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Hagerty
Hagerty Employee

Piston Slap: Poppin' caps on a disc brake conversion?

I put disc brakes on the front of my '56 Nomad a while back, and the passenger side grease cap keeps popping off. What would cause that? Sajeev answers: That's a good question, but did you mean the bleeder screw's cap on the brake caliper?
https://www.hagerty.com/media/advice/piston-slap/piston-slap-poppin-caps-on-a-disc-brake-conversion/
6 REPLIES 6
audiocage
Advanced Driver

Sometimes if there's an air bubble that gets caught inside the rubber cap, when the caliper heats up the air expands and pops the cap off (especially if the cap and bleeder are wet with slippery brake fluid.) Clean the bleeder and cap with brake cleaner, and try to squeeze out any trapped air when you put the cap on. IMO, YMMV, etc.
TalkingPie
Pit Crew

I have zero experience with Nomads or even anything of similar vintage, but I took the part in question to be the grease cap on the wheel hub. If that's the case, aside from making sure the cap is fitting securely onto the hub - is it being tapped on sufficiently, is everything appropriately round and fitting together properly, etc.? - I'd ask myself if the passenger side is heating up inordinately compared to the driver's side. Presumably something is different from the left to the right, if the left is doing what you want it to. I'd try to find that difference.
Scoutdude
Intermediate Driver

Yeah I read it as the cap to access the wheel bearings. In which case you want to make the cap not round so it jams in the round hole in the hub.
Sajeev
Community Manager

Thanks @TalkingPie and @Scoutdude your answers are great, and could very well be what the OP needed to read! 

Swamibob
Technician

Hey Sajeev;

I believe the OP is speaking of the wheel bearing caps.
If it's just one side that's coming off; my first suggestion would be to switch sides with the other bearing cap. If that doesn't fix the problem, it might be time to get a new cap from a better manufacturer like Dorman. I have had trouble with some of the bearing caps that come in disc brake kits previously.
I suppose, a guy could use some sort of loctite or krazy glue and glue the cap in place.
DUB6
Specialist

If it's indeed the wheel bearing grease cap, it's a very rare condition, IMHO - as it's typically easier to break into a bank vault than to remove one of those caps.  So, my question is, does this cap just push into the hub, or are you having to tap it in (generally requiring a large socket or section of pipe that fits over the cap and transfers your blows to the flange).  If you're able to put the cap in without blows, I'd suggest that it's undersized for your hub - could just be a mis-pick from the parts bin at the kit manufacturer.  I agree with Swamibob to swap right for left to see about fitment, and you are likely going to find that you just need a replacement cap that is sized correctly!