How to go from self tapping to machine thread?

[Juggular 5169]

I wasn't thinking when I was building MF01X. I used self tapping screws for pogo-sticks. 

When I attached proper shocks, it came with these with machine threads.  Now I'm afraid that I'll have to cross thread. Is there a way to do it better?  Like using caulking or white glue or something?  (I'm not looking forward to using superglue...)

 

 

[El Gecko 1631]

10 minutes ago, Juggular said:

(I'm not looking forward to using superglue...)

Can I ask why not? I've been using superglue for years to firm up machine threads in plastic, especially those that are already stripped out or getting close.

[Willy iine 18842]

If that was my car, I'd just leave it with the screws you already used.  

I've had terrible luck with threadlock + plastic in my youth so I recommend not doing that.  

I think superglue will work okay (I've used it many times in the past with success for the same application), but again, I'd just leave it the way it is.

[InsaneJim69 1587]

Over many of my Tamiya cars I have went from self tapping to threaded machine screws with no issues. Just screw the machine screw into the old thread hole. If anything the machine screws are alot stiffer to screw into the self tapping hole in my experience. As long as it's a one time change it should t be an issue at all.

James.

[BuggyGuy 1144]

You could try using Tamiya thread forming tap #54232 to get M3 threads

[InsaneJim69 1587]

Just now, BuggyGuy said:

You could try using Tamiya thread forming tap #54232 to get M3 threads

Or an old m3 turnbuckle in a drill at super slow speed. 👍

James.

[Juggular 5169]

1 hour ago, El Gecko said:

Can I ask why not? I've been using superglue for years to firm up machine threads in plastic, especially those that are already stripped out or getting close.

Would it be okay if I wanted to undo?  

In the past, I've just used machine thread on tapped hole. It held okay, but I am nervous because the hole is already threaded once.  If superglue would strengthen it, I'll use it.  The A-arms being flexible, I don't think I would have any trouble backing it out.  But the upper support arms are more like ABS plastic, and I'm afraid not being able to unscrew.  (though I can't think of any particular reason to unscrew them...)

 

[El Gecko 1631]

52 minutes ago, Juggular said:

Would it be okay if I wanted to undo?  

In the past, I've just used machine thread on tapped hole. It held okay, but I am nervous because the hole is already threaded once.  If superglue would strengthen it, I'll use it.  The A-arms being flexible, I don't think I would have any trouble backing it out.  But the upper support arms are more like ABS plastic, and I'm afraid not being able to unscrew.  (though I can't think of any particular reason to unscrew them...)

 

Yes it should be fine. The superglue seems to bond better to the plastic vs. the shiny slick metal threads of the screw, so generally it will stay in the hole rather than coming out with the screw. The superglue helps fill the voids--the differences between thread types--and gives the machine threads a bit more surface area to bite into. Never had an issue with flex or anything backing out. I use superglue to prevent screws from backing out

The only thing I will note is that for any chassis with shiny ABS, you may get some ghosting or white residue if any glue gets outside the screw hole, so less is more--apply the glue lightly. It would be the most visible on a dark black chassis like the Hornet.

[Juggular 5169]

I think I'll go with superglue!  Thanks! 

 

[djmcnz 755]

Hmm, I'm not convinced Super Glue is the best option although I'll admit to not having used it for this purpose. I would have thought it was either too permanent or too brittle.

I'd use an epoxy putty such as JB Weld (Plastic Weld) which is pliable upon application but then hardens so you can drill/tap it. Because it's an epoxy it forms a polymer bond with the surrounding plastic so it effectively becomes one solid piece of plastic.

You could use it with one of two methods:

1. Fill the hole, let it cure, drill a pilot hole then tap the 3mm hole and insert the screw.
2. Fill the hole, before it cures screw in the machine screw then remove it (forming a threaded hole), after it cures inset the screw.

Super Glue sounds like an unnecessary and potentially inferior (if not convenient) way to address a problem for which a proper solution already exists.

[jonboy1 3736]

I'd agree with @djmcnz, superglue isn't structural, whereas something like JB Weld is. If you mix superglue with baking powder it becomes a bit more like a gap filler, but I still wouldn't trust to the degree I would JB.

On a few restorations I've done where people have spent years screwing in self tappers over and over and completely stripped the hole I've mixed up JB Weld and popped it in a small syringe and squirted it into the hole to fill it completely, let it dry then either drill the hole for a self tapper or tap it for metric. Never had a problem with these lasting.

All that said (yes, it has been said I enjoy the sound of my own typing) if you have just screwed a self tapper in and left it there and now want to put a machine screw in there I don't think you'll have a problem. The issue comes (as hinted just previously) when the self tapper has been removed and reinserted (tapped) over and over. The root diameter of a 3mm self tapper is 2.1mm and the outer diameter is 2.9 with a pitch of 1.1mm. An M3 thread has a root of 2.5, an OD of 3 and a pitch of 0.5mm, so you will only get a cross thread every other turn but it should still be in pretty much full contact.

 

[Big Jon 732]

I've always just taken out the wood screw and installed the machine screw. If the plastics weren't stripped in the first place, you'll be fine. If the threads are a little soft, a drop of medium CA fixes them right up. If it works on cars that get driven hard and worked on frequently, it's good enough for me.

[nowinaminute 3342]

In my experience, there seems to be enough meat left for the machine screw to form new threads successfully as long as the self tapper hasn't stripped at some point. 

 

[Zealot 398]

I have used a thread forming tap in holes that previously had self-tapping screws and replaced them with machine threads successfully, even where the self-tappers were stripped.

Just did some today, in fact, on a Tamiya factory assembled WR-02 gearbox. I disassembled it to install ball races and found 4 out of 5 10mm self-tappers to be stripped upon reassembly, which is quite disappointing in itself for a factory assembled unit.

