head machined so it can be reused, but most of the time it needs to be replaced. A word of warning about the big bore kit - there's already something amiss in the engine - it won't go away by itself.

Thanks. The carb is off and slated for a tear-down anyway. There are overbore pistons and rings available for it. It's an 02 KX125. I'm not sure about the timing, I'll have to see if it's adjustable. Kawasaki lists 4 plugs. Standard is NGK BR9EIX, with R6385-9P, BR8EIX & BR10EIX as "options". The plug that was in there was an NGK R BR9ES - not any of the plugs listed by kawi. Possibly the problem? That was the plug that was in it when I got the bike and I never thought to check it. I've even replaced it, and just replaced what was in there.

I won't treat the big bore kit as a fix, I definitely need to find out what caused the problem. I am just wondering if it would be any more expensive than an overbore top end, all the gaskets & whatnot and sending what I've got out to a machine shop. Any chance that cap is salvageable?

["Dangerous" Dan K]

Don't bore it. Fix it. Replate cylinder (if needed), regrind head, new top end, check jetting and timing. Ride and enjoy. Send the head to someone who works on lots of kawi 2strokes. I'm sure that can be cleaned to an acceptable level. You're looking at about 300 bucks +- to get things sano.

I wasn't referring to boring what I have when I said big bore kit, I meant ordering an entire new top end that's already overbore with jetting, etc. i.e. Athena kit. We're talking twice $300 though, so if I can get what I have fixed in the 300 neighborhood that'd definitely be the way to go.

Appreciate all the help, I'm definitely in over my head on this one. It's not looking like the simple top end kit I expected it to be.

[6 Fingered Man]

Knowing the background on that bike it could be that it went WAY to long with out proper maintenance, it should have had a top end a while ago. Could have had some shit get into the combustion chamber at some point. I would wait to change timing or jetting until you have the new top end in. You will be able to tell if the jetting is right after you run it. And the compression will effect how you want to time it.

[SSearchVT]

The BR9ES plug certainly didn't help things - the EX plugs have a much finer electrode, and gives the engine the ignition pulse it was designed for. Do a quick check on the jets you pull out of the carb to make sure they are in the right range (a lot can happen to a 2 stroke in 7 years). If the previous owner put a pipe or silencer (or both) - check to see what the recommended jetting is for them.

["Dangerous" Dan K]

The BR9ES plug certainly didn't help things - the EX plugs have a much finer electrode, and gives the engine the ignition pulse it was designed for. Do a quick check on the jets you pull out of the carb to make sure they are in the right range (a lot can happen to a 2 stroke in 7 years). If the previous owner put a pipe or silencer (or both) - check to see what the recommended jetting is for them.

It has both. FMF Platinum pipe and FMF Q series silencer. Can't dig up the getting info on their site, I'll have to give them a call tomorrow.

["Dangerous" Dan K]

Knowing the background on that bike it could be that it went WAY to long with out proper maintenance, it should have had a top end a while ago. Could have had some shit get into the combustion chamber at some point. I would wait to change timing or jetting until you have the new top end in. You will be able to tell if the jetting is right after you run it. And the compression will effect how you want to time it.

A good point, and something I should have mentioned. I got this bike from a kid who knew nothing and had gotten it in trade. I think he had it 2 years, adn did no work at all. He did burn the rear off doing burnouts in his driveway, I think that was about what his riding was about. God only knows how many hours were on this top end - that's why I tore it down in the first place.

["Dangerous" Dan K]

I'm not sure how this managed to slip my mind for so long, but I just remembered this morning that this bike also had a serious cooling issue a while back. It was overheating something fierce after a little bit. I flushed the radiator and coolant hoses, pulled the side cover and impeller, gave everything a looking over and put it back together. It has not overheated since, but it has not had a real long hard ride since either. Anyway, I’m thinking that could have contributed to the damage.

[drop]

I'm not sure how this managed to slip my mind for so long, but I just remembered this morning that this bike also had a serious cooling issue a while back. It was overheating something fierce after a little bit. I flushed the cooling radiator and hoses, pulled the side cover and impeller, gave everything a looking over and put it back together. It has not overheated since, but it has not had a real long hard ride since either. Anyway, I’m thinking that could have contributed to the damage.

running too lean will make the engine run hotter. the overheating might actually be a symptom of your original issue, rather than the cause.

["Dangerous" Dan K]

running too lean will make the engine run hotter. the overheating might actually be a symptom of your original issue, rather than the cause.

Yet another angle. Thanks for pointing it out, it's something else to watch and consider when I put this thing back together.

It's funny that the old timers and woods riders/shade tree mechanics I know all say "pfft, that doesn't look bad, if there's nothing in the cylinder to catch a fingernail on replace the piston and ring and ride it" while the guys who are apparently more into their bikes and generally ride more complicated machines are trying to help me determine the cause of the issue to keep it from happening again, and thinking along the lines of machine shops and new cylinders. I wouldn't be in the least bit surprised if the shade tree mechanics were right and I could just scuff the one male divot off the head, replace the piston and ring and run it.

Thing is, whatever caused it to happen this time would happen again, and I'm not a fan. Of course the bike got some insane amount of hours as it was without ever really blowing up (couple seasons at the least). Still, if I'm going to take the time to do something (which apparently I am, based on the stack of top end parts sitting next to my desk at the office) I figure it's worth doing right, assuming cost/benefit doesn't get unreasonable.

It'll be interesting to hear what a shop has to say about the cylinder and head, but I'm actually more curious as to the cause of the issue. Jetting, timing, plug, overheating, or just a gazillion hard hours on a 2 stroke top end? I'm guessing a combination of at least a couple of those things.

[Paul_E_D]

The guys you're talking about are the same one's who sell a bike instead of doing a simple top end, and ride around jetting issues instead of sorting out a sweet running 2-stroke. They likely don't even know what they are missing.

Overheating didn't cause what happened to you bike, although now you better make sure to fix the head as overheats tend to warp them causing poor sealing

["Dangerous" Dan K]

The guys you're talking about are the same one's who sell a bike instead of doing a simple top end, and ride around jetting issues instead of sorting out a sweet running 2-stroke. They likely don't even know what they are missing.

Overheating didn't cause what happened to you bike, although now you better make sure to fix the head as overheats tend to warp them causing poor sealing

Nah, they are guys that do their own work and do two stroke top ends somewhat often. Can't speak with regards to the jetting thing though

It will suck if the head is indeed warped. Is that something I can check myself, can my local shop do it or does it have to go to a machine shop? I'm wondering if I should just send the thing out for a rebuild, resleeve, whatever it needs at this point. I was more than ready to do a top end kit, but this is getting pretty involved. I'd like to do whatever I can myself, both to learn and to save $, but it might be getting to the point where sending it out would be cheaper.