piston/cylinder clearance?

[xTHUNDERCATx]

Ok i have a pro-x cylinder, 72.5 mm wiseco piston. Almost everything i can find says that wiseco pistons were generally set at .003". I got my piston in today and the new piston says to set clearance at .002". So the question is what do I go with? Do i stick to the old clearance tolerances or do i go with what came on this new piston? Did wiseco change something that im not aware of?

[udontknowme]

.002 sounds about right but i think that all depends on the cylinder casting material and design and piston composition of course. seems like  i read something about esr cylinder needing alittle more clearance but maybe it was bs. perhaps .003 will work better for that particular cylinder

i dont think theres any general rule that say a 125 should have X amount of clearance and a 250 should have Y clearance. theres alot of variables involved. seems like my 300 was a hair over .002 and the 250sx was real similar. one of my ktm 500cc bikes is .0019-.0023 depending if you have mahle or wiseco. so theres really no general rule

[xTHUNDERCATx]

Indeed I understand that, It just made me raise an eyebrow because most of the info i found on the wiseco pistons were all saying .003 give or take a little depending on the goal with it.  the wisecos had clearance recommendations sent out with them and i actually think i found a thread with neil saying those pistons called for .003 with slight changes. But .002 and .003 seemed like a decent jump so i was wondering why they changed.

[xTHUNDERCATx]

http://www.trx250r.net/forum/engine-transmission/20687-proper-piston-wall-clearance-78-50-pro-x-cylinder.html
1st reply, this is why i am asking for clarification. also thanks for the reply, interesting knowing your setting up a 500cc at those tolerances.

[udontknowme]

i dont think its so much the size of the engine but more so the material and design of the cylinder and piston. no time now but tomorow i could probly show some pics

[jcs003]

machining quality and build quality also play a role.  

john

[rsss396]

.002 seams tight for a forged wiseco , the 78mm wiseco makes for CP and their sphynx/puma cylinders calls for .004

[udontknowme]

we have to remember the clearance on the box is only a specification for when the parts are cold. when the engine gets hot everything changes. what the true clearance is when everythings hot i have no idea but i think that is whats most important. you may have .004 on a sphynx or .002 on a 500 but at some operating temp range im sure the true clearance will be about the same between both engines. from these numbers it appears the sphynx and wiseco have quit different expansions as the piston will grow quit a bit more than the cylinder, why else would it need .004 cold clearance. .002 on a 500 seems to have expansion similar between piston and cylinder and there for only a small cold clearance is needed

these are some pretty big pistons yet only need small clearance. if your not familiar with mm then .05mm is just under .002". (.0019" or something like that. ) .065mm is about .0026"