Jump to content
Sign in to follow this  

How many hours do you get on a bottom end ?

Recommended Posts

Hi all,

Just wondering how many hours the average rod and main bearings last barring catastrophic failure?

My 06 KX 250 has about 130 to 140 hours on it, clean filters etc, trail ridden and raced in a few enduros, but I'm old and slow, and the bike has never seen an MX track.

I tore it down to do the top end (re built yearly at about 60 hours) and found the big end had vertical play. I am now replacing big end and main bearings, seals, etc.

Was just idly curious as to how long every one else's bottom ends last between rebuilds?👍

Share this post


Link to post
Share on other sites

I run 32:1, like a lot of oil for the bottom end, and other than a catastrophic failure I have never had to replace a bottom end. Even when I was doing 15-20 D-37 desert races a year I never had to replace one. That would work out to about 1600 race miles / year at high speeds along with all the related practicing / play riding on Saturday

Edited by BrokenSpoke2

Share this post


Link to post
Share on other sites

A good bottom end build should last as long as you own the bike, so long as everything else is in order. Jetting plays a big part in this, believe it or not.

Share this post


Link to post
Share on other sites
A good bottom end build should last as long as you own the bike, so long as everything else is in order. Jetting plays a big part in this, believe it or not.

i disagree. it completely depends on the state of tune of the motor, its geometry, the rider, terrain, oil, maintenance, etc etc. to say that any of them should last as long as you own the bike is false, maybe a kdx or an xr if you own either for a relatively short amount of time. but how about a kx100 for example? they eat through cranks. high rpm modern 4 strokes? even the most well maintained, correctly jetted kx100 ridden even modestly will need a bottom end within a few years or whatever

Share this post


Link to post
Share on other sites
A good bottom end build should last as long as you own the bike, so long as everything else is in order. Jetting plays a big part in this, believe it or not.
The big-end rod bearing is a known weak point for the KX250, and has been for many years. And the crank seals often wear out before any other part of the lower end, requiring a case split.

I rebuilt the bottom end on my current KX at about 120 hours, because the seals were shot. The crank seemed fine, but measurements showed the radial rod play to be excessive. My current crank has about 80 hours on it and is still tight.

Share this post


Link to post
Share on other sites

Juliend,

What would your recommendations concerning jetting in this context be, to prolong life of engine internals? I don't mean specific jet sizes, just a guide.

Chokey,

Thanks for the comment, this is my first KX and I was not aware of the weak rod big end bearing.

Just as a matter of interest for the Gurus, piston measured and looked OK after 60 hours of trail riding, but was near the limits. This in my humble opinion seems a reasonable rebuild schedule for a trail ridden KX250 (ie top end at 50 to 60 hours)

Share this post


Link to post
Share on other sites

I have way more than 200 hours on my bottom end ( oand I'm not slow. I also run like 24-26:1...

I think the weak rod end bearings were in the 94-04 engines?

I think the key is running lots of oil and clean airfilters, which I do. I think 200 hours on a bottom end should be easy for most big bikes. But some get lucky and some don't.

Share this post


Link to post
Share on other sites

True, it is a lot about luck. I wasn't aware of the weak big end bearing in the 250. Good info to have!

As for the jetting, you want it on the rich side of being perfect. Running too lean can oil starve the bearings causing them to overheat and fail sooner.

Share this post


Link to post
Share on other sites

i've never had to replace bottom end parts on any bike. currently have 210 hrs on my 07 kx, 550 hrs on my 08 200xcw. We have an 07 250xcw with around 800 hrs on it. I'm going to check the play on that closely next time we do a top end.

Share this post


Link to post
Share on other sites
210 hrs 07 kx, 550 hrs - 08 200xcw, 07 250xcw - 800 hrs

must be nice.... -BIG DAN:thumbsup:

Share this post


Link to post
Share on other sites

I do not want to inadvertently start another "Oil Thread War", so without getting into brand names, assuming a 32:1 or more (eg 26:1) mix, does the quality of the oil have a significant impact on bearing life assuming that their is sufficient oil in the mix?

Share this post


Link to post
Share on other sites

i think it does yes..oil can be cheap as hell, crap stuff, or u can pay the same amount of oil for around 20 bucks...sythetic to... iunno ill have to agree, everyone has there own thoughts on oil wars...

funny thing is guy at a nkown good bike store told me his kx250 94 lasted him around 500 hours, i was DAMN WELL AMAZED..

weather hes lieing or not, he gave me some awesome tips.

Share this post


Link to post
Share on other sites

Okay, I've gotta jump in here with a question for all you guys. It's probably stupid, but humor me!

How do you keep track of your hours? Are you keeping a detailed log of each ride, or are you just swagging it? I'm just curious and learnin' - new to 2 strokes and just putting together a new bottom end for my kid's kx100. The main bearing failed on the flywheel side, everything else looked really good - no idea how many hours were on the bike, but I think I'll want to track it now.

Jack

Share this post


Link to post
Share on other sites

funny thing is guy at a nkown good bike store told me his kx250 94 lasted him around 500 hours, i was DAMN WELL AMAZED..

If it's not being ridden and revved super hard, that's not really that unbelievable. As mentioned above we have multiple bikes in the garage right now that have over 500 hrs on the bottom end, untouched.

I put over 600 hrs on a ktm 300 before even doing the top end, but I was running it a little too lean off idle, and the carbon blowing back through the exhaust port wore the cylinder bad enough that i chose to replate it.

How do you keep track of your hours?

I have an hour meter (mounted in the airbox) on my kx250.

My ktm and husky and husaberg all have electronic odometers that keep track of total miles and total hours. These are 'rolling' hours as opposed to 'running' hours, but the difference isn't enough to stress about.

Share this post


Link to post
Share on other sites

To keep track of the hours if you are real cheap, get a bicycle computer, attach the magnet to a spoke, and the sensor to the fork guard with a zip tie.

Lengthen the wire from the sensor to the mount by cutting and soldering in about 2 feet of twin core wire (anything works), and wrap around the front brake line.

Mount on handlebars and hey presto, you have average and max speed, odo, tripmeter, hours, speedometer and clock all for about $10, and it's waterproof too.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...