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Piston-Cylinder clearances...Help
Well, I thought I was doing real good, My cylinders looked to be ok but my pistons are really confusing me. I have a bore diameter of 3.1500" and a piston o/d on 3.145" (.005 clearance) However the east-west piston dia shows 3.135" (.015" clearance to the bore) The diameter is measured around the top of the piston with the rings removed. But the north-south diameter is only showing .005" clearance. Why would the east-west diameter of the piston be that much smaller? I thought the wear would be on the upper and lower sides of the piston. What clearances should I be looking for. What can I get away with on the clearances. I was only doing a quick head job to get the car on the road while I spend my money on a bigger engine. Don't want to do a piston and cylinder job! Any ideas?
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Steve F 69 911 71 911 87 Carrera 2004 RAM 1500 4x4 "Hemi" |
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02-25-2002, 02:32 PM
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Location: Marysville Wa.
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pistons are oval. the only place to mic a piston is across the skirt (north/south in a 911). within an inch of the bottom of the skirt. any other place will tell you nothing.
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https://www.instagram.com/johnwalker8704 8009 103rd pl ne Marysville Wa 98270 206 637 4071 |
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02-25-2002, 05:09 PM
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Location: So. Calif.
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According to the '69 spec book:
Piston clearance specs are 0.0039" (.1mm) Piston OD is 3.148" (79.97 mm) Cylinder bore is 3.1496" - 3.150" (80.00 - 80.01 mm) You should measure the piston OD about 0.826" from the bottom of the skirt perpendicular to the piston pin axis. You should measure the cylinder at the top and bottom of ring travel and also in two directions to determine out-of-round spec. All this is on page 34 of the spec book. Get some reference books to help you decide what's worn and what's okay. There are other things to measure as well. Sherwood Lee http://members.rennlist.org/911pcars www.seinesystems.com |
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02-25-2002, 05:09 PM
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Thanks, I'm learning all the time, feel a bit stupid this time. I think a spec book sounds a good idea
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Steve F 69 911 71 911 87 Carrera 2004 RAM 1500 4x4 "Hemi" |
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02-25-2002, 05:58 PM
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Location: Los Alamos, NM, USA
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One also needs to be using a 3"- 4" range micrometer and an inside "snap" gage to make these measurements which should be done to the .0001" level. It takes some experience to learn the correct feel (drag) of an inside snap gage required to obtain an accurate inside diameter measurement. My references state that the pistons and cylinders should be replaced when the running clearance exceeds 0.15mm or .0059"; based on the measurements you listed your P/C's may be close to the limit. Jim
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02-25-2002, 07:00 PM
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1969 and 1971 Porsche 911T/E 2.0L and 2.2L
