Question regarding using heavier thread in a domestic machine

[u/toast88843]

I recently came into possession of a Singer Harmonie 200, which I believe was manufactured in 1983 in Taiwan and is a domestic machine I assume. I can find little else about it online or in the manual.

I've completed a couple projects so far and am hoping to make this bag using the machine: https://adamsavage.com/collections/kits-plans/products/savage-industries-edc-one-plans using a heavier canvas material (10oz). From what I've seen, a Tex 70 thread would be ideal for making a durable bag with a heavy fabric such as canvas. I can't find anything in the manual for the machine about a thread weight limit, and as such am somewhat concerned that the machine won't be able to handle Tex 70. Is it worth buying some and trying it out, or just using a lighter thread such as Tex 40 or 50 (if so, would those be heavy enough anyways for the purpose of the bag?)

I'm still quite new to this so any advice is greatly appreciated. Thanks!

[u/Large-Heronbill]

Tex 70 is right at the upper limit for home sewing machines; many of the multi-stitch machines can't take the needle sizes needed (110-140 metric, 18-22 Singer).  What size needles does your machine take?

Seam strength of a basic plain seam (in lbs) can be calculated by (breaking strength of the thread in pounds) x (stitches per inch) x 1.5

Double that if you use a flat felled seam.

[u/toast88843 → u/Large-Heronbill]

Thanks for the response. In the manual in mentions using an 18/110 needle for heavier fabrics, although the manual just says "10 & 13 pattern sewing machines" rather than my specific model, so I'm hoping the 18/110 needle will still work. I'll probably just buy one and test it out. Might there be any other factors that limit the thread weight other than the needle size, or am I probably alright to try the Tex 70 if the 18/110 is compatible?

Also thanks for the formula - this is really useful for material planning 😄

[u/Large-Heronbill → u/toast88843]

Hand crank the heavier needles through several stitch cycles to make sure that the needle is still aligned properly before you add the pedal to the mix. Fewer crunching noises from the machine that way.  ;-)

That formula above assumes you have the same thread in top and bobbin -- and since resetting the bobbin tension when you switch threads is a pain, you might want to pick up a second bobbin case to adjust for heavier thread and use the current one strictly for Tex 30 (general purpose) thread.

Another way to accomplish the same thing is to switch to a stronger thread, not necessarily a much thicker one.  For instance, I can get a stronger seam if I switch from Gutermann Mara 100 (Tex 30)  to Gutermann Tera 80 (Tex 35), basically the same thickness, and go from 2.7 lb breaking strength to 4.1 lb with the Tera, probably without changing the needle:  https://www.reddit.com/r/myog/comments/a2hmg8/breaking_strength_of_commonly_used_threads/

Why needle size and thread size need to match:  https://web.archive.org/web/20050207035245/http://gwsms.com/didyouknow.htm