Can the gauge of the thread change your seam allowance. I use
Connecting Threads but have been told a smaller gauge thread will give me a more accurate seam allowance. I really hate to pay $8 a for a spool of thread if it doesn't matter.

I use the Connecting Threads thread and I just take a scant seam allowance then. If you do the same amount of seam allowance for all the blocks you will have the same outcome. Buy and use the best thread you can afford.

I use Connecting Threads 50wt thread all the time. I also use Superior Bottom line thread 60wt in the bobbin. Using a scant 1/4" seam allowance I get perfect seams each time. I feel that Connecting Threads thread is the best top thread for the price.

It's the thickness, which is in general, tied to the weight, that makes a difference. I've got a nice stash of 50 wt cotton Aurifil, bought from Tristan Threads in BC, Canada for about $3/spool on clearance, but I'll try CT sometime, I bought a set of neutrals. But 40 or 28 wt would definitely add bulk to a seam.

I bought superior Bottom line at the quilt expo. Can i use in both in the bobbin and top thread?

It is the weight that determines the thickness in the seam.

Great info....I will continue watching for answers and education!!

Ditto :-)

it most certainly can; I'll tell you how i know. Last year I was doing a Civil War quilt w/6 1/2 in blocks; I couldn't get them to turn out for anything; I was so careful. My teacher suggested Aurifil; it's a 50 wt but it has only 2 strands so it's thinner; now all of my blocks are the right size. Plus my Vikings and Janome love the thread even when I'm cross hatching quilting; not one problem. Viking actually recommends this thread to use with the Mega Quilter.

It's also the # of strands; Aurifil only has two most have 3.

Sorry, had to edit to correct mistake.