Yanmar 1200 Tiller - what size shear pins?

[Tb22]

Was given a YamMar 1200 tiller to use behind our John Deere 790. Worked great tilling up the garden area two times. Worked another area and hit a rock, which broke the shear pin. I have replaced it with Grade 8 bolts and it continues to break, even when tilling up the garden area where there are no rocks.

Does anyone know the exact shear pin size or why I keep breaking them????