Unexplainable Problem with Trailer Hub Dust Caps

l008com

Original poster
Member
Feb 19, 2016
919
Massachusetts
I have been fighting the weirdest problem with my boat trailer the past few years.

So the trailer is probably around 40 years old. We've been towing our little 15' boat with it since the late 80s. Maybe 90s.
We never had a problem. We were doing short trips from the house to the lake in NH. Then around 2010 I brought the boat home to MA. Here in MA, I drive a lot further with it going from home to the boat ramps in Medford or Newton. And I have no slips so every single boat ride is a drive. So for over 10 years, the trailer handled that no problem.

Within the last few years, the caps on the axles started to fall out, so I took that opportunity to replace the rear seals, clean the bearings, regrease everything, and retighten everything. The hubs are rolling perfectly now.

Here's the problem:
I keep replacing the caps with new caps, and they keep poping right out and I lose them on the road. I can't figure out why. I'm DEFINITELY not using caps that are too small. I know this because they are so tight that I've ruined about half of the new ones I bought during install, where the medal just starts peeling back and splitting. It can take an hour to get a new cap on, they're so tight. But once I get it on there, I am not exaggerating when I say I can drive ONE mile and they will back themselves out. Another 5 miles and they're gone.

This is happening on both sides. There's plenty of grease in both hubs. Yesterday I hammered them in perfectly snug, drive exactly one mile, and pulled over to check. Both had backed out partially in just a mile. I don't get it?! Nothing inside the hub (aka the retainer clip) is rubbing against the caps. They're super tight. Yet any amount of driving and they almost right away start to back out. I can't figure out how? There's no grease fittings so its not like it's over-pressurized with great. I can't figure it out!!

I even tried putting a ring of liquid nails around the ring before I hammered one tight to see if that could hold it tightly in place. It had ZERO effect.

Any ideas?

Here's a photo of what I'm dealing with. Note this is the first time the cap fell off, and it sat that way all Winter, which is why the grease looks rough. But I took the whole hub off and cleaned and regressed everything before reassembling and putting a new cap on.

trailer hub.jpg
 

Mooseman

Moderator
Dec 4, 2011
25,661
Ottawa, ON
Maybe pressure builds up and pushed them out? Maybe try using bearing buddies instead of regular caps. They're greasable and are made of heavier metal than the caps.
 

l008com

Original poster
Member
Feb 19, 2016
919
Massachusetts
How could pressure be buildling up so fast? I drove 1 mile on suburban side streets.

Bearing buddies are much heavier than simple caps, I'd be afraid they would pop out even faster.
 

cornchip

Member
Jan 6, 2013
638
I'd try a complete replacement hub kit on one or both sides. Every part new.....can't go wrong.
 
Last edited:
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flyboy2610

Member
Aug 24, 2021
498
Lincoln, Ne.
Is the hub properly seated on the axle? Maybe there's a bit of play and that's knocking the caps off.
 

Mooseman

Moderator
Dec 4, 2011
25,661
Ottawa, ON
I just thought of something. Have you checked the temps of the hubs with an infrared temperature gun? Maybe they're getting hot enough for the hub metal to expand and let the cap loose. Are the hub bearings adjusted properly with some free play? The nut shouldn't be tight on the bearings.
 
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l008com

Original poster
Member
Feb 19, 2016
919
Massachusetts
The hub nuts are properly tightening, meaning hand high, backed off 1/6th of a turn, and pinned in place. With the trailer jacked up, they spin totally freely.

I don't have a way to test driving the trailer around without the boat on it though.

I also don't have one of those thermometers so I can't really test the hub temperature. I can touch them with my hand, and they are warm but certainly not burning hot.
 

Redbeard

Member
Jan 26, 2013
3,535
About the only other thing I could think of is if the bearing were to tight and there was some water in the grease (which isn't uncommon on boat trailers) that the water boiled creating enough steam to push the caps off, but with you able to touch the hubs I doubt that would have occured.

In jest place some Lock-tite on the cap before installing them.
 

l008com

Original poster
Member
Feb 19, 2016
919
Massachusetts
Not only that, but they were wiggling off after a SUPER short drive at very low speed. I would have had to crank those hub nuts on with a breaker bar to make them get that hot in such a short, slow drive.
 
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Mooseman

Moderator
Dec 4, 2011
25,661
Ottawa, ON
How about trying to stake the caps outwards before installing them on the hubs. With a straight punch, add some punch marks.
 

l008com

Original poster
Member
Feb 19, 2016
919
Massachusetts
So heres the thing. These caps are SO tight as it is, that half the time they get torn up as i'm taping them in and i have to pull them out, trash them, and try another one. So it feels like if there were any harder to install, they'd be basically impossible to install, they are SO tight already. Which is why its so crazy that 3 minutes of slow driving and they're popping off.
 

flyboy2610

Member
Aug 24, 2021
498
Lincoln, Ne.
Not only that, but they were wiggling off after a SUPER short drive at very low speed. I would have had to crank those hub nuts on with a breaker bar to make them get that hot in such a short, slow drive.
Did you see any evidence of metal-to-metal contact on the back of the caps?
 

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