Very interesting question, I'm experiencing the same thing RIGHT NOW. I just finished separating my hitch head from the shank. We purchased a new truck 3 days ago (see my signature below) and when test towing the trailer with this truck, it ran so quiet, I could really hear the bang-bang of the slack. At the time I didn't realize it was that slack, I thought maybe the pin was too thin, or that insert to fit the larger receiver was causing the movement.
After unhitching from the test drive, I realized and felt the slack between the shank and the head. So, I've been on a campaign to do something about that.
I checked with Modern Trailer in Anderson, IN, and basically, to get a hitch with the larger shank to fit my truck now, will require purchasing a whole new WD system. I really don't want to do that. They said most people are just using the hitch insert.
I then asked about the slack between the 2 parts. Funny, I NEVER noticed this before until 5 days ago during the test drive, so I don't know when this started.
Evidently, it's not a real problem. The last trip we made was over Christmas when we went to Florida for a week, towing with the old truck. No problems or anything, but the slack had to be present, I just didn't know it.
So, the guy at Modern Trailer suggested putting a washer in there to take up the slack. Only problem, it takes a very, very thin one, and then it's going to have to be wedged in there.
Well earlier today I took the two pieces apart to see if any washer would fit. I don't have anything that thin. So I came up with the following idea.
#1. I'm taking the time to wire down the shank and the head and getting rid of all the old loose rust and then going to repaint them with some good old fashioned Rustolium. I'm hoping the paint may be all it takes to fill that gap.
If that does not work, my next idea is to use a bit of old inner tube and cut out a make shift washer, spacer with that. I know that probably will not last long, I expect it to wear if there is any movement at all, but if it helps quiet down the clang, clang I'm OK with that.
One thing I also thought about, just like you, was to compress the two sides of the hitch head that wraps around both sides of the shank. I DID rule out that idea though, think that could potentially weaken the head, and the last thing I want is a broken hitch at 60 miles per hour.
So, your post is absolutely right on time! If I have any success, I'll definitely do a follow up on this post and let you know what happened.
Do you think paint will be enough to do the job?