Just went out and checked. It's 42 mm. I use about a foot long 3/4" extension on 3/4" drive with 6 foot piece of pipe to torque it properly, e.g. about 80 # pushing on it to tighten.
The bolts have an extended nipple shoulder that keeps them from rotating. Try to get the nipple to go in straight and true; otherwise, you'll be tightening it longer because it'll bind.
You may have to rap the ends of the bolts with nuts on them (to keep from spreading the ends of the nut) to get the bolts off the nipple when loosening them. Just loosen the nuts a tad and give them a rap with the socket extension.
Ralph