Help 283 pistons

[Charles M.]

Long time member, first time posting. Trying to get around 9.5 to 1 compression on original base motor 61 with stock 60cc 692 heads. Block is bored .030, pistons are sealed power cast flat top with 4 valve pockets. Installed 1st piston and measured .040 below deck. Compression ratio calculator comes out to 8.2 to 1 with .040 head gasket. Talked to Keith Black tech, he said their hyprerutics pistons (although currently unavailable) would be about .030 below the deck. What pistons can be used with original deck height. Thanks in advance Chuck M

[Joel F.]

Re: Help 283 pistons

Did you cc your heads? Even if you have had no porting or valve work done you may find them inconsistent and this of course will affect your compression.

[William C.]

Re: Help 283 pistons

The use of factory .020 head gaskets would help, as would about .030 off the heads. Have you actually measured the pin to crown distance on the pistons you have? I'm assuming you want to keep the original block surface which precludes decking the block, but head gasket and the head cut should pick up enough cc's to get you close to where you are going.

[Charles M.]

Re: Help 283 pistons

Thanks for the replies, Havn't cc,d the heads or measured piston crown other than measure installed height. Was hoping to find original spec. pistons rather than cutting the heads and maybe have to cut intake because of changed angles. Chuck

[Don Y.]

Re: Help 283 pistons

You could try Egge, they have a large catalog of replacement parts they manufacture for vintage engines.

[Charles M.]

Re: Help 283 pistons

Thanks Don, just called Egge, all they offer is 11 to 1 FI piston, probably wouldn't work with 300 HP hyd.cam

[Duke W.]

Re: Help 283 pistons

What's the part number, specified compression height, and net volume of those pistons? Four notches indicates that the pins are offset.

Likely the decks were cut high relative to the nominal 9.025" dimension.

Felpro offers a polymer coated shim gasket - 4.100 x .015" compressed thickness - 1094.

I don't have any specs for 283 head chamber volumes, but I thought they were in the 55-57 cc range.

You must have accurate specs and dimensions to calculate the CR. What CR calculator are you using?

Duke

[Don Y.]

Re: Help 283 pistons

Sorry, I almost forgot! All of the '57s were 283/283's, sorta like the '67 427/435's! I used the Keith Black hyperutectic pistons in my car but it was a fuelie. You would think there would be more of a market for the base piston. Have you tried Summit?

Thanks Don, just called Egge, all they offer is 11 to 1 FI piston, probably wouldn't work with 300 HP hyd.cam

[Timothy B.]

Re: Help 283 pistons

Charles,
If you are sure about head volume how about a thin head gasket .020 and pistons with two valve reliefs. There are probably left side and right side pistons because of the pin offset with just two valve reliefs. Try to find the piston volume and run that through your calculator with the thin gasket and see if that gets close.

[Charles M.]

Re: Help 283 pistons

Pistons are sealed power #235np-.030 only other info on box is "size point 2.88 from top of head" .Havn't been able to get specs from website. heads are cast 3774692, chevy by the numbers says 59.669 cc. Used Keith Black website for calculators. even with .015 shim gasket only gets it to 8.52. how much can I cut the heads before getting into intake manifold problems?

[Duke W.]

Re: Help 283 pistons

The Keith Black calculator and some others I've seen online make some assumptions that can trip you up. I use the following calculator and recommend it to all. It is straight forward and requires all the relevant specs/dimensions independently. Some lump gasket thickness and deck clearance together into one dimension or make other assumptons. This calculator allows you to work with each relevant spec/dimension independently and makes no assumptions about anything, but it's accuracy is only as good as the accuracy of the data you input.

http://www.csgnetwork.com/compcalc.html

One thing that appears to be missing is net piston volume. With four notches I would guess negative 2 to 4 cc, but get an exact number from Federal Mogul or measure them yourself.

I would also recommend that you seek out another independent source of head chamber volume, find some actual measured data, or measure them yourself.

Duke

[Duke W.]

Re: Help 283 pistons

www.federalmogul.com is not easy to navigate, but some parts are in online catalogs. You need to follow a product path and then the "catalogs and training" link.

I looked up your pistons "by specification" and found a listing for 235P, but not 235NP. If you click he part number the data comes up for just that part and you can also do a pdf to print it out.

The 235P has a compression height of 1.78", which indicates a nominal deck clearance of 0.045". I suspect this may be for the low compression 283s. Your base engine is "high compression", but the nominal CR was only speced at 9.5:1, and most Flint built engines were lower than nominal.

Unfortuneately the specs don't list the notch volume, and this piston does have the OE .060" pin offset, which is why it has a double notch set. You need to ensure that the pistons are installed properly IAW the service manual. Offset pistons were common in that era. They theoretically reduced piston noise and major thrust surface load, but the industry has pretty much abandonded the idea ,and I see no problem with installing non-offset pin pistons link Keith Black.

Anyone comtemplating a rebuild should familiarize themselves with the F-M online catalogs. Their OE replacement brands like Sealed Power are top quality OE equivalent replacement parts.

This is the page for looking up Sealed Power parts by specification:

http://www.federalmogul.com/en/AftermarketSolutions/NorthAmerica/TechnicianResources/Catalogs/Catalogs.htm

[Charles M.]

Re: Help 283 pistons

Duke, thanks for response, put the following # into your reccomended calculator bore 3.90, stroke 3.00, gasket bore 4.10, gasket thickness .040,head volume 59cc,piston volume 6cc, deck clearance .040,=compression ratio 9.45 to 1. Just what I need. Inputting same info to several other online calculators come up with 8.2 to 1. Are the pistons I have correct?
Chuck M.

[Joel F.]

Re: Help 283 pistons

Chuck,

I think that piston volume should be a negative number (not sure where you got the 6cc's) if they are flat tops with valve reliefs.

Joel

[Duke W.]

Re: Help 283 pistons

Yes, read the instructions. A "dome" volume is input as positive - a "dish", which is what a flattop with valve notches is - is input at a negative volume.

I'm not sure if those pistons are correct OE replacements. Corvette 283s were "high compression" (9.5:1 except some FI engines), but 283s were also available in pass. cars with lower compression to run on regular fuel, which implies different pistons and I'm not sure which one the 235P replaces. I'm not that familiar with the initimate details of 283s. I work mostly with 327s and the nominal compression height of all OE Corvette 327 pistons is 1.675", which yields a nominal deck clearance of .025".

Try the KB pistons. Their catalog should show both compression height and volume. If the KB compression height is different than the OE 1.78, you will have to adjust the deck clearance.

Be very careful to get the signs correct. Some piston mfgs list a doomed piston as negative net volume, some as positive, but the CR calculator requires a "dome" to be input as positive even though it subtracts from net chamber volume. This is not rocket science, but it can be tricky, and drawing a sketch can be helpful.

The "dome volume" is the net volume of the piston. An absolutely flat piston has zero volume. A flat piston with valve reliefs will have a net negative volume as far as the calculator is concerned, but it adds to net chamber volume, which drops the CR relative to a flat crown with no valve reliefs. A domed piston with valve reliefs has both negative and positive volumes and the mfg's specified volume is usually the net volume - dome volume minus valve relief volume.

For example the KB157 327 piston has a compression height of 1.678, which is .003 more than OE. So if the deck clearacne measured .025", on disassembly with the OE pistons, it should measure .022 with the KB157s.

Duke