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If the current Federals are being made in the same place now that the speed pro forged versions are that quote from them is a bunch of knowing what I know now about the the current new stock of speed pro forged pistons , but then again they will be great for NOT having to overbore a block that only needs a rehone and the taper is still in spec

Otherwise I'm with Dave , the current offereings for 383s AND 400's are completely OUTDATED , as are most of the current offerings in forged units ..

YMMV .

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Other than poor selection for 383s, what exactly is wrong with the inexpensive sealed power/federal/speed pro pistons? They are extremely widely used (especially in the SBC stuff I play with) and I have never heard of a problem with them.

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My local shop did a 440 for a board member here , he swapped in a set of brand new 6pk replacements for a set of the 2266's that sit .100 in the hole he originally had this shop install, that motor was fresh he should have been able to check weights to confirm they were the same , which they usually are , and rehone the block , fresh rings and go . the pistons were 45 grams heavier ... rebalance rotating assembly .... and when measured they were EXACTLY the size of the overbore , where the clearance is usually built into the piston ... rehone the block oversize and buy file fit rings to get the proper end gaps.

I can't remember the exact story of who sold what but the pistons are another quality product from the land that gave us crab rangoon.

The plus is I have 2 engines that are at an odd oversize after honing with a torque plate when they weren't originally and these should be a cost effective way to freshen these engines

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My local shop did a 440 for a board member here , he swapped in a set of brand new 6pk replacements for a set of the 2266's that sit .100 in the hole he originally had this shop install, that motor was fresh he should have been able to check weights to confirm they were the same , which they usually are , and rehone the block , fresh rings and go . the pistons were 45 grams heavier ... rebalance rotating assembly .... and when measured they were EXACTLY the size of the overbore , where the clearance is usually built into the piston ... rehone the block oversize and buy file fit rings to get the proper end gaps.

I can't remember the exact story of who sold what but the pistons are another quality product from the land that gave us crab rangoon.

The plus is I have 2 engines that are at an odd oversize after honing with a torque plate when they weren't originally and these should be a cost effective way to freshen these engines

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If the current Federals are being made in the same place now that the speed pro forged versions are that quote from them is a bunch of !*&$ knowing what I know now about the the current new stock of speed pro forged pistons , but then again they will be great for NOT having to overbore a block that only needs a rehone and the taper is still in spec

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Also, would a shorter compression distance mean more or less compression? Wouldn’t the lower number mean that the piston sits higher in the bore?

KB-162 - Compression Distance (in): 1.908 in.

Federal Mogul #STL-366NP30 - Compression Distance (in): 1.848 in.

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a 1.908 piston is taller then a 1.848 piston.
therefore it will have more compression then the shorter one.

if your looking for every last 1000th of a horsepower a dome might block it, otherwise its them or the sealed power forged flat tops and then have NO valve reliefs... I went with the domes because of the reliefs.

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My local shop did a 440 for a board member here , he swapped in a set of brand new 6pk replacements for a set of the 2266's that sit .100 in the hole he originally had this shop install, that motor was fresh he should have been able to check weights to confirm they were the same , which they usually are , and rehone the block , fresh rings and go . the pistons were 45 grams heavier ... rebalance rotating assembly .... and when measured they were EXACTLY the size of the overbore , where the clearance is usually built into the piston ... rehone the block oversize and buy file fit rings to get the proper end gaps.

I can't remember the exact story of who sold what but the pistons are another quality product from the land that gave us crab rangoon.

The plus is I have 2 engines that are at an odd oversize after honing with a torque plate when they weren't originally and these should be a cost effective way to freshen these engines

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what brand pistons were these? Even the cheapie pistons will have the size on the box, which even with cheapie sealed power pistons is undersize to set up normal running clearance to fit a std size or std over bore. I have never heard of a piston being made, say 4.0000 for a 4.0000 bore. Something ain't right there.

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a 1.908 piston is taller then a 1.848 piston.
therefore it will have more compression then the shorter one.

if your looking for every last 1000th of a horsepower a dome might block it, otherwise its them or the sealed power forged flat tops and then have NO valve reliefs... I went with the domes because of the reliefs.

