Pre-drilling holes in fiberglass for screws

[Snotter]

For the past few weeks I've been working on replacing the teak trim on my boat that covers the hull-to-deck joint at the stern. I thought the hard part would be fabricating the curved pieces (bent in 2 dimensions). As it turned out, fabrication was a piece of cake.

Last week I tried mounting them. It was a disaster. I broke screw after screw trying to drive them into the deck. When I went to the next size bit in my index the screws would strip. I finally gave up and and spent last night epoxying in bungs in my trim piece in the holes that now have a broken drill bit or screw underneath them.

What do ya'll recommend for a drill bit size for #12 ss self tappers? Is there a lubcricant I should use on the screws when I try to drive them? If so, will the lubricant interfere with the boatlife caulk I am using to bed the teak to the glass?

This has been a humbling experience.

 

[Tim R.]

You should be using wood screws. These taper and have no thread up near the head which allows you to snug the wood down without having the thread bite into the wood. Use a good quality tapered drill bit that is matched for the #12 screw. Use a collar to get the correct depth and drill the holes through the wood and FG at the same time. This way you get a tapered hole that is the correct depth and matches the taper of the wood screw. The screw should get tighter as it screws into the tapered hole. The caulk should be lubricant enough.

It also could be the quality of the SS that the screw is made from. Make sure they are 316 and not 302.

 

[Ross]

Sometimes I find a piece of wood that is so hard that I can't drive screws into it with a proper size pilot hole. In those cases I set to work with a grinder and grind a cutting lip on the point of a screw the size I will be using and use that as a tap ot make the threads. The screw that I use has to be hardened and just of the some diameter and thread pitch. Self drilling sheet metal screws work if you can find the right size.

 

[Bill Roosa]

Screws and fiberglass

I run into this all the time when I try to fasten stuff back into fiberglass using the holes that they where previously attached to.
I have given up re-drilling to the next size..... stuff.

I have had great success using the plastic inserts (saw it at the boat show a few years back) for putting screws in concrete or brick walls. Drill for the insert, install the insert, install the part, enjoy.

 

[Ross]

I run into this all the time when I try to fasten stuff back into fiberglass using the holes that they where previously attached to.
I have given up re-drilling to the next size..... stuff.

I have had great success using the plastic inserts (saw it at the boat show a few years back) for putting screws in concrete or brick walls. Drill for the insert, install the insert, install the part, enjoy.

Good idea. One of those "why didn't I think of that".

 

[sailingdog]

I'd recommend silicon bronze screws over stainless steel, since it is a better material for such an application. Also, I'm a big fan of through-bolting where ever possible, rather than using screws. Fiberglass laminate is a lousy material for screws, as it is far to brittle to hold screws really well.

 

[Ray T]

rere drilling

As you can see there is more then one way to skin a cat. All of the suggestions have merit I favor the through hole method if you can get to the underside. In second place I would use SS sheet metal screws and size the drill so it was somewhat larger then the core of the screw, I eyeball this. If you feel you need a lubricant you might try a bar of soap, I moisten the screw and rub it on the soap filling the threads. My question is how are you breaking so many drill bits? Ray T

 

[Ross]

As you can see there is more then one way to skin a cat. All of the suggestions have merit I favor the through hole method if you can get to the underside. In second place I would use SS sheet metal screws and size the drill so it was somewhat larger then the core of the screw, I eyeball this. If you feel you need a lubricant you might try a bar of soap, I moisten the screw and rub it on the soap filling the threads. My question is how are you breaking so many drill bits? Ray T

I have found that brush on teflon pipe dope is an excellant screw lubricant that also helps to keep water from the hole. I also wondered about the broken drill bits.

 

[kendall]

If you decide on using screws, use a drill bit that's the same size as the root on the screw, or just barely larger, and make sure that the screw is long enough to get the straight section into the fiberglass.

Don't know how often I'll see something loose and find that someone has used a 3/4 inch screw to fasten 1/2 inch material, leaving just the tapered tip of the screw to do any holding. If the screw loosens up at all it means that there's less thread engagement so far more chance of tearing out the threads in the hole. With the straight section in, you have more time to notice it's loose before it starts eating the threads.

Tapered threads work on things like wood or the soft plastics because there's enough compressabilty in the material that it will remain snug on the screw for a while, fiberglass is to hard so will not hold the same way.

Sorry if it seems basic, but I see it often enough to think people just don't think of it.

Ken.

