Crankshaft self destruction?


4 replies to this topic
  • jimbob69

Posted June 23, 2006 - 06:56 PM

#1

A while back I was at my yamaha dealer, picking up some valve shims and such for my yz400f, and the parts guy asked me if it was for a yz or a wr, i said a yz. Next thing, he warned me about them bikes, you either have to be accelerating or decelerating, or the crank will go into a harmonic imbalance or somthing of that sort, and said the bearings on the rod will basically self destruct. He said you shouldnt be cruisin at a steady speed or your crank will go. Is any of this true?

That scares me a lot cause theres a kid that goes to my school that has supposedly put 2 cranks in his 98'.

Now when im riden on big open straights or anything, im always paranoid and i speed up and slow down slightly but constantly.

Is this total BS, or is there some truth to the cranks going on these bikes from just putsin at steady speeds?

  • kid on a 426

Posted June 23, 2006 - 07:44 PM

#2

A while back I was at my yamaha dealer, picking up some valve shims and such for my yz400f, and the parts guy asked me if it was for a yz or a wr, i said a yz. Next thing, he warned me about them bikes, you either have to be accelerating or decelerating, or the crank will go into a harmonic imbalance or somthing of that sort, and said the bearings on the rod will basically self destruct. He said you shouldnt be cruisin at a steady speed or your crank will go. Is any of this true?

That scares me a lot cause theres a kid that goes to my school that has supposedly put 2 cranks in his 98'.

Now when im riden on big open straights or anything, im always paranoid and i speed up and slow down slightly but constantly.

Is this total BS, or is there some truth to the cranks going on these bikes from just putsin at steady speeds?

That harmonic crap is BS. But if stay at a steady high rpm as in near the rev limiter for a long period its a good chance that a connecting rod will let go

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  • Xolotl

Posted June 23, 2006 - 10:36 PM

#3

Out of curiosity, this "good chance" is it more than .1 percent chance? :excuseme:

  • bnio

Posted June 25, 2006 - 09:24 PM

#4

I am glad you are asking this question. There are no stupid questions just stupid people and that parts guys is currently ranking very high on the dumbass meter. The main problem with the 400 is that the wrist pin or the piston pin was a little on the small side for sustained high rpm output. Thats why the 426 and 450 has a bigger wrist pin and that is why there are no issues with connecting rods failing. With the 400 you need to make the piston pin a part you change out with some regular interval, especially if you like to rev the bike in the upper rpm range. Now the problem is figuring out how long of an interval before you can run the engine before you have to change it, thats the real trick here. I have a 400 and I noticed my new piston pin was coated with some sort of high pressure coating now I don't know if the originals were coated or if this is something new to help combat piston pin failure. Also don't forget to check the oil pump for proper operating clearances and also change your timing chain regularly. These are the three main deadly killers of these bikes. It is a whole lot cheaper to buy a head gasket, cylinder gasket and psiton pin than it is to rebuild a whole motor. I hope this helps.

  • kid on a 426

Posted June 27, 2006 - 08:58 PM

#5

Out of curiosity, this "good chance" is it more than .1 percent chance? :excuseme:

Well good chance as in there is definetly a possbility that a wrist pin or something of that sort will let go after being on the throttle for hours straight. I would do a search but the site is too busy right now and it won't let me but there are several cases of engines grenading during high sustained RPM's





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