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Old 06-08-2006, 09:44 AM
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Piston tech

I have a tech question
As pistons run in a bore I know the skirt face centres the piston in the bore but I'm wondering what centres the top of the piston. I know the compression rings
float in the top grooves as any engine develops carbon behind them. I've always assumed the oil ring expander does but I don't know for certain. I have a 383 with KB pistons installed to the min specified spec. # 6 cylinder has an odd wear pattern after just 700 miles and a slight piston rattle cold. The pattern could be best described as two polished diamonds about an inch square one above the other on the thrust face of the bore. I'm running total seal rings with the gapless ring in the second groove as specified. The engine runs well and normally doesn't smoke but if you idle for long periods it will oil this plug slightly. The pistons measure fine for skirt clearance and groove width so I'm wondering what else could cause it to rattle

Potts

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Old 06-08-2006, 12:20 PM
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Sounds like Ring Flutter


Its a matter of opinion but I always run my second ring with more gap then the top, and would never use a gapless ring on the second groove.


By the way are you sure the clearance is correct? KB pistons are setup differently then motors with forged

Last edited by brainsboy; 06-08-2006 at 12:25 PM.
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Old 06-08-2006, 01:47 PM
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The piston clearance is set to the min speced by KB. The total seal rings are the street version which specify that the're for the second groove. The ring end gaps are to the KB spec which is differant than normal. but the KB installation instructions are specific about using there specs as the piston design demands it. I had heard that KB pistons can be noisy in a street engine if not installed at the tight end of the clearance range they spec. Unlike a forged piston motor it is only # 6 that rattles the others are perfectly quiet. The mystery has me stumped but the wear pattern I described is only in # 6 bore as well. The question seems to me is it the piston , bore or rings?

Potts
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Old 06-08-2006, 05:03 PM
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My guess is that it is a bore geometry problem. Was the block bored and honed with torque plates? If it was, the bores will mike about .0007-.0010"" smaller in a relaxed (head off) state than in a heads installed state. If the block was not finished with a torque plate, the head installation will make the bore slightly bigger in the top third of the cylinder than it measures head off. If this is the case and the bore was finished a little big, you will have a little more bore clearance than planned on.

tom
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Old 06-09-2006, 07:58 PM
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The block was honed without deck plates as the shop had none for the big block dodge. The machinist was not excessively concerned due to the thick walls of the older chrysler big blocks. If I understand your comment then bore irregularity after assembly is causing the piston to chatter in this one bore. Would this explain the wear pattern and the oil on the plug? Will this problem continue or will a few thousand miles wear it in ? In other words is there anything I can or should do?

Potts
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