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I ride a WR250R and have been think about getting a YZ125 to start racing. I would realy like to know why it is that a 2 stroke in a sled can go 10,000 miles (if you don't screw with it) but with bikes you must rebuild every so many hours. I wasn't having much luck finding an answer so thank you for all ideas.

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I ride a WR250R and have been think about getting a YZ125 to start racing. I would realy like to know why it is that a 2 stroke in a sled can go 10,000 miles (if you don't screw with it) but with bikes you must rebuild every so many hours. I wasn't having much luck finding an answer so thank you for all ideas.

lots disagree with me, but, i think most people over rebuild there 2-strokes. I just recently got into the dirtbike scence, but i used 2 race a honda trx250r quad in hairscrambles. I used amsoil 50:1 and i swear to you, it lasted 4 years. I rode that quad so damn hard to keep up with the 450's! I took the top end apart every to put it back together because it looked fine. I think if you have a fast wearing bike, them you have to change 2 cycle oils or check your getting, a hot running motor wears fast. I also know this post of mine will stir up this topic. It is very debatable. Snowmobile engines are very similar to water cooler dirtbikes and atv's!

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i don't know anything about a trx or sleds but a 125 dirtbike, ridden like they should be ridden, and that is the statement of the year, will wear out rings relatively quickly. they are ported aggressively and this wears rings and subsequently pistons out sooner. And a dirtbike is ported conservatively relative to a roadrace bike. The maintenance schedule on a TZ125 roadracer is pretty brutal. course that's why they make 40hp stock!

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cause you're proboly not riding your sled at 10,000 rpm and you sled is also no race bike.

if you look at the yamaha dt 125 it could also last 10,000 miles riding on normal circumstances

sleds do run at 10,000 rpm.... almost constantly since they have a belt transmission.

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cause you're proboly not riding your sled at 10,000 rpm and you sled is also no race bike.

if you look at the yamaha dt 125 it could also last 10,000 miles riding on normal circumstances

watch this video and listen to the sounds. This is not a modifed snowmobile.

Edited by nickpetri
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most sleds run closer to 8grand, anyhow....they operate in the same rpm, which they are balanced for, have undless supplies of cool clean air, next to never see shock loads from hard shifts or big landings.......so they do have a bit easier life. I have and have had a sled since 1980......I have never seen one with 10k miles on it. my yammy triples have gone over 6....well one did, one pooped a crank at 4k. I am in the PM crew....fix it before it pops. I lost a silly shim on my D&D 1010 and that ate a crank, piston and jug.....about $1500 with me wrenching it. all over a stupid little washer that I was too lazy to pick up......chit happens on sleds too.

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Motocross engines are in a higher state of tune. Furthermore there is much less strain on a multi-cylinder engine moving at a constant rate of speed (a snowmobile on a trail) than there is on a moto bike that is going from closed to full throttle 15-20 times in a single lap of the track.

No snowmobile is running 10,000 rpm constantly, the variable clutch transmission is the reason they run at lower RPM, not higher. Centrifugal force moves the two halves of the angled clutch face together making the diameter of the "cog" that the belt must travel around larger, therefore increasing speed and reducing RPMs.

Having never torn into a snowmobile engine, since they are practically useless with Wisconsin winters as they are (too little snow), I cannot vouch for the materials used in the cylinder liners, pistons, or piston rings.... Suffice to say that I am sure that snowmobile engines are built from much tougher (read: heavier) materials.

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sled motors are very similar to bikes in construction. todays hot rod sleds can make over 300hp from a 80lb engine. most are more like 140hp. there is WAY more technology in sleds today then bikes.

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Yes, good points........however, many of my friends have sleds that pop at the 2,000 mile mark. And like was pointed out, when they do go, it's as costly as a 450 4 stroke. Sleds have the advantage of running cool (the 800R powertek from Ski-doo is designed to run <90 degrees F with it's cooling system)....try that on a dirtbike? Never capable. Also, there's no dirt up in the mountains in the winter. It's all clean, cold, (relatively) dense air. Dirty air can kill a motor in a very small amount of time.

Mountain sleds are routinely run very similar to an mx bike, in that there are just as many idle-WOT power transients. The drive system does cushion the drag on the motor to some degree I suppose, but the sled motor has to turn a heavy clutch system, attached through a chaincase to a big driveshaft, which in turn drives at least a 50# track. That's a heckuva load itself!! Most people I know swap their topends every other season.....the ones that don't, well, they were mentioned at the beginning of this post...

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