Chimney Flue Pipe installation question.

I have installed footer, cleanout and approx 5 feet of grey chimney block. I have lined from footer up to thimble with red 8 x 8 flue pipe. My questions are about internal clearances between flue and grey block.
1) Is there a recommendation to keep the flue centered in the grey block? (Can this joint be packed with refractory cement, or filled with vermiculite, or do I need to keep 3/8" free air space between flue and grey block?)
2) My mortor seams currently line up between grey block seams and refractory cement tile seams (should I cut 4" with diamond saw off of red 8 x 8 flue pipe which would make my refractory cement seams in the middle of the 8" tall grey block?) Thus staggering the joints so flue seam is not parrallel with grey massonary joints?

This chimney is to be used on a AA-130 coal boiler.

Thanks in advance.

try and keep it centered, but doesn't have to be perfect. Use vermiculite and fill between tile and block, every foot or so put a few dabs of mortar around the corners of the tile to keep it from shifting, not much, about a tablespoon full, let it set on the vermiculite that you've already filled the chase with.

It's best if the joints dont' line up but not essential.

TIP: the ends of the tile are *NEVER* cut square, so take a square and mark the tile you've just laid as LL on the longest side and L on the next longest side, then mark the tile you are about to set the same, put the LL and L sides opposite on the joint so that the tile stack stays straighter - you'll spend a LOT less time shimming joints just to keep it straight and your joints will be tighter. If you have any tiles that are cut almost square, mark those as such and set them aside, also mark the ones with two parallel sides and one long side with an L and set those in a separate pile.

Berlin do you ever make it to MD, if so I could sure use your knowledge in building a masonry chimney

My footing was covered with refractory cement prior to laying the first tile. All tiles where set in refractory, and the tiles where backfilled with mineral insulation. That was 31 years ago and everything looks the same after all those years burning coal. Wipe the tiles with your finger and they look brand new.

I think vermiculite between the flue tiles and block is well worth the expense.