KX 80 connecting rod failures

[JonV]

Well, finally pulled apart the boys 2000 KX80 to see what failed.

Third connecting rod big end bearing failure in a row. the original, a factory crank assy and now a hot rod kitted crank.

what is the primary failure mode here? imbalance? bearing damage at assembly, poor materials? everything looked fine each time.

i would like to build it backup as a supermii spec motor but its not even making the 10 hr mark right now. i hear is is a weak link. what about a KX85 assembly?

jetting appears ok, no air leaks, no sign of dirt getting by. nothing different from last year, same lube (maxima superM) and ratio 32:1.

thanks.

 

[Ol'89r]

Jon.

Not experienced with the KX85, but, I have a whole box of toasted big end assemblys from a KX65. By increasing the side clearance on the big end thrust washers and grinding the oil slot in the big end of the rod to a vee shape does seem to help. Lets more oil in the bearing. Most of the failures on the 65 were due to heat/lack of lubrication.

We ran Yamaha lube R at 32:1. Used as much side clearance as possible. Check your manual for maximum allowed clearance.

Finally cured the problem by going to a KTM65. So far, so good.;)

 

[JonV]

My dealer told me there are upgrades like this to the 85 assemly when compared to last years 80 and 85.

it appears the thrust washer clearance does just go away with theral expansion and gag up. when we put the hot rod in we went right to the max clearance thinking that was it. this one failed different, with the radial bearing going down. everything still rotates and there is not as much thermal damage (white con rod end) only blue this time. no bearings in rod left (big yucko for the rest of the internals.

will try to dig up this post and update after we get some time on this setup.

 

[mkandl53]

JonV,
Have you seen my post about our brand new kx65 that failed the lower end rod just like your description here?

Click Here and go to reply with pics!

I noticed that the rod has kx80 stamped on the side! Your post does not give me much hope! Take a look at the pics of crankcase missmatch we had which was clearly visable. I believe that since the small kx failures seem to be plentiful on only certain bikes, the case miss-match may be the cause. I am getting parts from kaw but it has been like pulling teeth and they refuse to relace the cases even though they see the offset and the rod made contact (ground into) with the cases.

Let me know your current status! Thanks.
mkandl53

 

[ecy063]

I have had to bypass the dealer and go to the regional rep. I got paid for an engine rebuild do to a faulty oil pump.

 

[griffbones]

Check your cankcases real close! I don't know about the 80's and the 65's but the 85's and 100's have had a lot of crankcase porosity problems. There are three cresent shaped spots in the castings (both right and left cases) around the main seal area, often times these are porus and on some cases are prous all of the way through to the inside. This causes the engine to suck some air, simiallar to a bad crankcase seal, if so they have a habit of tieing up the big end of the rod! A very good two-stroke mechanic pointed this out to me and said that he had customers that had been to other shops and replaced several cranks, they came to him and he epoxied these areas and put their engines back together and they never lost another crankshaft again! I found my sons 85 had this problem and yes it did loose a crank from lack of lubrication!

 

[Chili]

Guys this is a 4.5 year old thread, I'm betting they got the issue resolved.

 

[griffbones]

Obviously I didn't look at the date, boy I feel stupid! ;)