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Old 11-15-2012, 04:10 PM
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Originally Posted by Mr. P-Body View Post
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

While I have a great deal of respect for Jim, let me make this a little easier for you fellows who have domed pistons. The piston needs only to be lower than the block deck to find the dome volume. Putting a short dome down in the cylinder one inch will work, of course, but you'll spend additional time refilling the burette, increasing your chances for error.

If, for instance, the dome is 0.200" above the edge of the piston, then you only need to put the piston down in the bore by 0.200", just enough to miss the clear Lucite, Plexiglas or Lexan cover plate. Hold the cover plate down on the block deck by hand, bring the piston up in the bore and push it up against the cover plate. Lay the cover plate aside and use your dial caliper to measure from the edge of the piston crown at the top ring up to the block deck. Let's say it measures 0.210". Write it down. Use Q-tips and Vaseline to seal around the edge of the piston to form a liquid-tight seal of the piston to the bore. Be careful not to push the piston down in the bore any further. Smear Vaseline all around the bore on the block deck and lay your cover plate down with the fill hole right at the edge of the bore and slightly uphill so the air will bleed out as you fill the void with alcohol from your burette.

Use math to figure the cc displacement of the void as if there is no dome on the piston, as if the piston were completely flat on the crown with no valve reliefs or dome. If we measured the bore at 4.031" (for instance) and we placed the piston down in the bore 0.210", then we have all the numbers we will need. Do the math thusly....
(.7854) x 4.031 x 4.031 x .210 x 16.387 = 43.91 cc's, call it 44.
Let's say you fill the void with alcohol, using 38 cc's to do so. Subtract 38 from 44 and find that the dome is worth 6 cc's. (Remember there are valve reliefs in the dome too, so it will take a lot of dome to make any real difference over a flat-top piston.) Here's a "for instance" of a 6cc dome piston....

I would advise against using water to find displacement in any voids. As Jim said, water ain't wet. You can improve the flow of it by adding a few drops of dishwash detergent and mixing it around, but alcohol is cheap and works very well. Go to Walmart in the pharmacy section and buy the largest bottle of 70% Isopropyl alcohol you can find, then go to the cake mix section and buy a bottle of food coloring so you can see the meniscus in the burette. Your choice, red, green or blue. It only takes a little food coloring to be able to see the fluid in the burette, so add a few drops at a time into your bottle of Isopropyl until you get the desired shade of color.

I agree that if you're going to be serious about knowing what you're doing, you need the proper tools. While there is absolutely a need for a dial indicator and magnetic stand, quite a lot of measuring can be done with a dial caliper and you'll use it more than you will the dial indicator. Go to ebay and bid on a used Starrett, Brown & Sharpe or Mitutoyo 8" dial caliper. Used is fine for your purposes. I recently bought a used Mitutoyo 12" with wooden case on ebay for 85 bucks. With an 8", you can measure the block deck height on most small blocks. With a 12", you can measure the block deck height on any block. A 6" caliper is the most common and you will use it every day, but you can't do block deck height with it. This would be a good representation of a caliper I would recommend....
Mitutoyo 505 676 Dial Caliper 001" 0 8" Stainless Steel w Case | eBay
For instance, on a Gen I SBC, we know that the main bearing bore is ~2.641". Half of that figure (radius of the hole) is 1.3205". Hook one jaw of your dial caliper on the I.D. of the end bearing bore at each end of the block and hook the other jaw of you dial caliper onto the block deck. The key here is to get the caliper lined up parallel with the cylinder bore in "X" and "Y" axes. Let's say you measure at 7.6695". Add the radius of the bore (1.3205") to your
measurement and find a block deck height of 8.990". Since the blueprint dimension from the factory is 9.025", you have discovered that the block is 0.035" shorter than blueprint. That's probably not the only thing you will find. I suspect that you'll end up with four different figures from all 4 corners of the block.

As far as a burette, there are members on this board who will recommend a graduated device of some kind from the drug store. Here is an example of a 60cc catheter syringe that, while being better than having no measuring device at all and possibly doing a pretty good job at checking domes, will not be as precise as a burette.....
60ML EXEL (26304) Catheter Tip Syringe Only (Qty. 5) | 60 ml Syringes | Syringes Only | Syringes & Needles | Med Lab Supply

I am of the opinion that if you're going to do something, do it properly just as a professional would do it, with a graduated 100cc burette (with on/off spigot) and a stand so you can easily do cylinder heads....
Powerhouse Cylinder Head CC Graduated Burette 0 100cc Stand Plexiglass Plate | eBay.
If you put the word out that you will do a set of V8 heads for 35 bucks, you'll be able to pay for the equipment pretty quickly and begin making a little extra cash for yourself.

Last edited by techinspector1; 11-15-2012 at 04:37 PM.
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