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I was just communicating with the right guy from Gates, trying to get the right
belt for the TR3a. I have learned the following which may be of interest:
1. My old belt, which was a Gates 507, is no longer made. It measures 38 3/4"
outside circumference (OC), and the top width is 0.70 to 0.72" (a bit hard to
tell due to wear).
2. There are belts available in 11/16 and 3/4" widths, but there is less of a
selection in the 3/4" width.
3. There are SAE standards for the angles of the walls of pulleys. The idea is
that the smaller the diameter of the pulley, the smaller the angle, such that
the belt "wedges in" more tightly on a smaller pulley.
4. Said angles are:
up to 4": 34 degrees
4" to 6": 36 degrees
over 6": 38 degrees
5. The angle of the belt wall is 38 degress i.e. no wedging action on a pulley
6. The cords that are visible on the sidewall of the belt should run as close
to the outer circumference of the pulley as possible. If done perfectly, the
top of the belt should actually be just outside the edge of the pulley.
7. Notches, or cogs, have been implemented to allow the belt to go around small
8. Gates is in the middle of converting all their automotive belts to the
notched, or cogged style.
9. Since their distributors carry stock, there's no way to know if you'll get a
cogged belt or not if you order one.
10. A less popular size is more likely to be available in non-cogged than a
11. An 11/16" width is recommended. An OC of 980 mm (38.6") is the closest to
my 38 3/4". There are also 960 mm (37.8") and 990 mm (39.0") available in the
12. In 3/4", the only close one is 980 mm (which is really close anyway).
13. So, belt numbers:
11/16 x 980 mm: 15A0965
11/16 x 960 mm: 15A0945
11/16 x 990 mm: 15A0980
3/4 x 980 mm: 17A0960
I know you would expect the 3/4 x 980 to end with 0965, but that's the way it
I hope this helps. I still have a quandary about which width to go with. The
guy recommends the 15A0965, as it's closest. He said the difference is really
minor when you get right down to it.