Advice on replacing self-tapping screws with machine screws ...?

[Dire Wolf]

Some helpful soul on RCC suggested replacing self-tapping screws with machine screws. Seems like good advice, so I bought replacements for my spiffy new Axial Wraith kit.

My question is whether I should cut threads into the holes beforehand. I've already got the proper size tap ... to use or not to use?

My guesses are:

Threads make the screw harder to pull out.
Threads make it *easier* for the screw to vibrate out.
Threads make it easier to disassemble and reassemble multiple times.

But I'd appreciate advice from someone who's actually experimented with it.

(I did try to find a post on this topic & failed, - if someone can find a prior thread I'd be curious to see the search terms used)

[whitrzac]

No, just thread the screws in. They will form their own threads.

[high plains drifter]

As stated above... no, don't thread the holes. Allow the new screws to thread themselves. It should be harder to screw in the new screws but that's normal and it should assure you that these will hold strong. Also possibly worth noting... On certain parts, make sure that if the new screws are a bit longer than the self-tappers, that you're not risking going in too far and out the other side. As well, take care tightening slowly and making sure not to strip the plastic.

[Dire Wolf]

Hey, guys - thanks for the advice. It makes sense to let the screws make their own way in on any places I hope I won't ever have to disassemble. The extra grip should ensure they don't back out due to vibration. Cool.



But on the other side - a machine screw won't 'cut' its own threads in a virgin hole. It will push & mush & smoosh & tear the plastic around it. I'm worried that If i back it out & put it back in a few times it could degrade the plastic to the point where it won't hold any more. (oh, man, I could make some naughty puns here, but I'm way too polite.)



So, for those things I expect to dis/assemble (motor mounts, tranny housing ...) I'm leaning towards cutting threads - but I welcome discussion. (like, has anyone tried it that way?)

[Voodoobrew]

i have drilled holes in Petg which is not much different, the plastic gets hot, sticks to the bit, and doesn't want to come out of the hole. I don't think the tap will do much better. If you go slow the shaved plastic probably still wont want to come out of the hole.
Generally I drill the hole, then fire the screw in with a speed bit, this heats up the plastic and molds it to the screw. I would prefer pressure pushing in on the screw, then a lack of material in the hole from tapping.
Its not metal, its more like wood, and you don't generally thread wood.
Is this the best way? No Idea...
If it gets loose blue locktite.
If it gets really lose red locktite.

My suggestion is check your parts tree and do some test holes in that junk.

[celticsun33]

Don't do it either way (tap or force). Stay with the self tappers. The thread pitch on a self tapping screw and a machine screw are not the same. They are not compatible. If you re-tap the hole with a machine screw size it will not hold for long. Think of it like this, Take a wood screw out of a board and replace it with a bolt. Will the bolt fit? yes. Will it hold the boards together If you hit them with a hammer? No. All you will do is eventually pull out any threads there may be holding the screw in, in essence stripping the hole. You strip the hole on a plastic part for your truck there's no fixing it. You're replacing it.
My 2 cent's It's your rig.

[Dire Wolf]

I've been meaning to follow up with my own experience.



I built my kit, and found that for those holes in 'thin' plastic, tapping the hole weakened the joint - so not a good idea. But for deep frame holes, I found that if I tapped two or three threads the machine screw got a better start. Without the threads it spun a bit before biting in & so the first few threads didn't hold well at all.


celticsun33 - the holes do not have threads in them, so whatever screw used cuts its own. Many folks have stated that machine screws hold better & in fact you can buy complete sets of replacement screws for popular kits.



Just my experience, gonna go churn some mud before the snow sets in. (at which point, I'll churn some snow.)

[celticsun33]

Originally Posted by Dire Wolf

I've been meaning to follow up with my own experience.



I built my kit, and found that for those holes in 'thin' plastic, tapping the hole weakened the joint - so not a good idea. But for deep frame holes, I found that if I tapped two or three threads the machine screw got a better start. Without the threads it spun a bit before biting in & so the first few threads didn't hold well at all.


celticsun33 - the holes do not have threads in them, so whatever screw used cuts its own. Many folks have stated that machine screws hold better & in fact you can buy complete sets of replacement screws for popular kits.



Just my experience, gonna go churn some mud before the snow sets in. (at which point, I'll churn some snow.)

Gotcha, I didn't realize he was starting fresh with clean holes. The way the title read I thought he wanted to put machine screws in an already threaded hole. My bad.

[Natedog]

Originally Posted by whitrzac

No, just thread the screws in. They will form their own threads.

Originally Posted by high plains drifter

As stated above... no, don't thread the holes. Allow the new screws to thread themselves. It should be harder to screw in the new screws but that's normal and it should assure you that these will hold strong. Also possibly worth noting... On certain parts, make sure that if the new screws are a bit longer than the self-tappers, that you're not risking going in too far and out the other side. As well, take care tightening slowly and making sure not to strip the plastic.

Edit: new hole or re-used hole doesnt' matter, I screw them in using some pressure on the driver (not power driver, but that can work as long as you're very careful) so that they bite into the plastic instead of slipping. Don't over tighten always finish last bit by hand if using power driver and not too fast.

[mewalsh100]

Machine screws will thread their own way in just fine as others have stated. It's not metal, thread pitch is of zero consequence. The machine screws will hold tighter as well as they have a lot more thread in any give hole versus the self tapper. I have always replaced the black oxide self tappers with stainless machine screws. The benefit is two fold, the fact they hold tighter and the stainless machine screws don't get surface rust like that crap black oxide hardware.