Piston - bore clearance
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Intermediate
Joined: Jun 2010
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From: Australia
Piston - bore clearance
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
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From: Honah Lee
Picture?
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?
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Joined: Jun 2006
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From: san antonio
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.
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Intermediate
Joined: Jun 2010
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From: Australia
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
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Joined: Mar 2010
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From: Honah Lee
Picture
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?
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