I have a customer with two 496's. they were originally Pleasure Craft Marine with transmissions. he bought them to replace his 330hp in his Baja but after one had problems he pulled them and bought a set of 454 mags that dropped right in. the one with issues I have gone completely thru with new pistons bearings lifters etc and is a long block with intake and accessories but no wiring or fuel rails or injectors. the other I know nothing about but it was said to be running fine when he pulled it back out. he is wanting to sell as he has no need for them. I can see what he wants and put anyone in touch with him if interested.

Update:

I started digging and I pulled the right head off. Here’s what I know (I don’t know if some of this is normal):
A. the right head has about a 12” crack along the top of the exhaust ports external that can be visibly seen.
B. When I pulled the short bolts out of the block, the ones furthest out on the head, water came out of them. Not sure if that’s supposed to happen.
C. When running, the left side ran crystal clear, clean water out of the exhaust. The right side (side I took off) had water an oil (very milky) coming out everywhere.
D. The entire engine has milky oil in it below the pistons and in the valley. But the heads only seem to have it on the right head (the one I took off).
E. The right side most rearward piston had oil/water in it and in the exhaust valve (same side as the cracked head) it seems to be the only cylinder with this issue.
F. The spark arrestor had milk tester dripping from it on to the belt, howd oil get up into there?!

im really trying to convince myself the block is fine and I really only need a head. Can anyone else think of anything else to say differently or irrevocably tell me “if it’s doing this, the block is trashed...” from what I’ve detailed above.

one head is a lot better than one motor.

5 more posts and I can post some pics

Make that 4...

3 more... almost there

Maybe this is the 2nd to last one

Last one

https://cimg6.ibsrv.net/gimg/www.off...a8af0a751.jpeg
The crack is below the green line
https://cimg8.ibsrv.net/gimg/www.off...8f1cc7f97.jpeg
Milky chamber was the rearward one.
https://cimg9.ibsrv.net/gimg/www.off...e5aadbb27.jpeg
This is the milky cylinder after clean up. Walls seem ok. No visible issues.
https://cimg1.ibsrv.net/gimg/www.off...fe46b7ed9.jpeg
Nasty oil everywhere.
https://cimg4.ibsrv.net/gimg/www.off...a099e0e30.jpeg
This sucks.

That does suck
Don't know how the head cracked in that location

Summer is right around the corner, economy is booming, machine shops are backed up. I think you are out of time to really go through the motor like you might want to.

Buy two used heads already assembled, bolt them on this week, fire the boat up and see if the issue persists......