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KX85 bottom end rebuild (MAJOR UPDATE AND QUESTION)

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Completed a complete rebuild of a 2005 KX85 this past spring. I used a wiseco piston, and hot rods crank. The bike was broke in properly and ran perfect. Son moved up to 85 class in july, and during his last moto the Crank went out.. I tore it all down, and observed the piston and cylinder to look like new, but the main rod bearing was out in the crank. Motor did not appear to have gotten hot, so what would explain a brand new crank to go out with only a few hours on motor? I have since ordered an OEM crank to put back in the bike.. I had purchased the bike blown, when i rebuilt it i observed that it had a hot rod crank in it previousely, that had the same problem, slack in the rod bearing, and binded when moved. Is this a common problem with hot rod cranks?

Edited by troy306

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Was the crank true?

I bet that something happened to make it fail.....

Edited by moose_338

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Out of the box Hot Rod cranks are good. Have had some friends who only used oem and their engine builder said they were way out. Now that might just be an excuse to get more money out of them.

I split my own motors and do the rebuilds.

Other than sucking sand I've never had a hot rod crank fail before 150 hours.

30 to 1 I feel helps.

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I've never heard of a Hotrods crank being bad. Their QA/QC is extremely strict. I did have one go shortly after a rebuild once but it was due to my son getting stuck in the mud and just revving the hell out of it to get out. It overheated and locked up.

If you rebuild your own, I highly recommend a liberal amount of 2 stroke oil poured on the big end of the rod as well as poured or shot into the small holes on either side of the cases under the cylinders that leads to the main bearings. many early engine/crank failures are due to a dry, initial start.

2strokebottomend.jpg

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I used a complete Hot Rods Crank, with All Balls bearings and seals. I installed them myself and was very carefull not to get anything in a bind. Installation went very well and was really easy. Once the cases were back together I did use pre-mix oil on big end bearing as well as oil holes on main bearings. I turned the crank with the rod and it turned freely. The bike ran great with no vibrations, for approximately 10 hrs ride time. Ran great in two practice motos, three race motos and during the last lap of the fourth, Son said it started to make a funny screaming noise so he shut it down.. And no I did not have it checked before installing. I dont have a reputable machine shop close to where i live.

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Sorry to hear of the bad luck. Guess it's just one of those unfortunate things. After reading some stuff on other sites, it seems a very few others had the same issue with Hotrods. But it's 10 fold for Wiseco and even an OEM's have had issues.

Maybe that was the problem with ours. Hard to say.

But apparently you know what you're doing if you're oiling all the needed areas. So don't blame yourself. You're doing everything right.

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Thanks, I was beating my head against the wall wondering if I had done something wrong. New Crank is supposed to be in on Monday so I guess I'll try it again.. Thanks for the advise...

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I'm a big believer in oem bearings and seals. I've had 2 seals fail in an 85 motor one sucked air and one sucked oil into motor, this was only after 5 to 10 hours if I remember correctly.

No issues with oem seals yet.

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Parts arent do in till monday or tuesday, but it will be a week or so before i get a chance to work on it. Huntin season starts next Saturday. Thanks for all the advise though..

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Big Mystery! Pulled motor today, and when draining the oil, two ball bearings about 1/4" fell out! I was unable to find where they came from. To my knowledge there is nothing in the crankcase that uses that big of ball bearings. They are about the size of the bearings inside a main bearing, But my main bearings were good, and besides that there is no way, they could have gotten in the crankcase in the first place. Im stumped! I dont want to think that someone would have put them in there, but there is no other reason for it. Please tell me im wrong, and tell me where else they may have come from.

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governor for power valve has been known to lose bearings. Sounds like thats where they came from

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Yes power valve governor, put them back in and press back within tolerance or get a new one. My kid had a bad problem of revving the bike in the air and abrupt landings hard on the gas may lead to this. Once he quit doing it no more bearings flying out.

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Big Mystery! Pulled motor today, and when draining the oil, two ball bearings about 1/4" fell out! I was unable to find where they came from. To my knowledge there is nothing in the crankcase that uses that big of ball bearings. They are about the size of the bearings inside a main bearing, But my main bearings were good, and besides that there is no way, they could have gotten in the crankcase in the first place. Im stumped! I dont want to think that someone would have put them in there, but there is no other reason for it. Please tell me im wrong, and tell me where else they may have come from.

So, the only bearings in the crank case are the main bearings, and rod bearings. Since you said they came out when draining oil, I assume they came from the transmission, which is completely sealed from the crank case. Just trying to be clear.

As mentioned, check the power valve governor first.

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So, the only bearings in the crank case are the main bearings, and rod bearings. Since you said they came out when draining oil, I assume they came from the transmission, which is completely sealed from the crank case. Just trying to be clear.

As mentioned, check the power valve governor first.

This.

You need to have a good look at the transmission bearings, one of them came apart. Your going to have to look real good at you internals too, make sure nothing else got damaged. You should have torn into it before buying parts. I got ten bucks say's your going to be the owner of a spare crank......

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I removed the cyclinder and had severe play, and grinding in the big end bearing on the crank. So I ordered it first. Finished tearing motor apart today and found the bearings. After reading everyones advise, I went back out to the shop and sure enough they are from the Governor assembly. I got them back in, and it measured 1.778. I quit for the day and ordered the remainder of my seals and bearings.. Thanks for all the great advise!!!

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JBFL, can you tell me how to go about pressing it back within tolorance? It took a little enginuity just to get the bearings back in it..

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