The eye of the needle needs to be 40% bigger than the thread diameter to stop it over heating and cutting your thread.
To select appropriate sized machine needle for thread, put 30 cm of thread through the eye of the needle, holding one end of thread lower than the other. If the needle does not slide fairly easily (via gravity) down the length of thread then a larger sized needle is required.
For your 40 from the chart (in the web address below) It would be a "US" or "Singer" size of 14-16 or metric 90 or 100 you now just need the length to suit your machine. An old needle will have this length/style measurement stamped into it.
www.thethreadexchange.com/miva/merchant.mvc?Screen=CTGY&Category_Code=nylon-thread-information To test tension I do a test strip with the same thickness I'm about to stitch. (I have lots and just note the tension setting.)
You get to know after a while.
As the stitch runs I stop and change the tension setting and number the area according to the tension setting on my machine.
This is a test piece for my power kite repair. My Wife's machine to get triple stitch zigzag.
I then look on both sides to see what tension works the best. I am looking to see that the loops between the stitches are buried correctly. The front and back of the stitched material should look the same. If the bottom stitch is not buried like below, the machine tension is too light.
Very little tension change makes a huge difference. Also when you break a needle make sure the thread is completely rethreaded correctly and the offending broken needle tip has been removed.
Sometimes the thread will flick out of the tension discs, or off the light tension spring and you don't notice till you have destroyed another needle.
This is what stuffs up needles/ timing as it all gathers round the bobbin and stalls the hook and loop process.
Make sure you give the machine a blow out now and again, as it's amazing how much sand a windsurfer sail can contain.
Hope this helps. Chook