Engine life (250 / 350 / 450)

cwtoyota
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cwtoyota
Edited Date/Time 6/18/2015 5:06pm
I'm strongly considering a new four stroke MX bike. Assuming I do frequent oil changes and keep my air-filters clean, how many hours should I expect to put on a current four stroke engine between maintenance tasks: piston, valves & seats, cam-chain, Crankshaft, etc?

Obviously a 450F will outlast a 250F and the KTM 350 should fall somewhere between.

Most of my racing is in the 250B, 25+ and 30+ classes on a YZ125. I change pistons every 13 to 15 hours on the meter.
My other bike is a YZ250 with a 295cc kit, I get around 45 hours out of a top end.

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Paw Paw 271
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Paw Paw 271 6/17/2015 12:15pm
6/17/2015 12:15pm
From my knowledge base...The KTM350SXF has the best engine parts life of just about anything out there.
The main key to engine life on 4 strokes is to stay away from the rev limiter.

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731chopper
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731chopper 6/17/2015 12:55pm
6/17/2015 12:55pm
Kawasaki tells you on a new 250f to check the valves and replace the piston/rings every 15 hours. Kawi tells you to just inspect the head and the crank every 15 hours but don't specify a time period of when it should be replaced. But they also say to only do the oil and oil filter every 15 hours so...

I would say you should expect 15 hours at the absolute minimum. I think most people actually go well beyond 15 hours before even thinking about opening the motor up at which point they may open up their manual for the first time.
BobPA
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BobPA 6/17/2015 4:13pm
6/17/2015 4:13pm
My 2011 350 sx-f has been incredible engine wise. I ride +25A and Open A here and there...I tend to dance the bike around the rev limiter. I bought the bike brand new and have had an hour meter since new. I change the oil every 3 hours and keep a clean air filter. I have put a piston in approximately every 50 hours....because that is all I get to put on a year anymore. I check my valves every 15 hours or so. After 120 hours the valves never moved, and looked absolutely perfect. I replaced them to stay on the safe side, and have not had to adjust the current valves yet. My father has 70 hours on his 2011 350 (he is 66 and still rides pretty well) and he has not put a piston in his yet.

I had kawasakis previous, (250 and 450) they were also very reliable, but the 250 did require valves every 60 hours or so....took me one catastrophic valve failure to set an hour limit on 250f valve life.
MOTODAD422
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MOTODAD422 6/17/2015 6:50pm
6/17/2015 6:50pm
How do you do a compression test on a 4-stroke ?

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cwtoyota
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cwtoyota 6/17/2015 9:06pm
6/17/2015 9:06pm
BobPA wrote:
My 2011 350 sx-f has been incredible engine wise. I ride +25A and Open A here and there...I tend to dance the bike around the rev...
My 2011 350 sx-f has been incredible engine wise. I ride +25A and Open A here and there...I tend to dance the bike around the rev limiter. I bought the bike brand new and have had an hour meter since new. I change the oil every 3 hours and keep a clean air filter. I have put a piston in approximately every 50 hours....because that is all I get to put on a year anymore. I check my valves every 15 hours or so. After 120 hours the valves never moved, and looked absolutely perfect. I replaced them to stay on the safe side, and have not had to adjust the current valves yet. My father has 70 hours on his 2011 350 (he is 66 and still rides pretty well) and he has not put a piston in his yet.

I had kawasakis previous, (250 and 450) they were also very reliable, but the 250 did require valves every 60 hours or so....took me one catastrophic valve failure to set an hour limit on 250f valve life.
Thanks for the numbers, that's the kind of info I'm looking for.


I realize everyone will have different results due to so many factors including riding skill/style and conditions but a good ballpark expectation is something you only get from real world experience. I've never owned a four stroke motorcycle of any kind, so I lack that first hand maintenance experience.


Knowing some numbers will be a factor in which size bike I buy.
cwtoyota
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cwtoyota 6/17/2015 9:10pm
6/17/2015 9:10pm
MOTODAD422 wrote:
How do you do a compression test on a 4-stroke ?
There are compression testers with extended hoses. You thread the hose into the spark plug hole then connect the gauge. Next you turn the engine over with the kick starter (or e-start) until the needle stops rising.

Maybe you were thinking about a leak-down test?
Those can be done on four strokes as well, but it isn't as useful as on a two stroke.
slackinoff
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slackinoff 6/18/2015 9:31am
6/18/2015 9:31am Edited Date/Time 6/18/2015 9:47am
You should not rely on compression loss to determine when to do a top end on a four stroke mx bike. Piston fatigue and failure will happen because of the short "slipper" skirt design on these modern four stroke mx bikes before you see compression loss.
BobPA
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BobPA 6/18/2015 1:50pm
6/18/2015 1:50pm
MOTODAD422 wrote:
How do you do a compression test on a 4-stroke ?
cwtoyota wrote:
There are compression testers with extended hoses. You thread the hose into the spark plug hole then connect the gauge. Next you turn the engine over...
There are compression testers with extended hoses. You thread the hose into the spark plug hole then connect the gauge. Next you turn the engine over with the kick starter (or e-start) until the needle stops rising.

Maybe you were thinking about a leak-down test?
Those can be done on four strokes as well, but it isn't as useful as on a two stroke.
The auto decompression on four strokes really never gives you an accurate cranking compression...
cwtoyota
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cwtoyota 6/18/2015 3:42pm
6/18/2015 3:42pm
slackinoff wrote:
You should not rely on compression loss to determine when to do a top end on a four stroke mx bike. Piston fatigue and failure will...
You should not rely on compression loss to determine when to do a top end on a four stroke mx bike. Piston fatigue and failure will happen because of the short "slipper" skirt design on these modern four stroke mx bikes before you see compression loss.
I agree with that 100%. That's why I'm looking for an idea of real world teardown intervals. Even on a two stroke like the YZ250 you'll still have excellent compression long after the OEM cast piston is past the wear spec or even cracking on the intake skirt.

On my two stroke YZ125s and YZ250s I tear them down at safe intervals using the hour meters I've installed. Then I visually inspect everything, check ring end-gap, measure the bore at multiple locations, etc.

BobPA,
You have a good point, I didn't think about that feature. The old four stroke engines I worked on as a kid didn't have that feature but I've seen it in the new four strokes I've worked on. I suppose if you were planning to check compression, you would have to pull the cam cover off the head and wire-tie the little fly-weight in the running position to allow the exhaust valve to fully close when you kick the bike over.



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