Inliners International Forums General Discussion Engines GMC Head for Dome Top Pistons With Flat Top Piston

I have a military rebuilt GMC 302 with .020 flat top pistons, yet the head is a "477" head designed for dome top pistons.

Will there or is there a drop or significant drop in compression if a head that is designed for dome top pistons is used with flat top pistons?

- What will be the most noticeable effect if compression is lower using the 477 head with flat top pistons? Harder starting?, loss of fuel efficiency? poorer ignition?

Should I be searching for the flat top piston head? and replace this 477 head with an 819 head?

The engine will be used in a one ton truck for street use. Not a performance or modified engine. Snow free or warmer temp driving.

Hi Hunterdude . . .

Is the engine apart? Specifically, is the head off? If so you can calculate the C/R by measuring the volume of the combustion chambers and comparing it to the displaced volume.

In performing these measurements some details specific to the build need to be taken into consideration. First, has shortblock been zero decked? If the pistons are flush to the deck surface at TDC then the answer is yes - and therefore the very top of the cylinder need not be added to the combustion chamber volume. If the pistons stop short of the deck - the volume of this shallow cylinder needs to be added to the combustion chamber volume.

What head gasket will you be running and what is it's compressed thickness? The same shallow cylinder volume estimate applies here as well.

AFAIK the 477 is the most abundant head in terms of production volumes on the civilian side - which is why whoever assembled your engine may have chosen it in order to move ahead. If the head was milled the volume (compared to stock) may be considerably less.

Back to the original questions - if the head is off you can take a graduated cylinder and fill it with mineral oil to a round measurement mark on the graduations. Then set the head upside down on your bench with wood blocks/shims to level it in all directions. Pour the mineral oil into one of the combustion chambers until level to the top. The remainder in the graduated cylinder will reveal the combustion chamber volume.

I'd measure what's in hand before shopping for parts.

regards,
stock49

Hi, thank you. The engine is buttoned up and reassembled. I can take measurements from another 477 head I have on the bench.

TDC shows the pistons approximately 1/64" .015" or less from top of block deck (estimating from photos).

Regardless, the combustion chamber is larger on the 477 head. Still trying to determine though, what if any effect there would be if the compression is just a slight drop from an 819 head. And if it is worth investing $400.00 or more into a different head and fitting valves - if the effect is really just minimal and the engine will operate fine for street use.

Appreciate your response. Will see if I can measure the difference in volume between the to heads.

I have since learned that the 477 head will have about 6.5:1 compression and the 819 head 8:1 compression.

Are you referring to the published compression ratios?

We'll need to get a little deeper then those to get an accurate estimate. Here's a thread from Inliners Engines Forum circa '12

Deuce Coupe and others discuss the calculation - DC is backing into the combustion chamber and dome volumes based on the published C/R for the 270.

AFAIK the stock compression ratio for the 270 was 6.75:1. This is with a ~130cc combustion chamber, ~31cc dome, an ~.050 compressed gasket and a roughly ~.03125 deck (deck = Deck Height – Stroke - Rod Length – Compression Height).

If you add in the additional displacement (up to 304) – remove the dome – and tighten the deck to ~.015625 you are in the neighborhood of 5.8:1

That's why I would measure the actual head that is bolted to the block to see if it has been milled. With an accurate measurement of combustion chamber volume and deck, the static C/R calculation will be more accurate.

Ahhh… makes sense. I appreciate this. A good education for me. thanks.

Even though this is an "open" chamber without an official quench band, you create/enhance one around the annular edge of the new bore with the larger piston.
The estimated P-H clearances given above (.031" + .050") are right at the vanishing point for quench. I would certainly be looking for a thinner head gasket to close the TDC clearance to .035" minimum.
Resist any impulse you may have to radius the chamber edge out to the actual bore!