pistons for 250f...

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#1
Time has come to rebuild my 250f.
My question is: I have always used OEM parts in the past with my two strokes. However, I ended up using Pro-X, since my sources tell me that there parts are OEM. But, I have not seen any Pro-X top end replacement parts for the 250f, only Wiesco and Arias. I have no experience with either.
I get a much better, and I mean MUCH, deal on the the Arias or Wiseco parts as opposed to the OEM parts.
What has been your experience?

Thanks-
 
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#2
I personally have had bad luck with Wiseco, and I know others who have also had bad experiences with their products. The Wiseco pistons are great for 2 strokes IMO but for four strokes it is a new ball game. They must have closer clearances and be closer to "perfect" if you will! And the company I believe that has the best 4 strokes pistons is JE pistons, you can get them from White Brothers. I have used these in YZ400's, XR400, XR250's, XR200's and a few others (I will use them in my 426 whenever the time comes) and the fit and finish was far superior to Wiseco! I havent had a Arias piston before so I havent got the foggiest on that one. But you cant go wrong with JE in my opinion. Im sure others will disagree but hey that is what a B-board is for right? Hope this helps!

Garrett
 
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#4
I've put quite a few four-stroke engines with Wiseco pistons and they work quite well. But like any forged slipper skirt racing piston, if the person assembling the engine doesn't take care to set clearances and torque values correctly you'll have issues.

The majority of problems I've seen in these situations have more to do with the mechanic than the manufacturer of the pistion. Bad mechanics can turn any good product into JUNK.
 
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#5
Time to rebuild already??

Just curious if you mic'd anything to determine if it was time to rebuild, or if you just determined by loss of power/acceleration. I have been meaning to do a compression test on mine, but keep procrastinating. I got mine in nov. and it still seems as strong as day one. How long will these top ends last?
 
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#6
top ends- 250f

My understanding is that you should put a piston and rings in about every 5 hrs. Of course, depending on your riding style this could change. I was told that Brandes rode 4 nationals with the same top end. My bike has around 20hrs or so, easy. I would rather do the work rather than take a chance of breaking a skirt and roaching the engine. That can get rather expensive!

I am also hoping that Pro-X, or similarly reputable company, makes a rod kit as well. I do the bottom ends on my two strokes, but have never done a four-stroke. Other than the cam timing, I don't think that it will be that different. I am procrastinating as well.
 
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#7
Who told you every 5 hours? A piston salesman. You don't have to mess with the motors for at least a couple of years except for valve adjustments.:silly:
 
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#8
top end time-

Actually, I've heard from a couple of different sources:
1. The owners manual
2. Big Gun Exhaust
3. Pro Circuit R&D
4. Sources within YMUS racing

What I was told was that the piston skirts have a tendancy to break after extended hours. All of this of course depends on how hard you ride. But I can not afford to loose a bottom end at this point. That would run me about $2.0k plus, even after calling in favors!

I would rather due preventative maint. than risk a major break. And I race pro, 2 classes every weekend. SO I know that I have approx. 20 or so hours on my current piston.
 
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#9
sources

And I forgot-
another source of the 5 hrs is from within YMUS Japan-

So I feel that the 5 hrs is pretty reliable.
 
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#10
I just replaced my piston and cylinder at 20 hrs (big bore kit). I carefully measured piston clearance and ring gaps. Everything was well within spec. I would put them back in and run them for another 20hrs without hesitation. Keep in mind that I race this bike hard and change oil every three races.

Mark
'01 YZ267F
 
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#11
top ends- 250f

What is the pressure test amount? Maybe I could just pressure test the top end for now.

I also change the oil every ride.
 
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#12
I don't know why your so obsessed with your top end. Tear it down down once a season for complete inspection. Replacing the piston once a season wether it needs it or not is good insurance against catastrophic failure due to cracking or hidden flaws. If your name is Tim Ferry, then by all means replace your top end every 5 hours! An easy way to check your top end is to bring the piston up to the compression stroke and rest the wieght of your leg on the kick starter. It shouldn't move for a couple seconds. If it moves quickly or hisses, then you should definately look at your valves and piston/rings/cylinder. This isn't very scientific, but you can do it every time you ride, and you will have a clue as to the general condition of your engine. Personally, I think 5 hours is ludicrous.

Mark Boyd
'01 YZ267
 
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#13
obsession..

....my obsession is due to my 2-stroke having a catastrophic failure 1 week before the first national. It was only three months old. All of a sudden, my bike started vibrating badly, so I pulled off the track. Upon inspection, I found my bottom motor mounts had broken. So I yanked the engine out, and decided that I'de rebuild it. I discovered that the one of the crank bearings had let go, and in the process of those few vibrating seconds, had toasted my bottom end, left case half, and my cylinder. Total cost was about $2.0k. And my entire racing budget for the three nationals on the west coast. So I am very paranoid about it. Had I rebuilt the bike a month earlier, I'de be at Washougal this weekend.....

Upon closer inspection, I could not find any evidence of dirt in the crankcase.
 
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#14
Given the fact that you are racing this bike hard it's a very good idea to follow Yamaha's service schedule (in the manuall) for the piston and rings while keeping a close eye on the crank.

Using Mark's suggested feel-o-meter method is probably OK for guys just play riding or putting minimal race time on the engine, but it's a recipe for broken parts for serious racers. These are NOT XR250s and you can't treat them like it.
 
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#15
preventative maint

El Mucho Grachios

Thanks for your reply.

Now I pose another question. You seem quite knowledgeable in the world of four strokes. So, when I do have to feather my clutch, I feel modulation through the lever. I asked Doug Dubach about this, and his reply was that he would have a fix for this very soon. Fine and dandy, but what causes this? I thought it was due to the basket having ridges on the fingers from use, and that a simple replacement basket would solve this. But after Doug's comment, I am wondering if there is something more than what I thought it was.....:think