This is one of my dished pistons. For some reason I thought that this was a full dish and that half dished pistons would be, well, half dished – not so! This turns out to be a half dished piston which is simply shallower than the full dish. This one has a dish depth of about 9/32″
This is good for higher compression, but I had sent out the heads to be planed .010 to make up for the cleanup I did on the combustion chambers. I got the job stopped at .005 so not too bad. At least I can be sure of good flat head surfaces.
I checked out the dish cc and found them to be 18.2cc. Which translates into 1.11 ci. The gasket is about .018″ and with a bore of about 3.75″ it is 0.199 ci; deck at .023 & 6023.602 bore (.040 oversize) it is 0.234 ci; bore at 3.602 and stoke of 3.625 it is 36.939 ci. The average combustion chamber volume is 59.95 cc. Using an online compression ration calculator I came up with 8.1:1. Fine for low test gas, but not my target of 8.5:1. I would have to send the heads back and have them cut by .024. I’m not sure that the heads could go that far and still keep the rocker/pushrod angles within tolerance. Originally those heads, #1557570, had chambers of 54.5 cc. The valve work increased that by about 5.5 cc which wasn’t made up by the smaller dished pistons. So no more canaries for that cat. I’ll just live with the lower compression. Maybe some day I’ll stick on a supercharger 🙂
Here is the engine layout with the cc volumes shown – firing order at the top. The arrows show pairs of cylinders that fire close to each other. I figure any big mismatch from one side of the engine to the other will add to vibration at low speed and diminish power at higher speeds. If I were building a racing engine I think it would be good to have all the cylinders very close in cc. Here I have two problem cylinders, #2 and #6. I opened up #2 to 59.4 and #6 to 60.0. The average for all cylinders is now 59.9 cc. the range of variation between all chambers and the average is +6 to -5. This is well within the 1 cc that Jim Pepper recommends.
Here’s what the chamber looks like after some grinding on the sharp ridges between the valves. The spark plug hole if filled with dumdum and the surface around the chamber is smeared with some vasoline to seal the lexan plate for the cc operation.
Lexan plate is in place and the pipette is feeding water in through one of the two small holes at the top. I can take a bit of jiggling on the bottom of the plate to remove some trapped air at the bottom of the chamber where it tapers up to the head surface.
So time to clean everything up, paint them gloss black and do the complete assembly.