Possible cure for stray steering knuckle screws?

[nowinaminute 2815]

I'm sure we've all experience unwanted departures of those step screws from the knuckles at one time or another. 🤬

Whether a previous owner stripped the threads or you drive in dusty conditions a lot or they just decided to exit the vehicle because they felt like it. And sometimes the screw falling out can also lead to missing dog bones etc too.

It happens a lot in vehicles like TL-01, WR01/WT01/WR02/GF01 etc

Some people pop nail varnish in in the threads, some people use super glue but it can prove to be irreversible.

Alloy knuckles are a good fix because you can use threadlock but they cost a lot more. Adding them to an entire fleet of vehicles can soon add up!

Well, I think I might have found a cheap solution! Thermal threaded inserts! You see people use these a lot in 3d printing and similar inserts are often used in parts for electronics such as laptops etc.

All you have to do is drill the hole out slightly, put in the threaded insert (ones designed for thermal insertion are usually tapered to help) and then introduce a soldering iron. Once the insert is hot enough it will gently sink into the plastic. All you have to do is stop when it's at the necessary depth. You now have a metal thread inside the knuckle and you can use blue threadlock to make sure the step screw stays put but is still removable when necessary!

From various tests I've seen, the inserts are very strong and often exceed the performance of a screw directly in the plastic plist of course they can be threadlocked.

I'll have to try it out a bit more before I announce it a complete success but it seems very promising. It could save a lot of money and inconvenience for Tamiya users and for some old/rare/obscure models it could be a life saver. I once had a Nikko Dictator out of action for almost a year before I couldn't find another screw because parts are so rare. Nikko actually addressed the problem in later models by introducing parts purely to ensure the screws can fall put but for those oldest models, this mod could mean I can run them care free without fear of a screw getting loose and rending the car unusable for god knows how long!

[nowinaminute 2815]

I found this UK supplier who has a good price on a type that is both well regarded for quality and is quite compact so doesn't require as large a hole drilling out as some types which is preferable to removing a lot of material.

https://www.ebay.co.uk/itm/192308448492

You can get ones that are narrower still but they are intended for placing into moulds rather than being thermally inserted so they are not tapered. I don't know how that would affect insertion using heat.

The linked ones are ones with the smallest wall thickness I could find that still have a tapered end.

I've taken a few vehicles out for spirited runs so far with no issues yet. 

[isomer1 175]

It's a cool technique. I'd hoped to use it between the halves of the GF-01 chassis but there just isn't enough plastic. Using it on the steering knuckles is a clever idea, usually have a lot of plastic there for strength.

[nowinaminute 2815]

23 hours ago, isomer1 said:

It's a cool technique. I'd hoped to use it between the halves of the GF-01 chassis but there just isn't enough plastic. Using it on the steering knuckles is a clever idea, usually have a lot of plastic there for strength.

Yes the amount of material can be a limiting factor for sure. These knuckles were inside of the minimum stated in the spec sheets for the inserts though so I went for it.

It would definitely be nice to bypass the need to force tapping screws in into holes all the time, we all know how that tends to work out in a decade or 2 when the plastic becomes brittle, those areas of tension are always the first to go.

I'll have to keep an eye out for other possible places to use it. 

Another thing I've seen that can reduce stresses from screws is on some vehicles where it's possible to drill right through the screw "tunnel" or "seat" and out the other side and then use a tapping screw and nyloc nut.

I first noticed this on the Traxxas Cat which is a variation of the Nikko F10 buggy but where the Nikko just has tapping screws holding the gearbox together, on the traxxas they have machine screws going right out the other end of the case with a nut on, that way you don't have that horrible outwards pressure from the screw in the plastic, just a slight pinching instead.



Those areas are one of the first to go on Nikko F10 buggies and similar models like the Radio Shack Golden Arrow, I've even had new old stock spare transmissions that have cracked around the screws while still in the packet. As soon as I saw the traxxas, I went and drilled out all my Nikkos and added machine screws and nuts! 

It's not applicable to every RC and again, it depends on the amount of plastic "meat" available among other things although it needs less than you need to install inserts.

[Juls1 1661]

This works if you don’t loctite, because if you loctite it and it’s bond is stronger than the bond of the insert in the plastic then the insert spins in the plastic. Of course this depends on the plastic and the ability of the insert to grab. 
 

generally I just put a dob of medium super glue on the thread before it goes into plastic. Only comes out when I want it to then. But... you gotta be careful doing knuckles as you can glue the pin in the C hub accidentally if you slip.... can’t win.. 

[whahooo 465]

I know the Problem from my Wild Dagger.

But the Stepscrews in my Kits are all selftapering Screws. So i have to find a good Source of different maschine stepscrews.

[nowinaminute 2815]

50 minutes ago, Juls1 said:

This works if you don’t loctite, because if you loctite it and it’s bond is stronger than the bond of the insert in the plastic then the insert spins in the plastic. Of course this depends on the plastic and the ability of the insert to grab. 

I was worried about that too but it seems to be ok with blue threadlock. I suppose it makes sense, I've taken apart laptops plenty of times that have used inserts and blue threadlock. I think the red stuff would be a recipie for disaster though.

I saw some videos on YouTube too with m4 and m5 inserts and the bolts snapped before the inserts broke free. I expect the plastic would give way on the knuckle before that happened (the plastic parts in the test were chunkier) but I'm confident there's enough strength there for a little dab of threadlock.

[waterbok 1282]

I prefer Helicoils for fixing and reinforcing holes

You can get the insert cheap from china and I prefer it over the melting bit. its easier imo to have the thread sitting in the right angle which for the steering knuckles is important to prevent the stepscrews causing to much friction

M3 set

 

M4 set

 

[Juls1 1661]

If the knuckles plastic thread hole is already stripped I see no reason why this isn’t the perfect part recovery option. Particularly on rare hard to get bits. 

[whahooo 465]

I have the problem that the selftapping stepscrew that holds the upper trailingarm in my wild Dagger loosens.

To prevent that i put a drop of Revell glue in that hole and greased the step with some Geargrease.

Was this a good idea? I Hope i didn't destroyed the Plastic that way....

If not i will try that Insert techniuqe.

[nowinaminute 2815]

1 hour ago, whahooo said:

I have the problem that the selftapping stepscrew that holds the upper trailingarm in my wild Dagger loosens.

To prevent that i put a drop of Revell glue in that hole and greased the step with some Geargrease.

Was this a good idea? I Hope i didn't destroyed the Plastic that way....

If not i will try that Insert techniuqe.

I wouldn't have thought a little bit would do much harm.  I know model cement can kind of melt/deform some plastics if you use excessive amounts but unless you drown the thing in it I doubt it will be a problem.