I have told this story before, but now that the subject came up, let me tell it again. When I bought my first Volvo in 1959, I asked the dealer which was the engine part that was most likely to give me problems first. He said, "the timing gear." So I told him,"let me have one of them" because I was taking the car to a place where spare parts would not be readily available. I carried that gear in the trunk of two 122s for a total of 22 years, 20 of which in the second car, and it was still in there when I sold it. BTW, the engines in both cars got over-heated to the point where the paint turned brown/gray, but that didn't seem to have affected the timing gears. I don't know for sure, of course, but I suspect that factors other than the strength of the tufnol are involved in the gears' failure. First and foremost, a plugged or misaimed lubricator, but also, excessive engine fibration (tuning, wear with age), shifty camshafts, wrongly tightened retaining nuts, etc.
I now have two B20s, one with a fibre gear and the other steel, and I can't hear the difference either.
Bob S.