Hi all, I have a 1340 Evo that has a motor noise that was diagnosed as piston slap or crack in front piston. The noise was a metallic rattle and a stethoscope indicated its location. Lifters had been replaced and I removed and retorqued the compensator nut just in case. I now have the barrels off and have measured bores and pistons. Both cylinders measure "square" over six spots in each bore (I didn't have torque plates fitted while measuring) - the front bore is 3.498" and the piston skirt on the thrust faces 3.493" - so 0.005" clearance which is about on the service limit according to my manual. The rear bore measures 3.497" and the piston 3.495" - so 0.002" clearance. There was no noticeable noise from the rear cylinder. I don't think the motor has done much work - hone marks are still quite visible in the barrels and the pistons show very little wear on the skirts. Is it possible to get higher tolerance pistons? Thanks for any advice

 

Are these forged pistons you are refering to??
Scott

 

Hi Scott, I think the pistons are original, they are marked "STD 4" on top and inside the skirts "22091-83B" and "83075" and a $ sign over an "A"
Bill

 

You can't get accurate measurements without torque plates; .005" is too much.

If piston slap was the noise, you should see some scuffing on the cylinder wall; see photo.

 

Just a commit and I know pictures are deceptive but it does not appear that the cross-hatch pattern was done correctly. Looks like you also have and out of round condition here or are running short skirted racing pistons. What was your piston to wall clearance here?

 

No kidding! Why do you think they were pulled off the engine? Setup was supposed to be .002" but I did not measure to confirm; my mistake. Cross hatch is fine but the cylinder in the pic is "belled" in the center. Took 300 miles for the piston to start to make noise. The rear is worse. I don't see how this cylinder was setup at .002"; the guy that measured them and filled out the meausrment sheet must have been smoking some of that wacky weed. The rest of the story is too long and I don't want to drop any names but after two tries with the original supplier, I am going a different route for a fix.

Not looking for a fix. I was just posting the pic for the OP to show an example of how a cylinder might appear if piston slap is the source of the noise.

 

Thanks djl, annoyingly I cannot view your photo, but i have just discovered that I need to have 5 posts up before I can do so and this is number 5, so hopefully soon!! Do you know to what extent torquing the barrels alters the measurements? and if slightly oversize pistons are available? thanks

 

When I first looked at you picture I thought the cylinder was either out of round or the piston was too tight in bore an was seizing causing the wear pattern. When you talked about .002 clearance I had to get my book out since my experience is rebuilding car motors and I was surprised at how tight this air cooled motor is. Harley calls for .0014-.0025 clearance (max .003) between the piston and a stock bore 3.7500-3.7505 cylinder. I guess they can get away with it since its a Al piston and an Al cylinder (with I think a steel sleeve) It does have a steel sleeve doesn't it?

 

Hopefully I have attached a photo of the noisy cylinders bore and piston - even tho' there are some marks visible the bore still measures on new spec of 3.498"

 

hard to tell from your pics,but it looks like their scratched up pretty good and a bore job is in your future,go .005 (you do need t. plates to check the cylinders)