So, we started assembling the bottom end. Pistons and rods are in, but the pistons stick out about 0.5mm from the deck. Is this normal? If not, how much should it be?

I have CP pistons (+0.5mm bore), K1 rods. Block was decked, and mainline bored. The cylinder grades are still visible, so I doubt it was decked too much.

Deck height 204.8
minus
Crank stroke/2 41.8
con rod length 133
piston pin height 30
------------------------------------
combined height of rotating assembly 204.8

Your guy decked the block .5 and your formerly flush piston faces now stick out .5. You are surprised why?
whatever gasket thickness you were going to use you will need to get one .5 thicker to retain the desired compression ratio.

themadscientist wrote:Deck height 204.8
minus
Crank stroke/2 41.8
con rod length 133
piston pin height 30
------------------------------------
combined height of rotating assembly 204.8

Your guy decked the block .5 and your formerly flush piston faces now stick out .5. You are surprised why?
whatever gasket thickness you were going to use you will need to get one .5 thicker to retain the desired compression ratio.

What I really wanted to know is whether this can be remedied with a thicker HG. So, thanks for that!

Sure can.

We've measured the deck clearance more accurately, the pistons' slightly raised outer crowns (CP) are out this much at TDC:
0.28 (mm)
0.35
0.45
0.30
It seems the block was not resurfaced perfectly flat. Or I don't know what else to think. The deck is otherwise very smooth.
Would it be asking for too much, from a Cometic MLS to seal this much variation properly? I don't even know what thickness to pick...
Or should I go with an OEM or Reinz HG? I'm looking to boost 30psi on E85 in the long run. Can the non-metal head gaskets hold that much power?

OUCH! I'd be pissed at whoever decked that block. First off, why in the heck did they take that much material off? Secondly, you've got some pretty major variations in piston-to-deck height. Lastly, check the FAQ about the Cometic. There is a specific surface roughness that has to be met for the gasket to seal properly. If it's not met, it will leak, and from the sounds of it, whoever decked that block messed up.

PS- The thought just struck me that the problem could also be in the rods or the pistons. You should be able to easily measure the deck height at various places on the block and see if it was milled square, and then check to see if it was milled flat. If that all checks out and you find that they didn't take .5mm off the deck, then you need to check the wrist pin height on the pistons, and the center-to-center length of the rods, NOT the overall length!

Lastly, DO NOT assemble the motor as it sits. something needs corrected.

float_6969 wrote:OUCH! I'd be pissed at whoever decked that block. First off, why in the heck did they take that much material off? Secondly, you've got some pretty major variations in piston-to-deck height. Lastly, check the FAQ about the Cometic. There is a specific surface roughness that has to be met for the gasket to seal properly. If it's not met, it will leak, and from the sounds of it, whoever decked that block messed up.

PS- The thought just struck me that the problem could also be in the rods or the pistons. You should be able to easily measure the deck height at various places on the block and see if it was milled square, and then check to see if it was milled flat. If that all checks out and you find that they didn't take .5mm off the deck, then you need to check the wrist pin height on the pistons, and the center-to-center length of the rods, NOT the overall length!

Lastly, DO NOT assemble the motor as it sits. something needs corrected.

This is the same conclusion I was reluctant to come to... hoping there's something else I missed, being a newbie to all this. Thanks for the reassurance.
Then it's teardown, measure everything. It's not like I planned to use it much in the winter anyway...

I AM pissed at the machine shop, believe me. I was not impressed with the quality of the cylinder boring and mainline boring either, so I'm pretty sure they fudged it up. That explanation is way more likely than the rods (K1) or the pistons (CP) being out of spec. But we'll check everything and see what's what. Chances are I'm gonna need a new block, and definitely a new machine shop. A $250 mistake.