Scratchie

I had to use a dowel to fill in a bad hole in a wing mounting block. After tapping the hole and applying thin CA I wasn’t satisfied with the quality of the threads and would like to beef them up a bit. I thought of several ways to do this such as:

Apply a thin coat of epoxy to the threads and re-tap
Apply a small amount of wood filler, let dry and then soak with thin CA and re-tap
Mix fine saw dust and CA or epoxy into a paste and then re-tap

It doesn’t seem necessary to drill out the block and put a new dowel in, ideas?

Gray Beard

My Feedback: (-1)

When I drill the hole I make it undersized from what the tap calls for. I then CA the hole and let it dry then tap and CA again. Sometimes I run the tap through again. I have metal bolts that I run in when finished to clean up the threads. Never needed anything more.

MinnFlyer

My Feedback: (4)

The threads are probably just fine the way they are. No need to go crazy with them. But if you must, I would add some epoxy to the hole, coat a nylon screw with Vaseline and insert it into the threads - Then, keep an eye on it (This would be one of the few places I would use 5-minute epoxy just so you don't have to wait too long)

Once it starts to set, turn the screw a little each way, let it sit, then turn the screw again. Repeat until the epoxy is set well and remove the screw

Scratchie

Minn flyer, sounds good - thanks.

cpeisher

My Feedback: (13)

There really shouldnt be a need to use the epoxy. Becauseof the way CA penetrates the surrounding wood, it's actually better suited to fthe thread forming process than epoxy. The process of wicking the CA throughout the surrounding wood hardens the entire area and distributes the loads that would otherwise be concentrated on the load bearing face of the internal thread. This basically gives the wood a more uniform property like threads in metal. With the epoxy your gonig to get a "skin" around the tapped hole. On top of the wood, in the threads, and below the hole to some extent will be conected by a skin of epoxy, but that epoxy is not going to penetrate into the wood very effectively. There's not much use in using the CA twice, either.Once it's been used on an area, it really does no good to put more CA in after the first application has cured. Just tap,CA and then tap again...

I honestly think that putting vaseline on a screw and using it to help cure the threads in a hole most like wont help much either...seems like the process of installing the screw with vaseline on it will likely cause the vaseline to mix with the epoxy and leave pockets of vaseline within the epoxy. That's just an opinion, and i've never tested and if MinnFlyer says it, it's probably gospel...i do like the idea of tapping the hole undersize. The drills that come with most tap sets are really geared for the thread tolerances in metal applications, not wood, so having a tighter tolerance, and more wood around the thread profile will help beef up the threads almost as much as the CA.

FentonFlyer

My Feedback: (11)

Anyone ever tried to Heli-coil wood?

Rodney

For old worn out threads, coating with a slow epoxy, let it cure then retap. Works very well.

tcrowson

Heli-coil's work in wood. I have used 2-56 & 4-40 for cowl & canopy attachment, 1/4-20 for wing attachment. After setting the coil harden with ca then clean the threads with a tap.

Tas

vicman

My Feedback: (10)

I've had the mount break out and used epoxy and cab o sil to pot around a steel screw with some lube on it. It worked great.

Gray Beard

My Feedback: (-1)

There are threaded brass inserts designed to go into the wood. You just drill the hole to size then with a screw driver and a little epoxy on the outside threads you just screw them in and let the epoxy set up. Most any hobby shop has them in several sizes. Some don't have the slot for the screw driver so you djust install a bolt and use that to install them.

summerwind

you can also add another piece of ply behind the hole so you have a deeper thread bed. if i am understandiung this right, you have a 1/4" ply plate that was plugged, drilled and retapped right? so add 1/8-1/4 ply behind the hole and you are assured of good threading for the bolt.

cpeisher

My Feedback: (13)

The other thing to consider using is a threaded insert, or more commonly called a T-nut for wood. Normally, you would tap these in, but if you drill the hole loose enough and put the t-nut in from the backside, with the flange on the backside, when you tighten the screw, it will pull the barbs into the wood and seat itself. They come in boxes of 100 for sizes down to 4-40 form mcmaster.com. Just search t-nuts or threaded inserts.

cpeisher

My Feedback: (13)

by "tap" i meant with a hammer, and they are about $10 for a pack of 100 4-40's.

Gray Beard

My Feedback: (-1)

Also called a blind nut.

AERORICH73

I believe the suggestions of metal thread inserts, and blind nuts are a good answer to the question of having thread strength. Helicoil inserts do work as long as the dowel insert used to replace the oversized drilled hole. Keep in mind that the helicoil continues to try expanding with could crack out the installed dowel insert if the wall thickness is thin. I have been lucky in tapping threads in hard wood blocks, and using thin CA to harden the threads. A new tap does cut better threads.

aerorich73

Crash Campbell

Hi Scratchie,

The replies thus far demonstrate that there are many, many ways to "skin a cat". My method when faced with your situation is to epoxy a small piece of 4mm aluminium plate to the block, use the existing hole to centre the drill bit and then tap the alloy. Works for me.

Cheers,

Colin