A little top end advice?

yzinger
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yzinger
Edited Date/Time 4/8/2014 10:48pm
Hey guys, doing the top end on my 02 yz250. Got the head, jug, piston etc off and I am shocked how good of shape they are in. There are no burn marks on the piston nor the cylinder...no scratching or anything else that seems like an issue. A couple questions on things i need more knowledge in.

1. Considering the shape of this stuff i wonder if i even needed a new piston/rings. How do you know? (this bike has been ridden by me for almost 10yrs now with no top end work done Unsure )
(yes, i am older and not a pro but the bike is very often on the pipe perse)

2. I hear talk about ensuring the rings are placed properly (when compressed they have the pins in the [piston in the middle) so i wonder why you would even check them in the piston with a feeler...know what i mean here?

3. How does a fella know if his PV is working properly (obvious when i ride the bike) with the jug off the bike. Should i be able to move it with my hand? - What else should i check to ensure it is all good?

4. Cylinders walls look great as mentioned. Should i still take a bit of steel wool to it?
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Yamaharider1474 4/8/2014 7:58am
4/8/2014 7:58am
To see of the Rings are bad check their end gap (with a feeler gauge) when installed into the cylinder one by one without piston, That is outlined in the manual , Given the time on the bike i would do them just for good maintenance reasons. PV just clean it and be sure it moves freely before install. I would run a flex hone in cylinder just to freshen up cross hatch then go have fun.
yzinger
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yzinger 4/8/2014 8:03am
4/8/2014 8:03am
oh yes, i am replacing piston and ring. Just some questions i wanted peoples opinions on.
Cygnus
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Cygnus 4/8/2014 8:17am
4/8/2014 8:17am Edited Date/Time 4/8/2014 8:18am
Don't use steel wool. Find someone with a flex hone as stated above, And just break the glaze.if you don't have a solvent tank use some penetrating lube for honing. Trying to not remove any material. For a stock Yamaha piston you should measure the cylinder and match the new piston to the bore. They sell 4 pistons a,b,c,d I believe. That's a long life for 2T piston. So count your blessings it's not in worse shape. Clean and inspect the power valve, should move freely off the cases.

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yzinger
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yzinger 4/8/2014 9:41am
4/8/2014 9:41am
dont know the relevance of that pic but i love your thinking Smile
mxrose3
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mxrose3 4/8/2014 10:30am
4/8/2014 10:30am
TommyZ wrote:
[img]https://p.vitalmx.com/photos/forums/2014/04/08/50450/s1200_topend.jpg[/img]
That top end looks like its been rebuilt!
TommyZ
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TommyZ 4/8/2014 10:32am
4/8/2014 10:32am
yzinger wrote:
dont know the relevance of that pic but i love your thinking Smile
The thread was about top ends and her top just seemed to fit the mood of it.
crf250pilot
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crf250pilot 4/8/2014 12:06pm
4/8/2014 12:06pm
Cygnus wrote:
Don't use steel wool. Find someone with a flex hone as stated above, And just break the glaze.if you don't have a solvent tank use some...
Don't use steel wool. Find someone with a flex hone as stated above, And just break the glaze.if you don't have a solvent tank use some penetrating lube for honing. Trying to not remove any material. For a stock Yamaha piston you should measure the cylinder and match the new piston to the bore. They sell 4 pistons a,b,c,d I believe. That's a long life for 2T piston. So count your blessings it's not in worse shape. Clean and inspect the power valve, should move freely off the cases.
Like Cyg says, the best thing you can do with a cylinder tht old is get the biggest piston for it (the D size if available).
moto314
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moto314 4/8/2014 12:50pm
4/8/2014 12:50pm
To really know if your piston is worn out you would need to measure the skirts and hardness test it, but of course you would need to know what it should measure first. One easy inspection you can do is put the old ring in the cylinder bore, use the new piston to square it up and look for light gaps between the ring and cylinder wall, there shouldn't be any. Do this with the new ring when you check your ring end gap to make sure the bore is not out of round too.

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