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360 pistons

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drcrad View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote drcrad Quote  Post ReplyReply Direct Link To This Post Topic: 360 pistons
    Posted: Nov/27/2015 at 10:20am
had my '72 360 block bored .020 over, from 4.080 to 4.1. ordered .020 pistons and they measure 4.060. seems a little sloppy to me, local machinist agrees, but then measured stock piston and it measured 4.030 for the 4.080 stock bore.

any ideas?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tyrodtom Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 10:29am

Pistons expand as they heat, worn pistons will give you the worn dimensions.

Cast, forged, and hyperutechnic,(probably spelled wrong) all expand differently when they heat, and different batches of pistons will have different dimensions also.

Every machinist I've used, insisted on getting the pistons first before they would bore the block.

Edited by tyrodtom - Nov/27/2015 at 10:38am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 10:39am
Im with Tyrodtom.    A machinist gets the pistons before machining......
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Traveller Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 11:06am
Depends on where they measure. The skirts measured 90 degrees to the wrist pin, approx. 1-1 1/4 inches up from the bottom of the skirt, or wherever the manufacturer tells you to measure them. On a cast piston, it should only be about .0015 smaller than the bore, but also allow for the graphite coating if there is one. That's one and a half thousandths, not 15 thousandths. If they're measuring the piston crown, it will usually be quite a bit smaller than bore size, depending on the piston manufacturer with Keith Black stuff being one of the smaller crown size pistons.

The measurement that you're getting at 4.060 is .040 smaller than bore size that you're listing (4.100), so I'm suspecting it's being measured at the wrong location (crown). That's not where you measure pistons, or the measurements you're putting up are missing some digits.   No way in heck does someone run a piston with .040 wall clearance.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Boris Badanov Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 11:47am
The piston dimension will vary to the manufacturers recommendation.
Traveler is correct, measured at the gauge point the CAST piston will
have a clearance of around .0012-.002" at the gauge point.
A forged piston will be .0045+ and may be more for drag racing applications.
I used to run .007 in a 390 drag motor.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote drcrad Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 12:04pm
great, informative responses. I have been checking at the wrong place, many thanks for teaching an old dog a new trick will check and let you know what I find
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXFSTBK390 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 12:42pm
My 390 block was bored .030 Teflon coated pistons. The tops of the pistons are stamped .030. The machinist called me to say the pistons were too tight a fit in the bores. He called Packard Industries the manufacturer of the pistons and Packard told him the bores need to be honed an additional .0015 because of the coating. Guess what was added to my machine shop bill? $125.00 for additional clearance to block. Should the machinist have known this, or he was putting me together and overcharged me? Hmmmmm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Traveller Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 1:33pm
Originally posted by AMXFSTBK390 AMXFSTBK390 wrote:

My 390 block was bored .030 Teflon coated pistons. The tops of the pistons are stamped .030. The machinist called me to say the pistons were too tight a fit in the bores. He called Packard Industries the manufacturer of the pistons and Packard told him the bores need to be honed an additional .0015 because of the coating. Guess what was added to my machine shop bill? $125.00 for additional clearance to block. Should the machinist have known this, or he was putting me together and overcharged me? Hmmmmm


Nothing recent needs extra dimensions for the coating. The coating is usually considered sacrificial.   The official word from most of the piston manufacturers, is that it's largely gone after the first 1-2 thousand miles.   The pattern can still be visible on the piston, but dimensionally it's mostly gone.   It's considered more of a break-in coating than anything else.   Almost all of the newer pistons that use a graphite coating will have a small "spot" on opposite sides of the piston where you measure the skirt dimension. They are frequently oval shaped, and are at the correct height to measure the skirt, and have no coating in those small spots.   It's for using your dial mic to check skirt size. The pistons will usually go in snug at the proper dimension, but break in quick. You'll have to set your idle down a couple hundred within 50 miles or so, and maybe after initial cam break-in.

If the piston manufacturer says a piston is for a .030 bore, they've usually built in the proper clearance into the skirt. They are usually set up pretty darn close for recent pistons, but they always need to be checked.

Again, the coating is NOT supposed to be added to the dimensions typically, the machine shop should be measuring the skirt, not the coating. If you add .001 for the coating, your .0015 clearance at assembly will be .0025 in a thousand miles.   UNLESS, instructions are included with the pistons that say the bore size is supposed to be larger than the oversize listed. In that case there should be a spec on the box that says finish hone to .0015 over the oversize amount (.020, .030, .040, etc.).   That is unusual however. Usually pistons come set up for proper clearance for the oversize listed.   Example: .030 overbore 4.195 pistons for a 401 will come out of the box at .0035 under the 4.195 finished bore size (recent forged run tighter).   It will be close to the proper size for a true 4.195 bore, but we always set the dial bore indicator off the measured piston size, then hone to the fit we want.   Modern quality pistons are remarkably consistent from piston to piston in a set, mainly thanks to modern CNC manufacturing, but they all need to be measured.   It's possible to get a mis-boxed piston in the set that's smaller or larger than what you ordered.   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXFSTBK390 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 1:56pm
Traveller, I received a call from the machinist after he balanced the rotating assembly. He told me one of the pistons is 6 grams heavier than the lightest piston, and that is unacceptable. He called Packard Industries and they shipped to me another piston within 32 hours. In the end the rotating assembly was balanced within 1-1/2 grams from damper to flywheel. I just looked at the piston that was replaced and there is no smooth spot on the skirt for measuring the piston skirt size. I also called Packard Industries and was told that when they weigh the pistons they put a colored tag on them for the weight, and it is possible to get a mismatched piston from an assembly line error. He also said with their company you do need to hone an additional .0015" for their coated pistons.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jcisworthy Quote  Post ReplyReply Direct Link To This Post Posted: Nov/27/2015 at 5:00pm
Dont like the sound of Packard pistons. Like Traveller said the piston is made for proper clearance in the desired bore. Many blocks are bored and honed before the pistons arrive with no issues. If Packard does not have a foot note stating the final bore size must be increased by .0015 they need to. 
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