Measuring inverted piston done

[65dragster]

I am trying to confirm the volume of an inverted dome on a Chevy 400. The short block is assembled on a stand with oil pan in place and I cannot see a piston part number to verify. Does it make sense to put piston at TDC and use a light oil or liquid to fill the inverted area to validate CC volume? Is there a better way that I am missing? Any help or ideas will be greatly appreciated so I can get CR confirmation.

[rfoll]

If you pour the entire cylinder with the piston at tdc, you can mathematically determine piston volume by subtracting the volume as if it was a flat at whatever distance it is in the hole. A 4.125" bore contains roughly 0.2 CCs per .010" depth. That equates to roughly 0.2 compression point. You didn't say what you were after, but in reality the volume with the piston at tdc is the number required for the static compression calculation. The dish volume matters if you are considering a change in dish volume with a different piston, but in reality you really need to put it in the bore and measure because published volumes are not always correct, and compression distance can vary by manufacturer. KB pistons for sbc engines are typically .010" taller in the bore.

[65dragster]

Rfoll, Thank you. I am trying to calculate static compression. How can I determine if the liquid is actually level with the top of the piston, if I am only trying to determine the piston's inverted dome area? I believe the inverted area is either 16 or 22 CC's, but I need to know for certain. Would clay be a suitable alternative method to measure the volume of the inverted area only? Thanks for the guidance.

[Ericnova]

Do it similar to taking the cc measurement of a cylinder head chamber....flat clear plastic plate with a small hole to fill fluid through stuck down to top of block with light grease(Vaseline will work).

Put piston a set distance down the bore, such as .100" from flush to the quench band around the edge of the dish, then seal around the edge of piston to bore with Vaseline to prevent leakage.
Fill the space and record the cc volume, then do the calculation for how much volume should be in a .100" cylinder with your bore size.

Formula for volume is PI x (Radius squared) x length of cylinder(.100" in this example).

(3.1416 x 4.2359) x .100" =1.3364 cubic inches, or 21.9cc's in this example, for a .100" tall x 4.125" bore cylinder.
Any cc amount you measured bigger than that during your actual pour would be the dish volume in question.

[rfoll]

I calculate with a dial indicator and drop the piston with rings down the bore. With a little grease on the bore I bring it back up to tdc and it should be sealed. With a dished piston it is easy to fill at tdc, domes need to be in the hole a specified distance. Some flat tops will work at tdc if it is not a zero or close to zero deck.

[Fatman]

If you have an inverted dome with nothing about deck, what about just filling your inverted dome with clay, level it off flat with your piston deck, remove the clay out and roll it into a thin sausage, fill a burette to 50 or so ml, drop the clay into the burette, calculate the difference. Difference is you dish volume. Make sure there is not air trapped in your clay or around the clay when it is dropped into the burette.

[BillK]

If you have an inverted dome with nothing about deck, what about just filling your inverted dome with clay, level it off flat with your piston deck, remove the clay out and roll it into a thin sausage, fill a burette to 50 or so ml, drop the clay into the burette, calculate the difference. Difference is you dish volume. Make sure there is not air trapped in your clay or around the clay when it is dropped into the burette.

I have done that before to find the volume of a stock piston. Its kind of hard to get the clay trimmed off perfect but it definitely works. I never thought about rolling it out. I cut it into little balls and put them in the burette

[65dragster]

Thanks all! I think I will try the clay, and then the liquid method, and see how close they are. I recently signed up as a Hot Pass member and this community is the best investment I’ve made in a while (yes I would still say that if the stock market wasn’t pummeling me)!