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typically soft materials would use a larger thread pitch, such as a self tapping screw. You can certainly use whatever you'd like, a machine screw could hold just fine, as long as the hole is the right size and not too large. If you have already put a self tapping screw into a plastic hole, running a machine screw into it might be very weak because you have basically crossed threads.

as long as the screwed joint can sustain any load placed on it without fail, it's fine. but you won't know that until you try.

to make the joint as strong as possible, try and make sure you use the longest screw that will fit, so you grab as much of the material as you can. even better, if you can make the screw a through mount with a nut on the back side, it will be even stronger.

don't have much experience with schumacher cars, so i can't tell you specific things regarding those chassis'

Thanks for the tips, jwscab. Think I will have try with some machined screws and see how the cars get on.

Andy, yes this is fine mate, with new holes you can tap the plastic with a screw and light oil, where some have already had a self tapper fitted you may find a machine screw will not hold so well on fresh parts this won't be an issue

This is something that has concerned me after buying a set of RCScrewz (all are stainless steel machined screws) for my CYANide Optima. Most of the stock nylon/plastic Optima parts are meant to be used with self-tapping screws, but those were subsequently replaced by machine screws. The machine screws were initially more difficult to start threading them in those Optima parts.

My concern is that, with the finer, shallower threads of machine screws, it is logical that the potential for stripping out the plastic/nylon is greater.

Pro's and con's, results can vary depending on type of plastic used in the kit, Yokomo's for instance there is no issues with machine screws, but I can see with some plastics other kits use self tappers would be best to use. With plastic you need to make the thread so turn back and forth with screw lightly oiled when fitting machine screws into new parts.

Be careful not to overtighten a joint that was designed for a self tapper - you get more mechanical leverage with machine screws.

minichamps11 wrote:Be careful not to overtighten a joint that was designed for a self tapper - you get more mechanical leverage with machine screws.

I think overtightening is a worse situation--and more easy to do due to the finer, shallower threads--with machine screws. By design, self-tapping screws bite deeper and more firmly; machine screws would be better when screwed into something hard with threads also machined, such as aluminum, metal, or having a nut on the opposite end.

it would seem to me if you buy a corresponding tap for the machine screws that it could actually work better. The machine screw may actually have more physical contact with the material than the self tapper. It would be very cool if you could heli-coil all the holes....

gjeeper04 wrote:The machine screw may actually have more physical contact with the material than the self tapper.

That's true but the threads are much thinner and shallower--which isn't a problem when it's metal-on-metal, but when it's metal-on-plastic (or something else that's soft), it's probably more likely to strip out under torque.

Coelacanth wrote: I think overtightening is a worse situation--and more easy to do due to the finer, shallower threads--with machine screws. By design, self-tapping screws bite deeper and more firmly; machine screws would be better when screwed into something hard with threads also machined, such as aluminum, metal, or having a nut on the opposite end.

Precisely what I meant - you get more mechanical leverage with the fine threads so it's easy to overtighten.

Someone with an inch pounds torque reading screw driver should volunteer to do a study...

I don't even think that would be necessary. Just take 2 screws--a 3mm self-tapping screw and a 3mm machine screw--and crank them both into a plastic or nylon bit with 3mm holes in it, neither having had any screw threaded into it before. Once they stop turning, keep on cranking and see which one strips out and continues turning more easily.

I've already experienced this with AE white shock collars and the older-style black machine screws that thread into them. They strip out relatively easily, just turning and not tightening up anymore, whereas the 3mm tapping screws that go into the Kyosho-style black shock collars tend to crack the collar completely before they strip.

I've used machine thread screws to replace the more coarse screws for years. I have found that over time the threads in the part last longer because they dont back out as easily due to vibration and such. When you tighten and loosen the coarse screws several times (like removing and re-installing them for maintanence) they tend to strip the part faster than machine screws do. Just dont over tighten them.