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Keith Black pistons not the same size???

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danoflhrci

Member
I'm doing a 95" with KB's STD 348 Hypertuectics. The boring/honing is done (both with .0013" clearance) but I'm a little concerned about the sizes of these. One is as KB claims (3.8745) and the other is (3.8713). My rep where I bought them seems to think this is perfectly normal, but I think the freak piston should not have made it into a box for market. Wondering if their QC has over/under #'s that are tolerable. A lot of extra ring grinding on the smaller piston & I think if one was to get a piston that was larger by .0032" you would need to order an over-sized ring pack. The true piston required no grinding on the scrapers, a slight shave on the second ring & a wee bit more on the 1st to get .028".

Two questions come to mind: 1-Anyone aware of any specs KB's (United Engine & Machine) Quality Control might use? Anotherwords, maximum larger/smaller exceptable #'s before packaging. 2-Could there possibly be an issue running with one cylinder/piston .0032" smaller than the other?
Thanks in advance!!
 
I'm with Bodeen, you need to call. +/- .003/.004 is large for a non heat treated part.
 
As a past machinest and millwright .0032 ( 32 ten thousands of an inch ) is nothing to fret about. Could be wrong. have been before..:D
 
danoflhrci said:
I'll try one of the techs there. Thanks!
Click to expand...

Good call guys! The tech recommended not running it cause the piston most likely wasn't made that way & probably took a fall or something. Meaning there could be a deformity I couldn't find with the measuring tools I got. They're sending me another set (.010" over) unlike my rep at MidUSA. He sent me a copy of the instructions to fix the freak piston. Still can't figure how that works though.
Like I said, good call & thanks to both of ya!
 
Glad you got it or are getting it sorted out.
 
You Probably saved EARLY Piston Slap...

Tight on these pistons in their bores is a MUST...

I Loved KB pistons in a build on my 2000 FXDS... Properly bored cylinder clearance (TIGHT) and correct gapping of the rings is a Needed thing.

Looks Like You are doing a GOOD job all around here.:D

Using the proper torquing procedures is also a must...

signed....BUBBIE
 
Count the significant figures and see you are measuring in 100ths of a thousandth. Understand that a temperature swing of the part by a 10 degrees can cause 10ths of a thousandth to change. I would be very skeptical of the measurements and the accuracy of the devices used to take the measurements. Just the feel of a mike, the spot on the skirt used to gauge from, or a swing in temp a little could cause the 2 or 3 10th shift and anyone claiming to be measuring 100ths well that is likely not true unless it is a metrology lab with calibrated air gauges in a temperature controlled room. Most gauging is in .0001 increments but still is only accurate to 2 microns or in other words +/- .0001

If that were mine and I was 100% sure there was 1.3 thousandths clearance verified with calibrated gauging at room temperature and there was no damage to the piston I would run it with confidence as long as the KB ring gapping instructions are followed to the letter.
 
Interesting read Don. I'm waiting for the new pistons & still have these ready to go in. I'm 99.9% sure these are cleared by .0013 all the way around but what really worries me is what UEM says. I'm told that the same amount of metal is in both pistons. If one is .0032 smaller in the skirt that metal has moved somewhere else. He also said it's possible it could have been dropped during the manufacturing process. I can't see or measure any evidence of this but if install these I think it will always naw at me. I think I'll wait on the .010 overs just for my state of mind, which is what the rides are all about anyhow.
Thanks again Don!
 
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