Maintenance & Technical > KX500 Original

Cracked cylinder

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greencannon:
If your racing that thing then junk the cylinder and find a stock sleeveless cylinder and have it plated or coated from US Chrome or Max Power. That stuff holds up way better to race loads than a sleeve.
Did you figure out why you had a meltdown to begin with ? lean jetting will destroy a top end and or cylinder, maybe water pump failed. Are what appears to be cracks really cracks or casting marks ?
get some pics, the guy's on this sight are a wealth of help

ktmdude:
It is a stock sleeveless cylinder, and it is definitely cracked. I said "sleeve" for lack of a better word. We think it was jetted well, not sure about the water pump, I'll have to take a look. There was a couple of bad bottle necks on the harder rock garden sections. It boiled over at idle. Think it melted due to lack of water. I'll try to get some pics later, I'm a lot better on a bike than on a computer, I'll see if the gf can help me. lol I'm thinking of starting fresh with new (used but in good shape).

The rod feels good, no up and down play. Do you think that anything could have happened to the bottom end?

greencannon:
Maybe the plating is worn off the cylinder wall in that spot or maybe real thin due to age , that would cause major piston failure and overheating. you did say it was a 1991. do you know if it's ever been replated ? if not thats a long time on the junk factory eletroplate. Maybe the piston and rings were too aged to handle race loads. any idea on the age of those ? I once was stuck at a bad bottleneck at a GNCC race for porbably 10 min at idle with mud packed around motor and radiators and did not overheat. I was surprised I didn't break down, Four stroke bikes were dropping like flies. The steam from boiled over bikes was like a swat team smoke grenade. Most of the two smokers made it thru.
If your crank still cycles smoothly and has no noticeable play, run it....
get a new cylinder, im sure someone here could get you one for short money, once again go back to the water pump for certainty and double check jetting once you get cylinder issue fixed.
 I'm surprised no K5 experts have chimed in yet ??? anybody ???
Hope some of this info might help as I once experienced this with an older bike also. my problem ended up being worn plating and poor jetting ( the bike was an 87 K5)
Keep us posted with your results and good luck
PS - just thought of this, check your stator plate position to make sure it didnt move from normal setting maybe the screws loosend up and the timing advanced a little and overheated

ktmdude:
It had 35 hrs on complete rebuild, top and bottom, and fresh dip. He was running the some jetting that he ran last year at the Rubicon, witch is the same elevation as the race that we did. About the same temp. also. The bottle neck was just like how you described, 4st blasting tear gas, But rich 2st exhaust was pretty thick also. I was glad to get threw it. I re jetted my 300 to the factory chart and had no problem. But this post is not about me and my bike,... he is thinking about asking the Destry Abbott site if they would sell him any old race bike stuff(cylinder and head). Get a new piston and rings slap it back together and mix up some race gas and let her fly. I will take a look at the water pump, and stator. What should I be looking for? What are your thoughts on old race parts?

P.s. We road the Rubicon last year with 10 hrs on his fresh motor, so not much had changed.

snoopjonnyjon:
My cylinder was cracked, and the welding to fix it was only $60 on top of the replating. For me this was better than getting someone else's old junk. The left side in these is rewelded, the right side is original. It looks cracked as well, but that is actually just the parting line on the casting. From you description I think you are describing a crack in this location.





As well as a crack on the two webs shown on the underside here? Mine weren't cracked, but had the same grooved out casting flaw that sure looked like they would be prone to cracking there.

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