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Old 01-14-2004, 09:48 PM
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Unhappy Long skirt SBC pistons

I picked up a running motor from 89 F-body that made some slight internal noise. So I decided to take it apart to evaluate the condition. The engine had some performance work done to it. The engine had about 2000 miles on it so it's very clean inside with no build up what so ever. Oil traveled well into the heads via push rods. Now when I removed the pistons (block had no ridge yet or any scoring), I noticed two pistons #3 & #7 had the outboard skirts broken about half inch up from the tip of the skirt. The broken pieces where in the bottom of the pan still in one piece, no shavings. So I figured pre ignition or detonation did it. OK, I get the local engine builder chase me a couple of slugs. No problem he says, those are 345 NP. Well, five samples later and they cannot match the pistons. Obviously the engine have been rebuilt. Two bolt mains, one piece seal crank, 14101083 (64cc) heads ported and polished, aluminum roller rockers, flat tappet cam with gears not chain. The pistons have H 1479 96 casted on the inside. Pin diameter 0.927", compression height of 1.560", rod length 5.700" and an overall piston length of 3.220" The new pistons the jobber received average about 2.610". According to some piston manufacturers, the zero deck height with these heads give me about 9.7:1 cr. That's good for street performance using pump gas. BUT, what are these pistons and why do they have such a long skirt? Hate to buy a new set, but if I have to, then so be it. Anybody familiar with these pistons? Thanks.

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Old 01-14-2004, 10:08 PM
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can't answer your question, but if the engine had all that performance work done, why would they put in long-skirt pistons anyway? you'd think they'd use something with a shorter skirt for less weight, possibly less friction?

you might be further ahead to buy new pistons and get what you want.

JB
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Old 01-15-2004, 12:50 AM
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unless someone on this site can ID them, you likely will be stuck with buying a whole set, Ebay always has lots of deals on pistons. the 345NP pistons you mentioned, keep in mind there compression height is 1.540 instead of 1.560, most of the basic replacement cheapy flat top pistons are made this way and don`t be fooled if you get them off Ebay or any site that tells you the compression height is 1.560 since ebay is ate up with false advertisement. the coated skirt line Hyper pistons made by sealed power have the true compression height so I would either go with those or KB pistons.
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Old 01-15-2004, 08:33 AM
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Yeah, they are silvolite Hypereutectics.

Chris
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Old 01-15-2004, 10:32 AM
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Those pistons are a hypereutectic version of the stock flat-top piston. It mimics the older style long skirt 350 piston. It is NOT intended for performance use.

The skirts are broken because of too much piston-to-wall clearance or because of careless installation.

If the bores are too big, the shorter skirt pistons will rattle like crazy.
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Old 01-15-2004, 03:37 PM
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here is some information about them from the Beck Racing Web Site
http://www.beckracing.com/slvpg29.htm
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Old 01-15-2004, 05:51 PM
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maybe the block was preped for forged pistons
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Old 01-15-2004, 06:59 PM
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Thanks for all the responses so far. The actual measurement of the compression height is 1.560" I noticed that according to Beck Racing listing, H3437 piston is the same as H1479 (mine) except NOT destroked, hence 1.540 + .020 = 1.560". I also checked the bore to piston clearances of the broken pistons and the result is 0.0015" for both. As a matter of fact, only two of the eight pistons had 0.0020". So I don't believe excessive clearance was the factor. Is it possible that whoever installed the pistons did not have the ring compressor tight enough and had to hit them harder toward the intake manifold side forcing the piston to rock against the cylinder wall when pounded in? As you can see from the picture I attached, there is only one tiny scratch on the piston, maybe caused by a small speckle when the skirt broke. The picture was taken after I wiped the oil and removed the rings. No other cleaning was done. If nothing else, I just learned that these were possibly made for the early 350 around the early 70s. The engine ran good, but my suspicions were right. Now I have to save some cash for new slugs and hanging them.
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Old 01-15-2004, 08:21 PM
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Quote:
Originally posted by 85 regal
maybe the block was preped for forged pistons
This is a common misconception. Because the clearance is built into the piston, there is no need to bore the block any bigger for a forged piston than for cast or hypereutectic. The exception to this would be special forged pistons that may need extra clearance for a special purpose, such as blower or nitrous use.
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Old 01-16-2004, 08:02 AM
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thanks tom i always thought that the forged piston had to have extra cleance. now i know not to put my foot where it doesnt belong
denny
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