Thanks for your reply, Mickey. I have got similar experience.
I have used a range of needles from size 8/60 to 22/140 on my Singer 201k, Singer 66k and Singer 237 with no problem. I found no needle size specifications in the manual of the Singer 66. The manuals for both the Singer 201 and 237 specify needle sizes up to 21/130. The needle length from but to point vary with the needle size.
I have got a Bernina 910 as well, and the manual states needle sizes up to 18/110. I have tried to install a size 21/130 needle in it, but then something hits the needle. So for more reasons I would never try to use heavy thread with this machine.
For more layers of lighter canvas I have been able to balance the stitch with the normal beehive spring in tensioner and a size tex 135 thread. However with more dense fabrics of leather, vinyl and webbing I needed higher thread tension than the normal beehive springs of the tensioners can provide. If you try to squeeze the beehive spring, the thread tension becomes very unstable. So the right way here is to get a heavier beehive spring.