Pulled both engines, and had stock rebuild, 1st engine had hole burned in piston, rebuilt it, checked on lap top ok, but once I ran the rebuild for about 3 hrs had lost about 3-4 quarts out of the dipstick/pcv. Tore it down, and same piston melted down. Doesnt sound like the injector is working? Checking them out now, but could it be anything else? Also this engine would not heat up at an idle in both cases. Changed the thermostat, and it did, but lost the engine 10 minuites later. Any Help would be great.

Seems like it must be either the injector or an air leak at the manifold, also be sure all your plug wires and cap are good.

Once you get it back together with new injectors, i would run it up to say 2000 rpm in neutral for ten minutes and pull and compare the plugs if six is still lean, you should be able to see a difference.

Think about installing pyrometers also. You may be able to catch it before it gets too hot next time. Ive got them in # 6 and #5,they say 6 is always the hottest.
Check your fuel pressure. All 8 may be leaning out but because #6 is the hottest it goes first. If your injector isnt firing you would probably notice it right away because its down on power.

I havent got the results from the injector test yet, but is there anything on the manifolds that could cause this? I havent had them pressure checked as of yet. Once the engines are back in I will have it checked out by my marina, just trying to cover all my bases while I can.

hard for me to believe on a box stock 502 that having an injector go lean would be sufficient to burn a piston... ide be willing to bet that the if if ever got near that point the knock sensor would trip it back to safe very quickly... sounds to me... and i have seen this happen... that you got the cam and the crank in the wrong holes... and by that i mean, most of these gear sets have multiple holes and multiple dots... for advancing/ retarding the cam... and i have seen people put the cam in the " A " hole but use the "B" dot so to speak... and this will make the thing hard to start and run horrible and drive the egts out of sight... mostly because the timing ends up effectively wildly retarded... safe to say that lots of things get bad...

but frankly the fact that you melted the same piston twice, ide be looking a some sort of serious vacuum leak at or around that cylinder...

and if the injector isn't firing, then the cyl goes cold... so i doubt thats it...

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There is a possibility that you have a clog in that injector, just enough to lean the cylinder at speed and create high heat. On the EFi engines it is hard to detect a misfire since th IAC makes up for the loss of rpm.

well... i suppose ... but on an 8.5 to 1 motor if it EVER got close enough to hot enough to melt a piston into the pan, it would ping so bad you could hear it on the dock... i believe the electronics would go to safe mode long bef any damage... just my opinion of course but on stock , low compression efi, i just can't see an injector failure of any sort melting a piston.