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About to do top end on YZ250.

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The time has came do the top end on my YZ250.  I bought it brand new and I'm right at 50 hours and I ride pretty hard.  So....I'm going to go ahead and drop a new piston in there.

 

I've done top ends before (on 125 two stroke bikes) and I always use wiseco .  After doing some research I've seen some post about people having problems with the wisecos on yz250s.  I don't remember exactly what it was but I think rings were hitting ports (maybe??).

 

Anyway I was planning on just putting in a stock piston since it worked well for the first 50 hours.

 

Anyone talk me out of this?

 

Better choices out there?

 

I know this topic has been beaten to death but there is so much info I really would like a few opinions from yall for myself.

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Yamaha sizes pistons and cylinders by ABCD sizing. I would see what letter you have and go up one. Wiseco forged is one size fits all, they can be rattly in there. Stock/ProX offer the incremental sizing, but they are cast.

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I'm glad you mentioned the ABCD sizing.  I've never run into this with my RM125.  Allows dropped in a wiseco and the clearance is perfect.  So my piston will have a letter on it?  Then I need to go a letter up.  Is that because of the wear I have caused or is this just general practice.  If the bore still measured the same would I want to stick with the letter that is in there?

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Yamaha sizes pistons and cylinders by ABCD sizing. I would see what letter you have and go up one. Wiseco forged is one size fits all, they can be rattly in there. Stock/ProX offer the incremental sizing, but they are cast.

 

Why would he automatically need to go up one?  He may just need a new, same size piston?  When you pull the cylinder you need to measure the bore and make an educated decision.  Am I missing something?

 

From the manual: Page 4-16 and 4-17 has all the info.

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Yeah that is what I was thinking.  This is the first top end job this bike has had so I was thinking the same size piston but I'm 100% open to other peoples opinions/suggestions on this.  If not I will just measure and see which will work the best.

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You need to measure the cylinder bore. The manual lists the piston to bore clearance range.

How many hours does the jug have on it? If it's a bunch, it's safe to say you're gonna need to send it to Langcourt or Millenium to have it re-plated. You can buy your piston from them and they will size the bore properly for you. Or you can send them your piston with the jug.

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50 hours total.  Bought that bike at the end of the season before last and I know about Millenium Tech.  They are going to do some work to my RM125 cylinder at the end of this season but yes bike is new...50 hours total first top end job on this bike.

Edited by DirtSmoker250

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Have any of you all re used a piston and just changed the rings?

 

On my Rm125 I change the piston/ring all at once. 

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Have any of you all re used a piston and just changed the rings?

 

On my Rm125 I change the piston/ring all at once. 

 

I'm sure it's been done.  But could you really trust it?  Okay, if it's a 50cc mini, but a 250?  If you have someone that can magnaflux the piston you might have some peace of mind that it's still sound, but to me putting in a new piston would be the way to go.  No telling what that piston's seen in those 50 hours.  And besides, do you want to be taking the cylinder off again in a few months when the piston decides to retire?

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Yeah your right and that's how I've always felt and did it on my RM125 so I'll stick with that.  I've just read about people re ringing around the hours I have and then doing the piston at 100.  I guess they are just taking a risk to save some extra money.

 

On a side note my friend bought a used KX250 with unknown hours on it.  I advised him that a top end would be good to do.  He didn't and about 30 hours later the engine is seized.  This just recently happened.  Guess we will have to open it up and see what destruction has been done.

Edited by DirtSmoker250

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Only 50 hrs...you can probably skip the re-plate at this point...but I would still want proper P-B clearance.

Measure the bore with a telescopic gauge in 3 different places and average your 3 results. Then, see if you can find measurements on "A" "B" "C" and "D" cast-al pistons and go with the one that will provide P-B that falls in the middle of the range that Yamaha specs in the manual.

Maybe when it gets 100 or 150 hrs, splurge and have the jug replated and throw in a forged aluminum slug, sized properly to the hole.

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Why would he automatically need to go up one? He may just need a new, same size piston? When you pull the cylinder you need to measure the bore and make an educated decision. Am I missing something?

From the manual: Page 4-16 and 4-17 has all the info.

You're right, that is the correct way to do it. Then, you'll see a bunch of guys that just throw a D piston in any used cylinder and call it good. Depends on your overall concern and tolerance of such things. Best thing is to make sure you're replacing cast pistons in a timely manner either way.

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Only 50 hrs...you can probably skip the re-plate at this point...but I would still want proper P-B clearance.

Measure the bore with a telescopic gauge in 3 different places and average your 3 results. Then, see if you can find measurements on "A" "B" "C" and "D" cast-al pistons and go with the one that will provide P-B that falls in the middle of the range that Yamaha specs in the manual.

Maybe when it gets 100 or 150 hrs, splurge and have the jug replated and throw in a forged aluminum slug, sized properly to the hole.

 

The re-plate is for my RM125.  Definitely not doing that to the YZ250 yet.  Thanks for the tips though.

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wossner would be my first choice.

Followed by pro x

wiseco is great, but at only one size available, depending on the size of the bore would dictate if it could be used.

 

cant go wrong with oem or pro x, they are cheap, come in different sizes and work great. Just dont let the cast pistons run to long, you risk losing the skirt.

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Yeah that is what I am worried about right now.  I was going to do it at 40hours but ended up going on a weekend trip and didn't get the bike done first so I'm at 50 now. 

 

I do have another question somewhat related.  The bike started developing a different exhaust note over the last weekend.  I was at a different altitude by 5000 feet so I don't know if that had anything to do with it and somehow my jetting was causing the new change but I doubt that.  I really think my muffler just needs repacking.  It is an FMF shorty with about 40 hours on it.  Time to re-pack or is different exhaust note from something else?  It has a slightly more annoying sound to it.  Hard to explain but definitely sounds different.

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Man you guys make this complicated. When i redid my top end, i first checked to see if the plating or cylinder walls where in good shape. They were. I got my I.D. mic from work, mic'ed the id, bought the appropriate piston(i think from vertex). Works fine.

 

Also my bike was bought early 2007. I was at least the 2nd owner, maybe third. Previous owner beat the crap out of it way harder than i ride for sure. It's just a top end job. Do it right, dont be scared. A top end is cheaper than when your piston cracks and destroys the bottom end, cylinder and head. Also inspect the crank for excessive play. Put all new seals as well(base gasket and head). never be cheap and reuse it. An air and/or coolant leak isnt worth it.

Edited by RagunCajun

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