I also was never told about nor had any knowledge of tightening the large nut.
YIKES! It was a bit loose, and called Terry to ask about the size for my PT-425. He said it is a 1-13/16" also.
After suffering sticker shock seeing the price of a large single wrench (Harbor Freight sets do NOT include that size; only by 1/8" increments), I found this much lower cost 1-13/16":
If you only need this size wrench once in a while, why not make your own wrench . A couple hacksaw blades, an air grinder with cutoff wheel, acy/oxy torch, plasma cutter, etc. Some auto stores will even loan you tools.
And keep in mind, in that size range, the fit doesn't have to be really precise. I found that my 2" open end wrench will tighten that nut with all of the torque my 170lbs can bring to bear. The nut wasn't even thinking about rounding off.
For the wrench try "Hanover Tools" in Ashland VA. Clint Blake got one there for about $20. Probably not the best quality, but it's big and you can probably not break it. The also had a more expensive one. Their telephone number is 804-798-9402. I could not easily find their website.
How to tighten the big nut:
Turn the PT as far right as possible. This exposes the nut for better access. From the left side of the PT, using the open end of the 1 13/16 inch wrench, slide it over the nut with the wrench towards the rear of the vehicle. Pull the box end of the wrench as hard as you can towards the front of the PT. Give it everything you got. (This is for normal people).
Is this is not clear, I can probably post a picture.