It was something on hand for making replacement tablesaw fences so v heavy gauge.
I think from memory it would be 40mm external in the small dimension.
In the later L5A Harrison chose to stand the 2 cast motor brackets off the lathe bade with stubby cylindrical spacers, but they also added slots in the base cabinet so you could move the brackets together or apart.
If you do it as I did on the L5, you do have to be careful with bolt placement because tightening a nut inside the end of a rectangular tube gives very little swing for the spanner. Keep the holes as near the end of the tube as you can and a wide tube 80x40 is easier than a 4040 square section.
Couple of shots below of solving the same problem for the L5A after swapping the motor.
With a different spindle height the modern motor still needs tube.
As the standoffs when I got mine were a stack of washers with a bead of weld to keep them together, I made new ones about 40mm dia and about 20mm thick.
I needed the lathe to fix the lathe so temp standoffs were offcuts of 25mm mdf
It is all about getting the spindle in the middle of the oval hole at the position where your belts pull tight.
I increased the size of the drive pulley as I had a 4 pole motor, this meant changing the belt lengths (to the nearest standard length) so there is often trial and error and need to buy a second set of belts when you figure out your mistakes.
My motor was delivered with a broken foot, so another thing to fix.
Mag drills and motorcycle scissor jacks are great !
