Author Topic: Wossner Piston (Read 8891 times)

hub97119 · « **on:** January 22, 2015, 10:08:32 AM »

Looking to drop in a Wossner piston. Specs call for a .06mm clearance. Has anyone set them tighter or is this a good clearance? Thanks Ron

Pumashine · « **Reply #1 on:** January 22, 2015, 12:32:35 PM »

Quote from: hub97119;49108

Looking to drop in a Wossner piston. Specs call for a .06mm clearance. Has anyone set them tighter or is this a good clearance? Thanks Ron

.002" clearance? You have to be kidding right? I guess forged pistons are run on the tighter side. From what I have read the .06 is tight. Some run .07 or .003 clearance. I would not go tighter but I am used to the Wiseco pistons.

FerrinMotorsports · « **Reply #2 on:** January 22, 2015, 02:09:38 PM »

Actually Puma Cast pistons run a tighter clearance....Forged pistons run a larger clearance due to the swelling of the piston....

Pumashine · « **Reply #3 on:** January 22, 2015, 05:13:55 PM »

Quote from: FerrinMotorsports;49114

Actually Puma Cast pistons run a tighter clearance....Forged pistons run a larger clearance due to the swelling of the piston....

I see Wiseco are forged also. My 85mm piston uses .004" clearance. Seems like .002 would not make it out of the parking lot.

udontknowme · « **Reply #4 on:** January 22, 2015, 08:56:48 PM »

this topic pops up all the time on every site. heres what i think. i havent found bore size to have much influence on what the skirt clearance should be. not long ago i posted a page from the ktm service manual. spec for 92mm mahle forged pistons are .002". the wiseco replacement is .0023" if i recall. so that that throws out the theory of a larger piston needing a bunch more clearance than a small one, not to mention the theory that forged need more clearance than cast, which also doesnt have much truth to it. the skirt clearance for wiseco forged piston for ktm 300 is nearly the same as the cast vertex skirt clearance. what most people dont realize is the piston is shaped like a egg and gets smaller as it goes up. the forged will probly be looser at the very top (thus having a slightly more egg shape) but regarding the bottom skirt clearance, modern forged and cast are close to the same

from what i can tell, the things that determine the skirt clearance are piston design, the ingredients of the piston and the cylinder bore material.

hub97119 · « **Reply #5 on:** January 22, 2015, 08:57:15 PM »

My bad...I meant more loose. Seems a little tight to me but .06mm is the spec on the box

udontknowme · « **Reply #6 on:** January 22, 2015, 09:08:21 PM »

if wossner designed the piston around a oem cylinder and your using it in something other than a oem cylinder, it may need more skirt clearance, then again you may be fine with even less skirt clearance. its difficult to say for sure.

i havent got around to it yet but one of my projects is using a cr500 piston (meant for a iron bore) and putting it in a aluminum nikisiled ktm cylinder. certainly the bore expansions will be different. eventually ill have to do some research on what skirt clearance will be acceptable

FerrinMotorsports · « **Reply #7 on:** January 22, 2015, 09:54:07 PM »

Again less clearance for Cast piston....More clearance for forged....Look in the Honda Manual......If I remember they said anything over .003 is worn out....Honda's pistons were cast pistons...I will check the manual again when I get home tonight to make sure of what I'm posting here...(The clearance measurements from Honda)...I'm positive on the clearance differences between cast and forged....

udontknowme · « **Reply #8 on:** January 22, 2015, 11:28:12 PM »

ferrin read again was i said earlier: from what i can tell, the things that determine the skirt clearance are piston design, the ingredients of the piston and the cylinder bore material.

