350x piston help!!

[sportsfinat]

bear with me on this one....well i tore down the x and am completely rebuilding the top end and i bought a wiseco which is .040 over off ebay. this is a 82mm piston compaired to the stock 81mm. anyways i gave th piston and cylinder to a local machine shop who is also a family friend to do the boring. well hear to find out the piston is almost .020 out of round which im not sure if thats normal for these pistons. i know in aviation that the pistons are like this, but i wouldnt think that this would have that steep of an oval shape. it is longer perpendicular to the wrist pin, i believe that his is where you would want it longer just for the simple fact that its going to wear and expand but im just kinda stumped.....be bored it to that length (the longest part of the piston) i was just wondering if im screwed and need to go up another size now or if thats ok.........also he put the rings it the cylinder and they are still within spec with the gap so far..so to shorten it up, the rings are in spec but only two sides of the piston are and the sides parallel are way out!?!?!?!?! is this right or what is going on thanks for the help in advance

[dcreel]

I don't think that the piston should be out of round at all. I'd say you need another piston.

[sportsfinat]

well that sucks case i cant return this one.......i didnt buy it directly from wiseco.....i dont know how it could be 'out of round' and be new in the first place....but idk ive read some stuff on the web and there are other machines that have new 'oval' pistons.....i just couldnt find anything definite

[Texas 200x]

Yes pistons are out of round and the head is tapered smaller than the skirts. When the aluminum heats up they expand more on the top and across it.

[dcreel]

Yes pistons are out of round and the head is tapered smaller than the skirts. When the aluminum heats up they expand more on the top and across it.

Wow, I didn't know that. What is to keep it from expanding evenly, and making a seize more probable? hmmm...

[sportsfinat]

well the hottest side is going to be the exhaust side so i would think it would get the most 'swollen' i go to school for aviation and in aviation piston engines the pistons are somewhat oval and its because they expand because of the aluminum. i wasnt sure if this was the saame or not but it kinda sounds like thats the case. i feel kinda releived now lol...thanks or the help so far.....im still wondering if the bore is correct now though.

[Dirtcrasher]

I would have immediately agreed with Dcreel but:

I just measured my brand new Wiseco for an 86 200X and it is .008 out of round. Longer perpendicular to the wrist pin also....

I've never even heard of this before to be honest with you guys?? The piston looks finish ct un a lathe, so how is it egg shaped unlace a tracer type device is used......

I'd like to hear more about this........It makes me wonder WHICH dimension gets measured to match the bore size?? Maybe this is why air cooled motors blow up so often due to poor measuring techniques and lack of knowledge.

[Danold]

I would have to say the measurement taken for the bore would be based from the measurement parallel to the wrist pin, that would let the piston and cylinder wear into a good seal. If the measurement was based off the perpendicular side it would cause the cylinder to were unevenly and blow-by would occur on the sides of the piston.

I wonder how hard it would be to get in contact with the machine shop that does the work for the dealerships? Somebody has to know what their doing out there or else we would be throwin pistons in these things 3 times as often as we have to.....

[3wheelrider]

Yes pistons are out of round and the head is tapered smaller than the skirts. When the aluminum heats up they expand more on the top and across it.
like he said -(they are not perfectly round).

[Dirtcrasher]

Danold - Thats what I'm thinking.......

Where do they measure as piston? are they aware of this? Has this been causing improper clearances in the past years?? It may explain some failures, esp with airfoolers....

Is this why Honda250SX may have (read that - MAY HAVE) had a clearance issue??

I can't believe we've been shooting the chit for 10 years and never talked about this before, :lol:

[dcreel]

How do you get a good reading or good ring seal when you are putting an oval peg in a round hole? I understand that the rings do the sealing, but what if the ring was sticking farther out and caught a port or something? Wouldn't it be weaker at that point where more of the ring is sticking out of the ring land closing the gap where the smaller side of the oval piston? It seems it would make the weak link (rings) weaker...

[Dirtcrasher]

^Again, I'm entirely with you on your thinking!

They place ring end gaps carefully where they don't line up with any of the ports on a 2 stroke. On a thumper, it's just a round hole, so like you said : why put an out of round piston in a there?? I'm lost, I'd like to see how pistonms are made now because a cast and turning it are chicken, but making one axis or another a mere .010 - .020 out of round?? I guess they know what they're doing, but I bet allot of machine shops don't!

Apparently they've done extensive research as far as where it expands and how much. If a piston were plain steel, would it be perfectly concentric??

If I had ever machined my own piston it would have been 100% round. Now I'm glad I never made one...................

[sportsfinat]

no you cant bore the cylinder to the small of the piston...(parallel to wrist pin) the piston will not even fit in the cylinder at all. cause thats what i had done first and checked it and the piston would not go in with out forcing the piston. and even at that it was so tight the piston wouldnt hardly budge. so it is .002 over the longest part of the piston like wiseco calls for. but unfortunately the machinist let the boring tool touch the top of the cylinder and there are scores in it....il post some pics here in a day or so and see what you guys think but im not sure what to do. it might still work but could smoke, or not hold compression or idk....ill see and like i said ill post some pics for youo guys to see soon

[3wheelrider]

page 19- jennings book. states this. sorry i dont know how to paste and stick pics and all that.