Anyhow, I ran my tap into the holes, which are well deep enough to accept a longer screw, and replaced them all with 12mm hex drive hardware. All of them took up fine and better than a self -tapper ever would without any glue or other work necessary. Obviously, this may not work in every case, but I think if you're careful, it should be fine unless the hole is already in very bad shape.

[silvertriple 4192]

Well... I read all the answers since i'm asking myself the same kind of things...

Metric in plastic could work, but if you have to unmount it later on, I doubt it will last. The time you screw in the machine threaded screw, it heats the plastic... See what some mentioned on Hotshot chassis with the machine threaded screws, when they unmount them. Ultimately, it doesn't look good on the long term.

That being said, if you really want to go for machine thread and having something durable, I would consider brass M3 inserts. You would need to drill the original hole to the proper diameter, and insert the M3 insert heated in the drilled hole. It won't move after that. One point to consider, it is not necessarily valid for all the parts : you need to have some plastic around to do this, it's not the case everywhere... But when this is possible, this would be the most durable solution you can find...

[foz75 95]

I had that problem with my hotshot. Am still undecided whether to build the new chassis with self tappers, or tap the thread and use the machine screws... Am starting to lean more towards the second option. Once built I won't be running it loads or racing so shouldn't have the need to open the chassis very often.

[silvertriple 4192]

1 hour ago, foz75 said:

I had that problem with my hotshot. Am still undecided whether to build the new chassis with self tappers, or tap the thread and use the machine screws... Am starting to lean more towards the second option. Once built I won't be running it loads or racing so shouldn't have the need to open the chassis very often.

That's the thing, normally, once built properly, you are not generally opening the car very often... But sometimes, it happens you need to open it. On some car, changing the servo saver is a nightmare. On other it is accessing something else... And sometimes it is during the first run you notice something... Ok, le me correct this: it is always during the first run that the screw you did not tighten correctly or you did not put loctite (for metal on metal) which will remind you it's existence :-D (on My Super Hotshot, that was the grub screw on the motor pinion, for example, and I was lucky I understood right away what was happening - Should I have forced on the trigger, the pinion was falling in the gearbox when I removed the motor, and that was all the rear gearbox to unmount at the track...)

[foz75 95]

On 9/11/2021 at 1:59 AM, BuggyGuy said:

You could try using Tamiya thread forming tap #54232 to get M3 threads

Is there any difference between the tamiya thread tap and a generic steel thread tap set? Just wondering if there's anything that makes the cost difference worth it? more accurate? better quality? ease of use? 

[Willy iine 18842]

24 minutes ago, foz75 said:

Is there any difference between the tamiya thread tap and a generic steel thread tap set? Just wondering if there's anything that makes the cost difference worth it? more accurate? better quality? ease of use? 

If you get a quality one there is really no difference.  Tamiya rebadges their tools in some (most?) cases so if you're a tool guy with quality tools, you probably already have the same quality if not better than standard Tamiya tools.  
 

 

[alvinlwh 7687]

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[alvinlwh 7687]

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[Nicadraus 6748]

The superglue advice is certainly a joke. Unless you have no plans of cleaning, restoring, upgrading, or running your car on dirt. Don't use it.

I've changed all my self tappers to machine threads with no problems. Old and new kits. I strip my cars' hard to reach parts every after dirty runs and clean them thoroughly before I set them on display again. I have no problems with the threads loosening. There's a proper way to screws them back (and vice versa) to not over tighten and avoid loose threads. Super glue is not the answer.

[SupraChrgd82 1434]

I suggest avoiding cyanoacrylate glue.  It is hard and brittle when it cures, not to mention leaves a hazy residue that is quite ugly.  

If you have already stripped or compromised the holes, it would be wise to replace the plastic component, however Tamiya does often use ABS plastic for chassis components. It is somewhat reparable with black ABS cement (found at a hardware store for black sewer pipe).

For good holes without compromised threads, a simple switch from self-tapping to machine threads is a seamless transition.  If there is a deep concern about threading, a tapered tap can be used to start a new fine thread (only partially inserted so as not to remove too much material deep in the hole). 

Have a look at some of Ampro's videos on youtube.  Alberto has a unique method for inserting screws in fresh plastics.

[rich_f 255]

2 hours ago, foz75 said:

Is there any difference between the tamiya thread tap and a generic steel thread tap set? Just wondering if there's anything that makes the cost difference worth it? more accurate? better quality? ease of use? 

Assuming you are talking about a normal thread cutting tap set, then yes, there is a difference. 

The tamiya tool is a thread forming tap and not a thread cutting tap. The latter type cuts material away and the former doesn't - it pushes material out of the way to form the thread. 

Which is more appropriate for the different types of plastic found in an rc car? I don't know, but tamiya don't appear to sell a cutting tap. 

[El Gecko 1631]

39 minutes ago, Nicadraus said:

The superglue advice is certainly a joke. Unless you have no plans of cleaning, restoring, upgrading, or running your car on dirt. Don't use it.

I've changed all my self tappers to machine threads with no problems. Old and new kits. I strip my cars' hard to reach parts every after dirty runs and clean them thoroughly before I set them on display again. I have no problems with the threads loosening. There's a proper way to screws them back (and vice versa) to not over tighten and avoid loose threads. Super glue is not the answer.

Oh HI! I'm so glad you decided to drop in and share your "expert" knowledge with us!

In fact, I do all of those things you mentioned above and have never had a problem with super glue.

I know you think I'm just some moron bush mechanic but you need to get off your high horse.

Super glue IS the answer if you want threadlock that DOESN'T destroy plastic.

Now explain to the class why we shouldn't use it.