piston to cylinder clearance

[pervO]

What do you guys think? My local machine shop made my Wiseco piston .015 inches smaller than my cylinder.

[Bren_downe]

What trike? What does weisco say? What does your manual say?

[ironchop]

If those figures are indeed accurate, it looks like you are running a 69.5mm piston. Of all the topend I've ever seen, the maximum piston-to-cylinder-wall clearance is usually no more than .0035 in inches. That means that if your piston is indeed .015 inches smaller than your bore, you got WAY too much clearance.

We could use some more details, of course, but it's not looking good. Are you saying he turned down the piston? Or overbored the jug that much?
"My local machine shop made my Wiseco piston .015 inches smaller than my cylinder."


Inches x 25.4 = millimeters

Millimeters/25.4 = inches




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[HairyJR]

Lets look at it in the positive as I did, the machinist measured the original cylinder bore to be 2.720", new piston measures 2.735", then he could rough bore out .015" with a finish bore or honing to 2.737" giving you a .002" clearance. Still might be tight if as Ironchop mentions if it needs .0035" clearance. Does it run an rattle?

"HJ" 240836 :beer

[RIDE-RED 250r]

If those figures are indeed accurate, it looks like you are running a 69.5mm piston. Of all the topend I've ever seen, the maximum piston-to-cylinder-wall clearance is usually no more than .0035 in inches. That means that if your piston is indeed .015 inches smaller than your bore, you got WAY too much clearance.

We could use some more details, of course, but it's not looking good. Are you saying he turned down the piston? Or overbored the jug that much?
"My local machine shop made my Wiseco piston .015 inches smaller than my cylinder."


Inches x 25.4 = millimeters

Millimeters/25.4 = inches




Sent from my Z958 using Tapatalk

Agree with Ironchop.... If those figures are not a typo, you have way too much clearance... Usually overbore pistons go in increments on .010"... Think about that for a second.

Piston/cylinder clearance is usually measured to the ten thousandths

And always go by the piston manufacturers spec for piston/cyl clearance and ring gap

[ironchop]

Lets look at it in the positive as I did, the machinist measured the original cylinder bore to be 2.720", new piston measures 2.735", then he could rough bore out .015" with a finish bore or honing to 2.737" giving you a .002" clearance. Still might be tight if as Ironchop mentions if it needs .0035" clearance. Does it run an rattle?

"HJ" 240836 :beer
I couldn't make sense of what was what written on that piece of box top. Hard to tell which figure represents what. Your interpretation makes more sense than mine.

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[pervO]

What Im thinking from looking at his numbers is

Bore was at 2.720" and target was 2.737" so he bored .015" to 2.735" and honed the last .002" to 2.737". I would assume the piston measures 2.735" (1/2" from bottom of piston skirt) and it gives you the correct .002"-.0025" piston to wall clearance. This would be the only logical way this would make sense. This would be a .020" overbore cut. figuring in the extra missing .003" is from wear of the previous bore.

[motor1066]

im a machinist by trade and yes i agree with perv0. that is the only way it makes sense. 015 thousands clearance is like a hotdog down a hallway and would run like Sh*T and knock like crazy. Rule of thumb .002 to .003 for cast pistons, and .003 to .005 for forged. forged pistons expand larger and need more clearnace

[John Tice]

All of the Wiesco pistons are now forged. The US power sports piston manufactures have come a long way with alloys & designs since they switched from castings to forgings. Modern power sports forged pistons take about the same clearance as cast pistons. Wiesco pistons typically print the recommended skirt clearances on the box. On occasion we question the recommendations on the box & give Wiesco a call to confirm the fit.
I’m now fitting a typical 100mm; nearly a 4” bore cylinder with a .002” skirt clearance.

The theory is that plated cylinders dissipate heat faster than cylinders with a sleeved cast iron liner. When we fit a new sleeved cylinder with Wiesco brand pistons, I usually give them an extra .0005” clearance just in case.

John Tice
503-593-2908 Alternate 541-595-0310
www.smallenginemachineworks.com & www.nwsleeve.com
Turning Custom Cylinder Sleeves Since 1971
http://forums.everything2stroke.com/threads/49513-How-It-s-Done-Projects-around-the-Shop.com

[motor1066]

yes wiseco does. i should go check the one i put in my 250x. i think they called for .0035 clearance. i agree with john, an extra .0005 is good insurance . i just purchased a ebay kit for a yamaha blaster. it came with piston cylinder wrist pin bearing and gaskets and rings. for 64.99 shipped. i was quite suprised when i looked at it. i chamfered the ports a bit more and then ran a ball hone down it before cleaning it up and installing. test fired the motor and broke it in a bit and i was amazed how quiet it was. no rattles. we will see how it holds up. for $60 cant complain if i even get a season out of it