Bottom bracket fixed cup wrenches are most commonly 32 mm across the flats with an oblong hole that acts as a box wrench so the needed torque doesn't spread it. A large adjustable wrench will work - sort of - but it isn't the right tool.

You position it over the fixed cup and crank it down VERY tight. The torque spec is in the 300 - 400 inch-pound range. The hard part is not having the wrench slip while you are doing this.

To prevent slippage I made a clamp for the fixed cup wrench out of a 5" long 5/8" bolt, a nut and two BIG washers. Use it as follows:

1. Thread the fixed cup all the way in hand tight then thread the adjustable cup (non-drive side cup) in a few turns.

2. Add one washer to the bolt and thread the bolt through the adjustable cup and out the fixed cup.

3. Place the fixed cup wrench over the fixed cup's flats, then place the second washer on the bolt and add the nut finger tight. That sandwiches and clamps the wrench between the frame and the washer.

4. TIGHTEN the fixed cup. Remember English bottom brackets have left-hand threads for the fixed cup.

5. Remove the nut, bolt and washers and complete the bb assembly.