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Old 11-14-2012, 12:05 PM
Mr. P-Body Mr. P-Body is offline
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Allan,

Deck clearance is the distance between the upper most portion of the flat part of the piston (usually at least the outer "edge") at TDC and the deck surface. Most manufacturers allow a deck clearance of greater than .010" to avoid incidental contact of the piston and head. Some are as high as .060". Measuring deck cleance requires a good magnetic-based dial indicator or a "bridge" (made specfically for the purpose). In our (machinists) lingo, TDC means EXACT top dead center, not "about". A degree wheel and indicator are required to determine that as well. Once TDC IS determined, one can use a straight-edge and "stack" feeler blades to get a rough idea of what the clearance is. Using the dial indicator is the proper way to determine exactly what the clearance is. Set the indicator to "zero" while on the deck. Move it to the edge of the pison closest to the pin. The difference in the readings is your clearance. In the vernacular, "in the hole" refers to negative deck clearance. The relationship of the center of the piston and the deck is of no concern. It's the shape of the dome and the chamber that must be compatible. Both a "dome" and "dish" will have different centers than deck clearance.

Most gasket makers supply that information, either in the catelog or with the product. A typical "stock" gasket for a 350 Chevy has a compressed thickness of .0394" (1 mm). We generally "round" that to .040". There are thinner and thicker gaskets, both "stock" and aftermarket for a variety of applications. For resistance to detonation, most builders agree .040" of "quench" is ideal, so using the stock gasket will usually get you as close as possible, depending on deck clearance.

Computing the dome volume from external dimensions is a tricky business unless it's a concentric shape. The engineers that made the piston have calculations that work, but "in the field", it's much easier just to measure it. At least one piston/ring assembly must be installed and attached to the crankshaft. Cam timing is not necesary at this point.

Find true TDC. Record deck clearance.

Using the dial indicator, lower the piston in the bore exactly 1". "CC" the cylinder in exactly the same manner as measuring the chamber volume in the head. Record the results.

Using the formula "Pi x R(squared) x H", find the "normal" volume of the 1" "cylinder". Where Pi = 3.14159, R = 1/2 bore size and H = stroke (in this case, 1"). Use the same formula to determine the amount "added" by the deck clearance where H = the clearance. Subtract the deck clearance volume from the actual volume. Subtract that from the "normal" volume of the 1" cylinder. To convert from "inch" to metric, multiply the results by 16.378 (how many CCs in one cubic inch). There is your "dome volume".

Doing this job isn't all that difficult. The tools aren't cheap. For a "once in a while" thing, I doubt it's worth the expense. A local machine shop can do the measuring for you at a reasonable charge if you do all the "prep" (subassembly, etc.). Call your local guys and ask. Of course, if you're up for it, buy or borrow the tools and do it yourself! You need a "CCing" kit, a dial indicator and base with at least a 1" stroke, a degree wheel and pointer and alcohol. Water will "do" in a pinch, but be sure to dry it up IMMEDIATELY when you're done. Nothing "thicker" than water.

Jim
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