Kx85 going thru cranks

[Foxx4Beaver]

if it's not smoking excessively, then it would/could only be the mag side crank seal.

I had an '83 CR125 where the crank bearings were so bad, it allowed the flywheel to run out of balance enough that it messed up the "pick up", and would break up on the upper revs.
A cracked or chipped magnet in the flywheel can cause the same effect.

I don't remember anyone implying the crank itself was the problem, and I don't see that being the culprit...I myself would never run Hot Rod anything, just based off what I've seen over the years, and I would suggest a better brand regardless...
I think the Doldguy could be on the right track with the MSV(maximum squish velocity)...since the head was changed, then you started having issues shortly after.
 

[KXDINO]

We had a hot rod let go on a kx 85 some years ago , broke at the big end side , lucky it happened at low revs and only cracked a case , and we could repair it , I stick with oem usually. Another thing I do to kx85 is to drill the holes that feed the main bearings 1mm larger as I had a problem there once ,seem to help it last longer , but I would look at the tuning side of things close, compression ,jetting , the casting behind the stator were the crank comes out ,were it dips in , they had trouble there to , is the crank cases sealed nice and tight.Hope you find out what it is  , and don't give up on it , the cure is out there. Try a stock head , see if that helps?

[85#371]

Cool guys thanks for the info. I never looked at the previous crank until today and had some guys from work look at it and it's in great shape so I don't know if it helps but the crank was fine the first time this happened why putting a new crank in fixed it I don't know but it did. I'm thinking that the seal between cases has a flaw and as time goes on it leaks just a guess. Let me know what you guys think. I'm off tomorrow and will be dedicating my day to the bike. Any suggestions I'm game. The other is the mSV makes sense kind of but wouldn't the bike do it right from the get go and not 10-15 hours later if that was the problem? I'll keep updating tomorrow throughout the day. Also shouldn't the compression be different with the head I have on the bike vs stock wich is 171. Will I hurt the bike running stock head with c-12 fuel for testing purposes?thanks guys

[85#371]

Ok guys I had to get the bike done today so I went through everything split the cases inspected everything all looked good replaced crank seals and cleaned crank breather.replaced piston and did see blow by on the exhaust side of piston looking at the head I found there was only one pin stud in the head going thru head gasket found it in the mrx02 head don't know if that was the culprit but sure dindnt help matters any. Cleaned powervalve and carburetor put it together and it rips again hopefully longer than the last time. Thanks guys I'll keep you posted on how it runs in the future.

[Foxx4Beaver]

good luck, definitely keep us posted.

[sandblaster]

Just curious, what brand of crank seals did you use?

[85#371]

Oem and all oem parts is all I'm sticking with for now and I did put the original oem crank back in it

[sandblaster]

When putting the seals in did you use some lubrication for the seal lips?