When the block has a lot of pieces I use 70wt or higher wt thread top and bobbin. Usually a 70 needle. I use 50 wt if the block is a nine patch or less. Always 70 or higher thread in the bobbin for me. Determining the correct thread and needle size is very important for accurate piecing. Many think any thread will work and it usually does but then the block size is off, the seams are bulky, and all around a mess.