[Mgs] Spanner/wrench size for MGA wheel cylinder

[Simon Matthews]

I am in the process of swapping out a leaking wheel cylinder. I was
able to remove the larger of the two bolts that hold the cylinder onto
the backing plate, but I could not find a spanner to fit the smaller
of the two. I tried both Imperial and Metric spanners, but the spanner
would either not go around the bolt head, or was too large to turn it.
Since this is on the back of the backing plate, I am working by feel.

What size spanner do I need? Is it a Whitworth size?

Simon