My intake manifold leaks like a sieve

d100clubcab said
I guess I need to go back and ensure I install the alignment sleeves correctly

I told you wrong.... alignment PINS, not alignment sleeves. They align the heads and head gaskets, which effects the intake surface positions. Those pins are in the block on the deck surface where the heads bolt on and should have been obvious; the head gaskets would have wanted to slip off! So if you laid the gaskets on and they just laid on the block, then then pins had to be there, unless you glued or RTV'd them in place. I can't tell you for sure HOW much of a difference it will make, but it is just another factor where thing can shift off a bit and be a part of a tolerance stack up that gets out of control.

Any small pins in the front or rear of the block walls above the valley should be removed... sound like they were before..

Did the heads get milled/surfaced when they were re-worked?

Murray said
I suspected as much. This seems weird, but aluminum manifolds can be working fine on a running motor. Now you work on the motor and you remove the intake. When you re-install the manifold it leaks- usually in one of the four corners. It is warped! This has happened to me twice. It seems they warp after they are removed. A machine shop can re-surface the manifold (hard to find around here). Or replace the manifold.

Interesting observations and experience!