283 Piston Question

[Mark P.]

I am trying to find a piston that will maintain the 9.5 to 1 compression for my .030 over 283 rebuild.

I would prefer a Hypereutectic or cast piston that is flat without the valve reliefs like original, but can't seem to locate one.

Should I go with a dome top piston instead to try to maintain the compression ?

With a decked block, surfaced heads and a shim gasket I am approaching 9 to 1 compression if I use the flat pistons with the 4 valve reliefs. Will I notice the drop in compression ?

Should I steer clear of the KB domed top hypereutectic piston with 2 valve reliefs to try to get the 9.5 to 1 compression ?

[John N.]

Re: 283 Piston Question

I am trying to find a piston that will maintain the 9.5 to 1 compression for my .030 over 283 rebuild.

I would prefer a Hypereutectic or cast piston that is flat without the valve reliefs like original, but can't seem to locate one.

Should I go with a dome top piston instead to try to maintain the compression ?

With a decked block, surfaced heads and a shim gasket I am approaching 9 to 1 compression if I use the flat pistons with the 4 valve reliefs. Will I notice the drop in compression ?

Should I steer clear of the KB domed top hypereutectic piston with 2 valve reliefs to try to get the 9.5 to 1 compression ?

I believe all Corvette 283s had valve reliefs. The only 283s I know of with no valve reliefs were the 57 2bbl & single 4bbl pass car.

What are the ccs of your heads? It seems you are getting a compression ratio lower than expected.

The KB domed pistons are simular to the 61 FI pistons which I believe gave a 11+ ratio with a 62-64 cc head.

For people here to advise you on piston selection, you might give your intended use of your car.
Regards

[Mark P.]

Re: 283 Piston Question

John - thanks for replying. I have the 692 heads so that is 59.7cc.

I wasn't aware that the 1960 Corvette used pistons with 4 valve reliefs. The pistons I have don't have them so I guess I have a passenger car 519 Flint block.

Thanks,

Mark

[Joe L.]

Re: 283 Piston Question

I believe all Corvette 283s had valve reliefs. The only 283s I know of with no valve reliefs were the 57 2bbl & single 4bbl pass car.

What are the ccs of your heads? It seems you are getting a compression ratio lower than expected.

The KB domed pistons are simular to the 61 FI pistons which I believe gave a 11+ ratio with a 62-64 cc head.

For people here to advise you on piston selection, you might give your intended use of your car.
Regards

John and Mark----

Yup! All 1957-61 Corvette 283 engines originally used pistons with valve reliefs. Most used the flat top style with 4 reliefs; 60-61 with SHP used the domed style with 2 reliefs.

A Sealed Power #W235NP will provide a configuration very close to original piston for Mark's 1960 application. However, this is a conventional cast piston, not hypereutectic. The original pistons used for your application were conventional cast so at least nothing is lost when using this piston. I don't know of a hypereutectic cast currently available with original piston head configuration.

The KB hypereutectic can also be used. It has only 2 valve reliefs, but they're a bit deeper than the originals. However, I think the resultant compression ratio would be very similar to the original "4 eyebrow" pistons.

[Mark P.]

Re: 283 Piston Question

Thanks Joe.

I am trying to calculate the CR using a link Duke recommended.

Does anyone know what I should enter for the "Enter Piston Domed Volume in CC's - Negative for Dished Pistons" if I use the Sealed Power piston Joe mentioned ?

Since those pistons will have the 4 valve relief cuts then I should enter a negative and that should bring the compression down to 9.5, right ? I am trying to find this piston on the Federal Mogul website and its not coming up.

Here is my attached calcs:

[Duke W.]

Re: 283 Piston Question

I use -5 cc (enter "-5" in the cgnet CR calculator) for the OE 327 flat pistons with four valve relief notches, and I expect the 283 pistons use the same notch geometry, but I'm not sure.

The KB data lists flat pistons with notches as a positive number, but this must be entered in the calculator as a negative number.

Assuming you are talking about a base cam 283 with the later ...929 base cam as a replacement you are okay to go as high as 9.8.

If you use KB pistons be sure that whoever assembles the engine FOLLOWS THE KB INSTRUCTION FOR MINIMUM TOP RING GAP, WHICH MAY REQUIRE FILING THE TOP RINGS.

Duke

[Mark P.]

Re: 283 Piston Question

Thanks Duke - the CR will be 9.542 to 1 using the -5 CC for the valve reliefs assuming the piston deck clearance is .005".

Does the .005" make sense if the block was decked .015" (typical to remove the stamp pad numbers I have been told) and a piston deck clearance of .020" is what I have been told is typical ?

Since I haven't ordered the pistons then I can't check for sure.

If the clearance is .020" instead of .005" then the CR drops to 9.19 to 1. This is acceptable isn't it ?

[Duke W.]

Re: 283 Piston Question

The nominal deck clearance for a 327 is .025". I'm not sure if the 283 is the same. Also keep in mind that decks are often high by as much as .015", and can vary .010" side to side. This is why deck clearance should be measured before the short block is disassembled. Either way, measuring deck clearance upon short block assembly is mandatory in order to select a head gasket to yield the target CR, and if one deck is more than .005" different than the other, different head gaskets can usually be chosen to minimize the variation in side to side CR.

Subtract (1.500+5.7000+piston compression height) from the nominal SB 9.025" deck height to determine nominal deck clearance, but remember, this is just a starting point. Each cylinder of each engine must be measured to understand and properly configure each specific engine.

If you can find the spec sheet on line for the Sealed Power OE replacement piston, it should have OE compression height; OE 327 pistons have a compression height of 1.675 and 350s are 1.560.


The KB piston might have a slightly different compression height. For example, the KB 157 piston that is used for my "Special 300 HP" 327 configuration is 1.678" with a 0.5 cc net dome.

I have no idea what the "correct" CR range is for the engine in question because all I know about it is that it's a "283".

Duke

[Charles M.]

Re: 283 Piston Question

Mark, I had same problem rebuilding 61- 230 hp 283. Most replacement pistons are made assuming a resurfaced deck and end up with too much piston to deck height. The best I could find was KB hyperutecs. Still had to use steel shim gasket with my 59cc heads to get 9.3 comp. ratio
Chuck M

[Steven B.]

Re: 283 Piston Question

Mark, check with Egge (egge.com) as they reproduce to original prints and will modify specs. to your needs.

[Mark P.]

Re: 283 Piston Question

I ran the calcs again using the Sealed Power Piston Joe recommended and Duke's guidance and I get to 9:1 which could be a bit higher considering my heads have been resurfaced twice. We will do a final check during assembly.

[Joe L.]

Re: 283 Piston Question

I ran the calcs again using the Sealed Power Piston Joe recommended and Duke's guidance and I get to 9:1 which could be a bit higher considering my heads have been resurfaced twice. We will do a final check during assembly.

Mark-----


You'll need to cc the combustion chambers to determine just what the volume is. I assume that valve jobs have been done each time the heads were re-surfaced. This will tend to increase chamber volume slightly by insetting the valves. However, the "normal" routine cylinder head cuts you mentioned should more than offset that. So, my guess would be that you're going to find yourself right in the 9.5:1 range. This all might be more trouble than it's worth, though. A few tenths of a point of compression one way or the other is not going to be significant in any way.

[Mark P.]

Re: 283 Piston Question

After having my 692 heads CC'd I found out they were at 62cc vs the 59.7cc I had expected.

I am getting the deck height measured so I can get the right piston.