Hello. First post, so probably going to drop alot of information to start. Such as what I have found in my "good 312 engine".

As I was told it had the top end rebuilt 2 owners back and was in running condition. I dropped it in the car and started it. Smoked alot but no odder and no distinguished color. Driving it showed blue smoke. Thankfully I didn't drive it far or run it for long.

Pulled the heads and found about an ounce of oil in a few of the cylinders. Had three spark plugs that showed a good fire, the rest were oil soaked.

Did I mention I heard a clattering sound at idle? Turns out the engine was bored .030 over and someone stuck a single standard bore piston in the block. Other things that were found upon disassembly were 5 of the oil control rings installed improperly, crank undersized .010/.010 with standard connecting rod bearings, number 3 connect rod cap installed 180 degrees out, oh and standard size rings installed. I did not pull the cam and really don't want to. I know I should but I'm already pushing my budget and the oil flow to the heads wasn't horrible.

Needless to say my mind was blown at what I found. I ordered a new set of crank bearings, connecting rod bearings, double roller timing set, .030 over rings and a new .030 over piston, but the weight is 20g heavier than the weight I gave to the supplier. How big of a deal is this and should I remove more weight from the pison?

Hopefully the question isn't lost in all of that info.

Thanks
Scooter

If you have a gram scale find some aluminum that weighs 20 grams to see what it looks like. With a dremel grinder and a burr you can find a lot of areas to remove metal. I have taken 15 grams out of one once and I believe left it 5 grams heavy and it ran many thousands of miles in a pickup. I had to fix a friends 292 YBlock that was .040” over and power honed one cylinder to .050” over which is a std 312 and he bought a replacement piston that was 2 grams over the one we took out. It ran like a top.
Sinceyou have it apart I would put positive seals on the valve and just relap them. Good Luck. 

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Thanks. How close should I get it?

Also don't think I can go with positive seal type valve seals. Heads are stock. Valves are probably at the limit for end play but they seal.