'87 cam gear problem

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Hi all,
I couldn't find the section for s15's so I'll post here and hope someone can help. I've got an '87 s15 4 speed 4cyl with about 200,000 kms on it. It's basically falling apart, so I drive it really gingerly, but in the past year I've destroyed 2 cam gears. I don't know exactly what happens (never taken the timing cover off) but the cam does not turn at all, the distributor doesn't spin and the gear needs to be replaced. Does anyone know why this is happening? I've been lucky enough not to wreck any valves are do any other serious damage, but the labour to fix this is incredibly expensive and I don't want it to happen again.

Thanks