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Okay, I see how the compression distance of the piston computes now. That is good to know and definitely moves the Federal Mogul pistons off the already short list of options. Does anyone know off hand what the compression distance is of the stock HP pistons for comparison sake? Also, is the compression distance measurement the distance between the centerline of the piston pin bore and the top of the piston?

Thanks everyone for all your help.

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Yes, for the longest time I was set on the KB-400 domes. But then I heard to avoid domes if possible as they mess with flame travel when compared to a flat-top pistons. Then I thought about using the KB-162s, but then I saw the following article in Mopar Muscle Magazine:

“The stock #906 heads were milled .020-inches to compensate for the thicker replacement head gasket, thereby reducing chamber volume from the stock 87 cc to 83 cc. The deep valve notches in the KB pistons (6 cc) required to run long-duration high-lift cams, coupled with the 84cc open-chamber heads and the 383's short stroke, gave us a compression ratio of 9.2:1. This compares to 9.7:1, which would have been achieved with stock-style flat-top pistons with no valve reliefs.”

According to that, the deep valve reliefs give up a half point of compression. Knowing this in addition to not needing valve reliefs with my stock magnum cam, I started looking at other possible flat-top pistons which consists of either using stock pistons or the Federal/Speed Pro slugs.

Also, would a shorter compression distance mean more or less compression? Wouldn’t the lower number mean that the piston sits higher in the bore?

KB-162 - Compression Distance (in): 1.908 in.

Federal Mogul #STL-366NP30 - Compression Distance (in): 1.848 in.

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If the current Federals are being made in the same place now that the speed pro forged versions are that quote from them is a bunch of !*&$ knowing what I know now about the the current new stock of speed pro forged pistons , but then again they will be great for NOT having to overbore a block that only needs a rehone and the taper is still in spec

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Johnrr, could you elaborate a bit more on this? What can you tell me about the current stock of Federals? Just curious...

So at this point, it seems that if given the choice between Federal Mogul cast and a stock Mopar HP pistons, the stock units would be a better choice.

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a 1.908 piston is taller then a 1.848 piston.
therefore it will have more compression then the shorter one.

if your looking for every last 1000th of a horsepower a dome might block it, otherwise its them or the sealed power forged flat tops and then have NO valve reliefs... I went with the domes because of the reliefs.

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Okay, I see how the compression distance of the piston computes now. That is good to know and definitely moves the Federal Mogul pistons off the already short list of options. Does anyone know off hand what the compression distance is of the stock HP pistons for comparison sake? Also, is the compression distance measurement the distance between the centerline of the piston pin bore and the top of the piston?

Thanks everyone for all your help.

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what stock HP piston ???

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a 1.908 piston is taller then a 1.848 piston.
therefore it will have more compression then the shorter one.

if your looking for every last 1000th of a horsepower a dome might block it, otherwise its them or the sealed power forged flat tops and then have NO valve reliefs... I went with the domes because of the reliefs.

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Okay, I see how the compression distance of the piston computes now. That is good to know and definitely moves the Federal Mogul pistons off the already short list of options. Does anyone know off hand what the compression distance is of the stock HP pistons for comparison sake? Also, is the compression distance measurement the distance between the centerline of the piston pin bore and the top of the piston?

Thanks everyone for all your help.

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what stock HP piston ???

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1968, 1969, 1970, and 1971 383 HP Engines. I may be wrong, but I’ve been assuming all these were the same dimensionally?

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Yes you are wrong , 68/69 is the same , 70 and 71 are different than 68/69 , at least with a LOWER advertised ratio from the factory.

The advertised ratio for the 68/69 is 10.0 but if you do the math on how it was actually delivered its closer to 9.3 , you have to build a 68/69 to NHRA blueprint specs to get the advertised 10.0, that would included milling the block .038ish to get the piston .020 above the deck (using the forged 2315) , run a steel shim gasket and cut a 906 down to 79.5cc from about 90cc ...

There is NO direct replacement for the 68/69 piston that I have yet to find , the closest is the Speedpro FORGED 2315 at .012 SHORTER in compression height . This piston is probably a closer replacement for a 67 down or a 70 71 , but it's NOT for a 68/69 . The 383 and the 400 are the REDHEADED stepchilds of MOPAR and POORLY supported by aftermarket piston manufacturers.

If you did a search in the Q+A you would find the info you seek as we have hashed this out a few times .

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