in some instances a cast piston might specify less skirt clearance. the honda manual is using specs of 30yo cast pistons. who knows what their composition was back then, probly not the same as it is today. forged today are not the same as they were 30yers ago either. the specs in modern times is nearly the same for cast or forged regarding skirt clearance, in many instances. i know this because ive got cast and forged pistons for the newest generation ktm engine. i also know if the piston is huge it doesnt meen it needs a ton of clearance like urban legend would suggest. ive put together engines with 92mm pistons at barely over .002" using wiseco. this wasnt guessing with feeler gauges slide between the piston and cylinder wall but using bore gauges and micrometers

fearlessfred · « **Reply #9 on:** January 22, 2015, 11:42:51 PM »

Quote from: udontknowme;49168

ferrin read again was i said earlier: from what i can tell, the things that determine the skirt clearance are piston design, the ingredients of the piston and the cylinder bore material.

in some instances a cast piston might specify less skirt clearance. the honda manual is using specs of 30yo cast pistons. who knows what their composition was back then, probly not the same as it is today. forged today are not the same as they were 30yers ago either. the specs in modern times is nearly the same for cast or forged regarding skirt clearance, in many instances. i know this because ive got cast and forged pistons for the newest generation ktm engine. i also know if the piston is huge it doesnt meen it needs a ton of clearance like urban legend would suggest. ive put together engines with 92mm pistons at barely over .002" using wiseco. this wasnt guessing with feeler gauges slide between the piston and cylinder wall but using bore gauges and micrometers

All this crap sounds like some the stuff the coc would make up.I would use the piston makers recommendations

udontknowme · « **Reply #10 on:** January 23, 2015, 12:08:16 AM »

fred you may be alittle confused. nobody said not to use the manufacturers recomendation. im all for using manufacturers recomendations most of the time, depending on the situation. i merely said if your taking a piston intended for a oem cylinder and using it in something other than oem cylinder, you may want to get some professional advice or do some research of what clearance would be acceptable. do you think the expansions between a oem cylinder and one of calvins cylinders would be the same ? i kind of doubt it. this may be why people run into problems, then blame the piston

FerrinMotorsports · « **Reply #11 on:** January 23, 2015, 02:17:21 PM »

Okay....whatever

Pumashine · « **Reply #12 on:** January 23, 2015, 03:57:13 PM »

Quote from: fearlessfred;49170

All this crap sounds like some the stuff the coc would make up.I would use the piston makers recommendations

I second that. I have seen .06mm and .07mm suggested for Wossner. Wiseco suggests .003" to .004"

udontknowme · « **Reply #13 on:** January 23, 2015, 08:35:25 PM »

seems like some of you guys refuse to see things outside of your obscured tunell vision. people that have no desire to learn anything never will learn anything. figure this stuff out on your own for all i care. grind down your rods, put your pistons so loose they rattle your teeth out, use your 50:1 magic oil

Jerry Hall · « **Reply #14 on:** January 23, 2015, 08:43:25 PM »

The OEM's piston skirt shape and recommended piston to cylinder wall clearance is determined through a lot of trial and error and expensive failures after all of the initial FEA and thermo analysis has been completed. The after market piston companies do not have the time to do a lot of field and dyno testing, or money to buy test engines for every model of piston they manufacturer. Computer analysis is all the development most of the aftermarket pistons get. Customers do the majority of the field testing at the customers expense.

I follow the piston manufactures recommendation until my testing guides me to use a different clearance. Normal cylinder and piston operating temperature is what determines a recommended clearance. Run the cylinder cooler that the normal temperature and you will need more clearance. Run the piston hotter than normal and you will need more clearance. Run the piston cooler than normal and less clearance can be used. Run the cylinder hotter than normal and less clearance can be used.

The big question is:.............What cylinder or piston temperature is normal ? I do not know, that is why a lot of testing and observation is necessary for each type of riding. Different types of riding typically control the cylinder temperature and the amount of power and the length of time spent at any one full throttle blast determines the maximum piston temperature and the necessary piston clearance.

It is always better to have a little more clearance than necessary than too little clearance. If it is too tight the piston will seize. If it is too loose the piston life is shortened.

I cannot offer any recommended clearance for your Wossner piston. You did not say what engine you are working on.