NMO Mount Hole not perfect

[RadioCheck]

Hey everybody. This is my first post and I'm getting off to a bad start but I'm hoping you guys can help me anyway.

In August I drilled a NMO Mount into my pickup almost dead center of the back portion of the roof.

I used a regular bit, not a "NMO hole saw"

I stupidly put the wrong end through the hole first, and could not fit the mount through.

Soooooo I filed the hole just a tad on one side so I can fit it through,

Today when I swapped my quarterwave 2m antenna out for a half wave (which I do vice-versa very often) the mount came out with the antenna.

Thankfully I was able to grab the mount before it fell into the headliner of my truck. I noticed that the washer was broken. I quickly reinstalled everything, minus the washer. Reeeeeal tight.

What options am I looking at here? Could I go to a body shop and have somebody "repair" the hole, so I can start over again?

Pic related... it's what the hole looks like. The washers break when I tighten down the nut, because they aren't 100% flush with the roof.

 

[W2NJS]

You might be able to make the "bad" hole work if you orient the "grab fingers" on the underside of the mount so that they grab the round part, not the elongated part, of the hole you cut. Take a careful look at the mount and you'll see what I mean. Even if this works you'll still have a possible water leak problem when you get the mount installed.

The best way, by far, to cut the 3/4 inch hole is with a Greenlee punch, but you need good access to the underside of the roof/deck to use this device.

 

[davidgcet]

how do you mean you put the wrong end in first? you feed the coax from the top going down, when you get to bushing it just pushes under one side then the other will drop in. the threaded portion should not fit thru the hole.

you can either bondo and repaint the existing hole or you can install a simple hole plug with a bead of silicone under it. then move forward an inch or two and drill a correct 3/4" hole.

 

[RadioCheck]

You might be able to make the "bad" hole work if you orient the "grab fingers" on the underside of the mount so that they grab the round part, not the elongated part, of the hole you cut. Take a careful look at the mount and you'll see what I mean. Even if this works you'll still have a possible water leak problem when you get the mount installed.

The best way, by far, to cut the 3/4 inch hole is with a Greenlee punch, but you need good access to the underside of the roof/deck to use this device.

That's how I'm doing it now. However it's hard to tell if the mount is completely centered on the hole. When the mount is being screwed together, it slides around.

how do you mean you put the wrong end in first? you feed the coax from the top going down, when you get to bushing it just pushes under one side then the other will drop in. the threaded portion should not fit thru the hole.

you can either bondo and repaint the existing hole or you can install a simple hole plug with a bead of silicone under it. then move forward an inch or two and drill a correct 3/4" hole.

I installed the radio first, and wired everything up from there. When time came to put the mount through the hole, I figured out it was backwards. I just decided to make the hole big enough to get the mount through the hole to the outside, so I can then screw it together.

 

[SCPD]

you could go to a electrical parts store and purchase what is called reducing washers, you can get any size, but for your app you would want a 1" to 3/4" reducing washer, we use the all the time, you use two of them, one on each side, the are dimpled so that they center themselves in the hole you are using them in, hopefully that and a little silicon would resolve your problem,

 

[kb0nly]

The reducing washers are a good idea...

Also if nothing else if you got enough room between the bottom side of the roof and the headliner get a NMO mount that has a N connector instead of coax on it. They call them a thick body mount and you can get that to stay in there. It drops in from the top, all one piece, then you slide over a washer and a lock washer and then a big nut, tighten it down, and use a right angle N connector on your coax to attach to the mount. I have fixed a few bad holes made by others this way. You can also go to a good plumbing store and ask for a flat rubber washer with a 3/4" hole, they are usually about 3/8" wider than the hole on each side, put that on the bottom side of the thick body mount for some extra "seal" on the top side of the roof. If your really worried about leaks put a rubber washer on both sides of the roof.

Thats how i fix em!

Oh and for future reference if you ever need to do one like this again, when i have to install one from the bottom side up with coax already fed up to that point i use a 3/8" NMO hole mount, gives you a lot more area for the mount to hold onto because of the smaller hole that it sits in. In my opinion they are better than a 3/4" hole mount, just have to get a good quality one. The cheaper ones are brass and they aren't very strong, i have broken a few while tightening. Get a good plated one that is stainless steel.

 

[SCPD]

Reducing washers

I'm thinking that using a reducing washer is going to make the thickness from the top washer to the bottom of the sheet metal too thick to get the NMO on. If it was me, I would go to the Lowes, get a rubber plug and plug the original hole. Put a little RTV around the inside of the rubber plug, and put that hole out of service.

Now pop another hole with (like someone else said,) a Greenlee 3/4" hole punch.
Using a hole punch is way easier, and has a much cleaner cut than drilling a 3/4" hole, even if you are using a stepper drill bit.
Then (again like someone else already said,) feed all the coax thru the hole from the outside, then mount the base.

I'm assuming, since you already put one hole in it, that another hole a couple inches away is not going to be too big a deal for you.

 

[fineshot1]

MOTOROLA part number = R 05-80379B46 MMA.

Flexible Rubber Hole Plug - Will fit 3/4" and 7/8" holes.

Not sure if that part number is still valid - its a few years old.

They are sold on ebay also - see link below, and I think Tessco sells them.

eBay.ph: Motorola NMO Rubber Antenna Hole Plug Set Of 3 Lot New (item 310271753549 end time Nov 24, 2010 03:00:13 PHT)

 

[BonziBuddy]

you could go to a electrical parts store and purchase what is called reducing washers, you can get any size, but for your app you would want a 1" to 3/4" reducing washer, we use the all the time, you use two of them, one on each side, the are dimpled so that they center themselves in the hole you are using them in, hopefully that and a little silicon would resolve your problem,

You could, but then the mount could possibly become loose over time. With a normal NMO hole, the bottom grabs the roof of the vehicle. If you put a reducing washer between the grabbers and the roof, no matter how tight you make it, the mount could spin in place when you go to tighten it.

 

[Dewey]

{SNIP}
Oh and for future reference if you ever need to do one like this again, when i have to install one from the bottom side up with coax already fed up to that point i use a 3/8" NMO hole mount, gives you a lot more area for the mount to hold onto because of the smaller hole that it sits in. In my opinion they are better than a 3/4" hole mount, just have to get a good quality one. The cheaper ones are brass and they aren't very strong, i have broken a few while tightening. Get a good plated one that is stainless steel.

A definate +1 on that. I know this will not help with your current problem, but it is always good to "file back" for the future. This is how I do my trunk mounts, and when I have gotten roof mounts done like on my current SUV (I'm always scared to drill into the roof), I will pop the inside dome light first and see if it can be done that way.

Dewey