Thread: URGENT! before I drill my firewall need advice :)

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Originally Posted by Itoldyouso

Everybody has been wondering where you got to.

You will be fine, Faith. The backer is a good idea, and I would drill the hole, then use a countersink bit to lightly dress the holes on both sides, to keep the holes from wanting to stress crack. Don't countersink deeply, just bevel it ever so slightly. We did this in the marine industry to keep hardware from stress cracking the glass.

If you were asking about a master cylinder, that is another matter, but a coil and the cover should be no problem at all.


BTW, people have been wanting to see your new bias tires. Are they on yet?


Don

Ok so this is the part where I come across as an idiot but what's countersinking?

Countersinking is just tapering the hole so a flush-headed, tapered bolt will work. Like a valve seat, kinda
Don't forget to run a ground wire to the coil, or to the inside plate.

Here's a couple of examples in some tools I made.

The first, a very deep countersink in 1" OD (Outside Diameter) aluminum into a 3/8" ID (Inside Diameter) hole.

The second, a medium deep countersink in the holes between two small holes in a wheel drill jig I made.

The last in the small (1/4-20) threaded holes between lugnuts in an aluminum wheel.


To accomplish these you'll need a "Countersink" (that's the right name).
They come in ether 87.5 degree angle or 90 degree angle.
The 87.5 degree angle is the best choice and usually what's available.

For obvious reasons, you'll need one with an OD larger than the hole.

To use one, mount it in your drill motor, hold the countersink in the hole and hit the trigger.
With fiberglass you don't want the countersink spinning before you contact it.

You'll just need to hit the drill motor trigger for a half second or less.
To much and you'll have too large a countersink.

One piece of advice when drilling fiberglass - new or painted, cured paint for sure - cover the hole area with a single layer of masking tape.
Mark the masking tape where you're going to drill.
A moderately sharp pencil point works well.
Put the pilot drill (1/8" diameter or so) onto the mark with the drill motor not running, hit the trigger and drill.
Next drill for the correct size and the pilot hole will guide your drill.
Use caution when getting close to the other side so as not to break the edges of the hole and make a ragged hole.

All that said, I wouldn't bother about countersinking fiberglass.
Drill it and use standard flat washers to spread the load.
The thinner stainless flat washers work well.

Even better are "Fender Washers".
Called such due to their large OD as compared to the hole ID.
These spread the load well and can be improved by laying the Fender Washer on a piece of wood and hitting the hole with the round end of a small-medium ball peen hammer.
This imparts a very moderate, almost flat cone shape to the washer and gets the loaded area out away from the hole in the fiberglass.

Never had any problems doing fiberglass this way and that's talking about over 200 holes in a fiberglass sailboat as well as several holes in my Wescott body 32.

The backup plate you plan to use sounds like the best way to go to hang a coil on a narrow bracket.

One thing I meant to add was, don't try to countersink with a larger drill.
A true Countersink is designed to run true.
You'll find them with 3-4 cutting edges - most common - as well as with a single cutting edge.
Get the single cutting edge version because it's the smoothest running one.

Countersinking with a large drill will have the drill chattering and in some cases walk right off the hole you're attempting to countersink which creates damage as it goes.
The chattering also leads to a ragged countersink in most cases.


However, and this is just for the heck of it.
You can countersink for common allen head bolts with a regular drill when you're making auminum brackets and the like.
Shown in the pic below.

The drill used to countersink is just a few thousandths larger than the head of the allen bolt.
Most times I go deep enough so the allen head is almost flush with the surface of the aluminum.
Other times, part way.
Makes for a nice finished product.

You may be wondering how it works out since the inner countersink has an angled surface, but what happens is, a few tighten/loosen cycles will create a flat ledge for the allen bolt and it will go no deeper.

I like this look and would rather do it this way than use FH (FlatHead) allen bolts sunk flush with the surface.