 

[sailaway78]

Have had this same problem

I have had this same problem, more than once. The idea of the plastic inserts sounds very interesting. I see no reason why this wouldn't work. Particularly when installing trim or some other not structrual part. Of course, the best alternative is still through bolting, when you can get to the back side. May have to give the inserts a try on some non critical project.

 

[sailingdog]

The only real problem I see with using inserts is that they're going to be harder to seal properly.

 

[Snotter]

Some good stuff here.

I try to be specific when posting but it seems I always think of new stuff immediately after hitting the submit button.

The teak is 1/2 thick. IMHO, this is not thick enough for counter boring and using bungs. Since the screws will be visible (flush with the surface), I wanted to use bronze screws, as someone already suggested. (I like the look of bronze in teak.) However, I couldn't find any bronze self tappers. Also, the reason I didn't use wood screws is because the non-threaded shank exceeds the teak (at least for the screws that I looked at in Home Depot). Besides, I already got one 6" trim piece mounted using SS. Hind-sight is 20/20. I am not going to beat myself up over the choice I made.

The reason I can't use the caulk as a lubricant is because I want to dry-mount the trim first. This is prudent because of the amount of curvature and because I want to use the trim as a template for masking with tape. The masking tape is for keeping the boatlife goop under control. I am in the habbit of using too much caulk rather than too little. It squeezes out everywhere and can make a mess.

I like the suggestion about brushable teflon. I've never seen anything like that. When I go to Home Depot tonight to find a letter bit that is between the size of the two drill bits I already tried, I will look to see if they sell the stuff.

I got to admit that I was busted on the freudian slip about the bits. I accidently drilled into two 1/4-20 bolt heads. I broke the bits trying to drill out busted screws that touched the bolts. Don't be too hard on me, I was on the hook and was ticked that I couldn't be off exploring with my two sons who took the dinghy. My work was a bit more hasty and sloppy then usual. The next time I will work at the slip with a bit more finesse.

Golly, I really like the idea of through-bolting. What a great suggestion. If I go that route I would have to wait for a weekend that I can get help. I suspect that I will be too impatient.

 

[Maine Sail]

(I like the look of bronze in teak.) However, I couldn't find any bronze self tappers. Also, the reason I didn't use wood screws is because the non-threaded shank exceeds the teak (at least for the screws that I looked at in Home Depot).

I think you'll find that the screws at Home Depot are yellow brass, quite soft actually, and not bronze. Bronze screws will need to be ordered online if you don't have a good local chandlery..

I like the suggestion about brushable teflon. I've never seen anything like that. When I go to Home Depot tonight to find a letter bit that is between the size of the two drill bits I already tried, I will look to see if they sell the stuff.

It's called Pipe Dope. I like and use Real Tuff by Hercules Chemical Company and HD has it.. It is not a replacement for bedding compound though!

 

[Ross]

When installing brass hinges with brass screws on oak, I always fasten first with steel screws the same size and the replace them with the brass screws one at a time.

 

[sailingdog]

You should never use brass fasteners on a boat's exterior, particularly in a marine environment. This is especially true for any wood work that may be used as a handhold or will be fastened down under tension. Brass tends to de-zincify when exposed to salt water, less so with fresh water, and loses most of its strength in the process—bronze does not.

 

[Pink Flamingo]

Another way for non critical items is to tap threads in the fiberglass and use machine screws.Add some caulk or putty and drive the screws.

 

[sailingdog]

IMHO, fiberglass is a lousy material to tap threads into—it is too brittle.

 

[Snotter]

I went to Lowes last night and got a drill bit

that is between the two sizes I was using. I couldn't find the brand of pipe-dope mentioned previously. I wound up bying a pipe lubricant that looked kinda greasy. I gues I will have to use alot of acetone to clean the area before bedding the trim.

Concerning the plastic inserts, wouldn't they stand proud, preventing the trim from resting on the glass surface?

 

[Ross]

that is between the two sizes I was using. I couldn't find the brand of pipe-dope mentioned previously. I wound up bying a pipe lubricant that looked kinda greasy. I gues I will have to use alot of acetone to clean the area before bedding the trim.

Concerning the plastic inserts, wouldn't they stand proud, preventing the trim from resting on the glass surface?

It doesn't require an abundance of lubricant on the threads to serve the purpose. You're not sealing a 2 inch iron pipe just a little screw so just use a small touch of pipe dope and work it into the threads.

 

[bffatcat]

Fastening to fiberglass

I would thru drill with a sharp carbon tipped bit then thru bolt with 316 stainless
and seal holes with 4200. This should give a good strong hold and hopefully no problems down the road