So 2018 F150 bought used two years ago. Right after I got it going through a car wash the air blower at the end cause drivers side wiper to flutter and fly off.

I thought odd, but looked at how the clip release worked and put it back on. Like everyone the little key for each are was laying in the plastic cowl.
'general opinion is don't need em.

So now our first snowfall of significance 2" of wet snow but not hard frozen, I hit the wipers and both arms popped off. The splines in the arms is either worn to the point they will no longer key lock onto the drive splines or the splines on the drives are worn or both.

Does not seem to be an aftermarket item the arms at typcal auto parts store, I plan on seeing what the Ford Parts desk has to say tomorrow in the
AM unless all wiper arms are backordered because of the recall on newer trucks.

 

The wiper arms have a little lock lever on the side. If a bunch of dirt has gotten under the arm it may be stopping the lever from being pushed right against the side of the arm. If that lever is in place and in good shape the arm should not be able to disconnect from the spline drive on the transmission pivot.

 

I can put the arms back on and lock down the spring loaded release levers, then I can swing the arms to and fro because either the splines are worn down in the arms, or in the drive shafts.

 

You said an arm had popped off when your first got the truck, and both popped off with your recent snow. 52merc was explaining what should be preventing them from popping off, not what should prevent them from spinning on the pivot shaft.

 

I am surprised it took two years for this to happen as there is a discrepancy between the diameter sizes of the pivot points on certain wiper transmissions and the wiper arms. I had bought a new wiper transmission to replace one that was seized - the original wiper arms were too large for it and returned the new part to the dealer. There was some discussion as to how build dates affected the sizes. I refurbished the original wiper transmission. The Dorman after market wiper transmission has large pivot point attachments on which all wiper arms sit snugly if you want to go this route. Previous related thread.

https://www.f150forum.com/f118/wiper...475462/index2/

 

oh yes you do, even with all new parts the fit is not great. that keys is probably the only thing keeping the arms from slipping again

 

I've been using my truck for about 4 years now without the keys. I agree with Mr. Boulette, there is a mismatch.

 

So I picked up a new set of arms from Ford dealership today, while the splines are sharper/pointier I measured with a caliper and the ID did not seem increased. The parts guy said most likely the pivot knurls are worn and need replacing but they did not have a transmission set in stock. The arms popped on fine and clicked in place, seem a bit less wobbly but also they came with the key that is going to fall out, so for now they work. Perhaps when weather improves and I can get inside not out below freezing I can replace the transmission assy. also