S21ZSN Rear Wheel Cover Issue

[FuzzyDriver]

The wheel cover on my S21ZSN will not stay against the rear wheel. With a lot of force you can make it snap into the wheel, but as soon as the wheel turns, the cover pops out except where the drive pinion washer holds it in. This makes the opposite side open about 1/4". I cleaned the areas where they meet carefully. I could put lubricant there, maybe WD-40. That would make it easier to push them together, but I'm think this would also make it easier for them to separate as soon as you turn the wheel. They don't seem to want to stay together, like they are slightly different sized. Has anyone ever seen this issue? Warpage from something? Does anyone have a couple of old covers I could get and compare to mine?

Update:
I put one of the wheels in my drill press and used my Dremel to 'machine' the inside diameter of the wheel larger by about 3/32" down about 1/4". Now the cover sits on the inner ridge with no problem, but the cover itself is about 1/8" from meeting the outer edge. Again, I can physically push that edge down into the wheel, easier now, but now it slides back out easier, too. Does anyone else have this type of mower? Doing a cross-reference, it looks like this system was used from 1986 to 1989 on models 8125, 8156, 8157, 8157A, 8157A1, 8440AE, S21BSN, S21ESN, and (my model) S21ZSN. Is the cover on yours flat? Mine curves inward toward the center, which is what pulls the outside rim out away from the wheel now.

 

[FuzzyDriver]

Rear wheel's reassembled SAT, and ready to roll. In case there is anyone else with this issue, I will detail how I fixed it. I put the wheel cover face down on top of a large, metal coffee can. I put a (large) socket over the center of the wheel cover, and then centered a sledge hammer on top of the socket to apply a constant even pressure around the circumference of the inner ridge. Then I used my heat gun on medium-low and heated the cover up for several minutes. I got it pretty hot; hot enough to soften the plastic, but not hot enough to melt it. When I started, the wheel cover center was sloping up about 1/4". By the time I was down with it, the pressure and heat were had reversed that slope so it iended up sloping down (i.e., towards the bottom of the can) by about 1/4". Once it cooled, I installed it and now it fits fine and stays in the wheel grooves.