The tool you want is a chaser, it looks like a tap, but doesn't cut. Instead it stands the threads up straight & cleans them.

I've never seen anything older than a B18, but it has to be UNC with an outside chance of whitworth, which stands out because it has a 55° angle, not 60° as with SAE & Metric. From what I've seen from other Volvo equipment they adheared to SAE standards early on and somethings stayed SAE i.e., truck bellhousing patterns.

Also with studs, make sure you don't bottom out the top thread when you put it back together. Screw it in finger tight & back it out a little, anti-sieze on the inside threads & oil as specified by the torque method.