when to rebuild top end

cbr1000rr
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cbr1000rr
Edited Date/Time 1/9/2018 9:33am
i have a 2016 crf250r i bought new, it has 57 hours on it currently. the book says top end at 20 hours under race conditions. i race but its my first year and i am very slow, never rev limit, roll jumps so on. I check the valve clearence every 10 hours or so, only one side is dropping low but still in spec. the service manual says compression should be at 87 psi, its currently at 91. I have everything i need to put in new piston and rings, my question is should i go by compression when deciding to put in a new top end or by hours riding?
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Acidreamer
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Acidreamer 1/8/2018 1:17am
1/8/2018 1:17am
I always get shit for saying this. I advise people to do what their service manual says and then once they get a forged piston go longer hours - at least double the original service limit. You want to keep the cam chain fresh too. TO ME 57 hours on a 250 top end is kindof high.

On my 450 the service limit is 15 hours. I put in a forged piston & do top ends every 50 hours. Usually about twice a year.
BobPA
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BobPA 1/8/2018 6:47am
1/8/2018 6:47am
Acidreamer wrote:
I always get shit for saying this. I advise people to do what their service manual says and then once they get a forged piston go...
I always get shit for saying this. I advise people to do what their service manual says and then once they get a forged piston go longer hours - at least double the original service limit. You want to keep the cam chain fresh too. TO ME 57 hours on a 250 top end is kindof high.

On my 450 the service limit is 15 hours. I put in a forged piston & do top ends every 50 hours. Usually about twice a year.
You only got shit because you said you changed your top end every 10 hours. Then advised people they were crazy for not doing the same...
Acidreamer
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Acidreamer 1/8/2018 7:00am
1/8/2018 7:00am Edited Date/Time 1/8/2018 7:09am
Acidreamer wrote:
I always get shit for saying this. I advise people to do what their service manual says and then once they get a forged piston go...
I always get shit for saying this. I advise people to do what their service manual says and then once they get a forged piston go longer hours - at least double the original service limit. You want to keep the cam chain fresh too. TO ME 57 hours on a 250 top end is kindof high.

On my 450 the service limit is 15 hours. I put in a forged piston & do top ends every 50 hours. Usually about twice a year.
BobPA wrote:
You only got shit because you said you changed your top end every 10 hours. Then advised people they were crazy for not doing the same...
If you go read what i said, i said i tell other people to follow their service manual. Which on a kx450 is 15 hours. Do you really think i spend $300+ on a piston once a month? Like really?

Im not gonna tell joe shmoe to run a piston for 80 hours and then him blame me for it blowing up.
TrailDigger 1/8/2018 8:15am
1/8/2018 8:15am
cbr1000rr wrote:
i have a 2016 crf250r i bought new, it has 57 hours on it currently. the book says top end at 20 hours under race conditions...
i have a 2016 crf250r i bought new, it has 57 hours on it currently. the book says top end at 20 hours under race conditions. i race but its my first year and i am very slow, never rev limit, roll jumps so on. I check the valve clearence every 10 hours or so, only one side is dropping low but still in spec. the service manual says compression should be at 87 psi, its currently at 91. I have everything i need to put in new piston and rings, my question is should i go by compression when deciding to put in a new top end or by hours riding?
Wait until the compression drops or valves go out of spec.

Also make sure you have a new cam chain and reset the tensioner.

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Tracktor
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Tracktor 1/8/2018 10:40am
1/8/2018 10:40am
If it was me with your description I would say throw in a new piston. It's cheap insurance. I do my sons at 30-35 hours on his 250f but he is a pretty aggressive rider. I go by hours on all our bikes as it's simplest to track multiple bikes. 87/91 for compression has to be a typo?..............
526
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526 1/8/2018 12:57pm
1/8/2018 12:57pm
Leak Down Test It...... that will give you an Honest answer,
yz133rider
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yz133rider 1/8/2018 4:10pm
1/8/2018 4:10pm
If you are slower, take care of it, ride in lower rpms id say every 60 hours is fine. On a 450 every 100 hours with that type of style.

Faster guys, revvers, and maintenance slackers changes big tine.
cbr1000rr
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cbr1000rr 1/8/2018 5:17pm
1/8/2018 5:17pm
Tracktor wrote:
If it was me with your description I would say throw in a new piston. It's cheap insurance. I do my sons at 30-35 hours on...
If it was me with your description I would say throw in a new piston. It's cheap insurance. I do my sons at 30-35 hours on his 250f but he is a pretty aggressive rider. I go by hours on all our bikes as it's simplest to track multiple bikes. 87/91 for compression has to be a typo?..............
no typo, the honda service manual says Cylinder compression 598 kPa (6.1 kgf/cm2, 87 psi) at cranking. my test showed 91 psi.
526
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526 1/9/2018 5:23am
1/9/2018 5:23am
Doesn't it have a Decompression button on the cam?
Acidreamer
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Acidreamer 1/9/2018 6:02am
1/9/2018 6:02am
4 strokes arent going to show 180psi like a 2 stroke would due to the decompressor on the exhaust cam.
Tracktor
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Tracktor 1/9/2018 9:32am
1/9/2018 9:32am
Tracktor wrote:
If it was me with your description I would say throw in a new piston. It's cheap insurance. I do my sons at 30-35 hours on...
If it was me with your description I would say throw in a new piston. It's cheap insurance. I do my sons at 30-35 hours on his 250f but he is a pretty aggressive rider. I go by hours on all our bikes as it's simplest to track multiple bikes. 87/91 for compression has to be a typo?..............
cbr1000rr wrote:
no typo, the honda service manual says Cylinder compression 598 kPa (6.1 kgf/cm2, 87 psi) at cranking. my test showed 91 psi.
Ha, I stand corrected. I am so used to 2-stroke stuff I've never even pulled compression numbers on a thumper. Learn something new everyday. I still say rebuild just to be safe..........And keep oil in it, lol-


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