I have just tapped 2 M8 holes into the 4mm steel plate on my chassis to mount my steering column. Will Mr. SVA like this or do I have to put a nut on each bolt as well?

4mm isnt very thick. I would put a nut on it as well! You only have about 2 or 3 threads in that thickness of plate.

David

I'd nut it too - as a general rule of thumb "to obtain the maximum effectiveness of an ISO thread you need the depth equivalent to it's nominal size. i.e. if it's 8mm thread you need 8mm of depth. That's why standard nuts are the depth they are.

A good rule of thumb is to allow at least the diameter of the thread for the nut thickness, I would drill the plate clearance and fit nylocs on the back.
Paul

Which column are you using? My Sierra column is held on with M12s. M8 would fall through the holes without a big washer.

The nuts/set screws on a standard sierra for mounting the column are M10 with very big washers.