Help 283 pistons

[Charles M.]

Re: Help 283 pistons

Thanks Joel , 6cc is just a guess based on similar pistons. I'm not 100% sure but I think a negative number is used for pop up type pistons in the compression ratio calculators. I just spoke with a different tech at Keith Black, he said the #kb165 has a compression height of 1.805 compared to 1.780 of sealed power 235np I have now. This piston used with a .020 gasket seems to come to close to 9.5 cr. Chuck M

[Duke W.]

Re: Help 283 pistons

The KB volume convention is the opposite of FM. FM would list volume as negative on a flat piston with valve reliefs. The 6cc volume of the KB 165 must be entered into the cgnet compression calculator as a negative number.

Assuming your measured deck clearance of .040" was measured with a piston that had a compression height of 1.780" the KB165's 1.805" compression height should reduce deck clerance to .015".

So with a piston dome volume of negative 6cc and .015" deck clearance you can try different gaskets to achieve a target CR.

If you are using the OE replacement 3896929 or exact equivalent camshaft I recommend you target not more than 9.75:1 for detonation free operation on premium unleaded fuel.

If you do install the KB165 pistons make damned sure that whoever assembles the engine adheres to the KB specification for minimum top ring gap. If the installed gap is less than minimum you are just about guaranteed to break a piston due to ring butting.

Duke

[Joe L.]

Re: Help 283 pistons

Chuck-----


The original pistons were of GM #3739886 (standard bore size). They are flat top and have 4 valve reliefs. I do not know what sort of deck heights were produced using these pistons. However, I would expect it to be in the 0.020-0.030" range.

You should be able to obtain replacement pistons from KB, F-M, or others that pretty much duplicate the above.

[Charles M.]

Re: Help 283 pistons

Thanks for everone's help. Looks like I will be going with the KB165 when they become available again.Wondering how can you calculate what compression height is needed other than trial and error? This is the second time a machine shop has supplied pistons that came up short of desired compression ratio. Chuck M.

[Duke W.]

Re: Help 283 pistons

Piston compression height is not a calculated number. It is a critical dimension that should be specified by the piston manufacturer, and it is easy to measure with a caliper.

Most OE replacement pistons will have the same compression height as OE, but some may be slightly different and OE pistons may have different compression height depending on the specific option.

In the case of 327s all OE versions are "high compression" and all the OE pistons have 1.675" compression height. The case for 283s may be different because there were low and high compression version 283s, and this can be achieved with different piston compression height, which is why I think the Sealed Power piston may not be the correct OE replacement for your 283, but it may be correct for the low compression 2-bbl. 283s that were used in the passenger car line.

When planning an engine restoration, one should become aware of the different available piston options and different available head gasket thicknesses in the planning stage, so you can run some pro-forma compression ratio calculations. You'll have to do this yourself. Few "engine builders" understand this subject, and/or don't want to take the time to do the work.

To measure the compression height of a piston, pull the pin out far enough to measure its diameter with a caliper - should be in the range of .9270-.9275" for a small block. Then measure the distance from the bottom of the pin to the piston crown, which is the machined flat area exclusive of any dome or dish or valve relief notches. The piston compression height is this distance minus half the pin diameter, and a typical mechanical caliper with .001" graduations should get to within a thou or two of the true dimension, closer if you have a vernier caliper.

Most "engine builders" don't pay any attention to measuring compression ratio. They just throw in "low compression" pistons and a thick head gasket, so you end up with something between 8-9:1 for a typical high compression OE engine, which leaves a lot of torque/power and fuel economy on the table, but the engine will probably run okay on regular unleaded, if that's what you want.

If you measured deck clearance prior to disassembly, and you know (or verify by measuring) the compression height of the installed piston, you can easily calculate what the new deck clearance should be by factoring in any change of compression height between the old and new pistons.

Once the new expected deck clearance is known, and you have accurate volume data for the piston crown and head chamber volumes, you input these numbers to the CR calculator and try different head gaskets until you achieve the target CR range.

When the short block is assembled, you then measure actual deck final clearance and rerun your CR calculations to make a final decision on head gasket thickness.

The above attention to detail can make the difference between a good engine and a stone!

Duke