I also have a Pfaff. Size 70 needles are not a problem, the needle shafts should all be the same regardless of needle size. I did have a spate of using twin needles & having them fall out in use. Since they were expensive Schmetz ones I took them back to the dealer I bought the machine and the needles from. I also took a micrometer to demonstrate to him that the shafts were undersized, and I sat there measuring his stock of twin needles until I found one with a shaft the correct size. He then refunded me for the broken ones. Cheap ones bought online didn't have this problem. This was a few years ago. Since getting a coverstitch machine I haven't used twin needles. So, in a long winded way, I'm saying there may be a problem with the needle. Dealer also told me if the problem continued he could replace the needle clamp in the machine. At that time my machine was barely a year old.