LT - Proffit told you right, you need to retest. Warm that engine good, say 15 min of running and rev it several times to blow carbon chips out and off ring lands and valve seat interface. Take at least 3 readings on each cylinder (dry), and throw out the high and low readings. Then shoot some oil ( a teaspoon or so) in each plug hole and take 3 more readings (wet) and throw out high and low. Take the average wet and dry and compare. This will tell you if rings are indeed worn as the wet readings will be considerably higher than the dry readings by 10 to 20 or more lbs. If you have a vacuum gauge connect to manifold (not ported vacuum on carb base) and watch readings. If smooth and steady, cool. If jerky, bouncing or irratic back and forth from high to low vacuum this points to valve and or guide wear. A cylinder leakdown tester is what you need to pinpoint which cylinder and or valves are the problem. Anyway something is puzzling in your comp readings that you stated and something does not seem right. 1. You stated that your 283 is "mostly stock" meaning what has been changed? Is it in fact a 283? Check block casting number on drivers side bellhousing flange, will be GM followed by 6 or more numbers. 2. Recently rebuilt by whom? And could they have possibly put the compression rings in upside down, or have excessive end gaps in the rings or have the rings out of position so the gaps line up? 3. Is the engine block leaking oil around the rear seal? Heavily? How about the front seal? The reason I ask is because the readings you stated follow the firing order and pulse up and down. As if a back pressure existed. Does the intake manifold have the stock oil fill tube? Is the crankcase draft tube in place behind the distributor? I think that was omitted in 1958 as part of the PCV mods. Stock 59 heads were the first year with the offset valve cover bolts. Additionally they have no accessory holes in the ends of the heads, and were not machined flat on the ends. Accessory mounting was from the exhaust manifolds and waterpump bolts. This was changed on the heads around 64 or 65 I think. Interesting that you stated that you have headers. So how are accessories mounted? Does your engine have breathers in the valve covers? Stock did not. If you are not venting the crankcase you cannot achieve correct compression readings because of back pressure from crankcase will not allow the rings to seat properly Its the same on the top side as compression rings actually need a little blowby into the ring land to properly expand the ring to the cylinder to seal correctly. Need more details? The readings you have do not suggest broken rings or blown headgasket but there is enough variance to be very concerned. Take more readings and get a leakdown test done to pinpoint trouble spots. Good Luck and Think Chevy!
