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[HILLSIDECYCLE.COM]

I see the words piston slap tossed around, like the 6 of clubs.
How much piston/wall clearence do you suppose is required to establish, the "piston slap".

[AXIL]

   Good question. by the time i see them thay are not worth measuring.

[Mr. Warlock]

I'm assuming this is one of those "I know" but "do you know" revelation threads 

No, I don't know but I'm assuming it's a lot smaller number than one might think.

[VANAMAL]

I was trying to figure that out too. my piston /wall clearance was .0045 according to jim at metal dragon. That was enough for slap.

[Twolanerider]

Most accurate answer for correct tolerance has to be "it varies."  Whether forged or cast is an obvious factor but a variety of other factors come in to play.  

Even forged versus cast doesn't always define.  TRW (and perhaps others) offers a product with what is called a "divorced skirt" for the automotive world.  That's a cast piston that is set up with nearly forged tolerances.  If the most accurate answer is "it varies" the most precise answer has to be "follow the specs of the piston manufacturer."

Decent source for some general discussion of piston choices:

http://www.mre-books.com/sa21/sa21_5.html

[grc]

Once you've determined the optimum piston to wall clearance, then you need to look at rod angularity and pin offset.  There are lots of things that enter into the equation.


Jerry

[HD Street Performance]

Yes Scott
Enlighten us and tell us what you believe.

[cahdbiker]

Hillsidecycle, according to 2009 HD service manual on chapter 3 page 79 HD sayes to replace piston and or cylinder if clearance exceeds .003. When I was having problems with my S&S 107 Evo style motor, it was knocking  like crazy and sounded like valve train noise, Bennett's  in Long Beach took it apart and said piston clearance was.0045 and.0050. at 2000 miles on a brand new engine. (I have already ranted about this in the Evo section of this site because it happened again after only  2000 miles and a complete rebuild by S&S under warranty). This was the second time S&S had a chance to get it right and screwed up. Third time Bennett's had S&S send new jugs, pistons and rings and set up at .0025. problem solved. (Still under warranty). Now there is no piston slap and it runs great with normal oil consumption for Evo engine.(3000 miles since Bennett's put it together) Turned out all that noise I thought was valve train related was piston slap. Question: what expands faster piston and rings or cylinders? If this is jumping a thread I apologize. CAHDBIKER

I see the words piston slap tossed around, like the 6 of clubs.
How much piston/wall clearence do you suppose is required to establish, the "piston slap".
[/quo

[cvobiker]

Yes Scott
Enlighten us and tell us what you believe.

. What he believes, is everybody his this problem and if you don't send him your motor to fix then you are in real trouble   

[hogasm]

. What he believes, is everybody his this problem and if you don't send him your motor to fix then you are in real trouble   

Would you insinuate a TROLL is on this site

[djkak]

The OEM EVO would begin to rattle around .0022”. EVO clearances out beyond .003” would become quite obnoxious. I rarely if ever saw an OE machine with clearances at .004 or more, unless there was some sort of damage; collapsed skirt, scored, etc.

Sometime around 1992, I did see an ’84 FXST from Ames that was apparently put together from parts lying around the barn. The machine had black highlighted cylinders that were bead-blasted to match the crankcase. One of the cylinders was bored +.010 and was running a standard size piston. The bike was being ridden without trouble, and it ran cleaner than I would expect it should; although it made more racket than a jackass in a tin barn.

[cahdbiker]

djkaka, thanks for the info. I guess what baffles me is how you get piston to cylinder clearance correct unless the piston, rings and cylinders expand and contract at the same rate. I am not a mechanic, but I used to do a lot of H#%da  basket cases for extra money way back in my college days. I am sure some of our more mechanically inclined members might be able to enlighten me on my question. Just something that crossed my mine when reading this post. Thanks. CAHDBIKER

The OEM EVO would begin to rattle around .0022”. EVO clearances out beyond .003” would become quite obnoxious. I rarely if ever saw an OE machine with clearances at .004 or more, unless there was some sort of damage; collapsed skirt, scored, etc.

Sometime around 1992, I did see an ’84 FXST from Ames that was apparently put together from parts lying around the barn. The machine had black highlighted cylinders that were bead-blasted to match the crankcase. One of the cylinders was bored +.010 and was running a standard size piston. The bike was being ridden without trouble, and it ran cleaner than I would expect it should; although it made more racket than a jackass in a tin barn.

[TXCHOP]

Had a set of forged knock at .005+/-...

[djkak]

...I guess what baffles me is how you get piston to cylinder clearance correct unless the piston, rings and cylinders expand and contract at the same rate....
Thanks. CAHDBIKER

The aluminum piston is going to expand at a greater rate than the cylinder. The piston’s skirt is not straight or round, and is designed in a manner which allows the piston to expand in the cylinder without overloading the thrust surface.

As the piston’s temperature increases, the contact footprint between the skirt and the cylinder becomes taller and wider. A piston that has been exposed to extreme heat will have a broader “wear pattern” on its skirt than one that has not been run extremely hot.

If anyone has a set of 110 “take-off” pistons lying around, it would be interesting to photograph the wear pattern on the skirt and make a comparison to the photo of Van’s piston above.

[HD Street Performance]

When I get time I will take a few. These have about 7k on them