Lister D bolt sizes

One of the top rocker cover bolts snapped off during disassembly, I managed to get the old broken bolt drilled out but now I need to find a tap set to fix the threads. I was just wondering if anyone where could tell me the sizing of the tap set that I will need. This happened on both of my Lister D's on the exact same bolt, maybe a coincidence or is there a weak point here?



Once again many thanks
Blair

The threads on Lister engines are Whitworth, measure the bolt thread OD to get your required size.

Maryalice