Using SS Screws in Plastic

[Martin109]

I'm replacing the cartridge on our Aqualisa shower.

The old one was held in by four 5mm diameter galvanized self-tapping screws, into a plastic housing, which has stood up well to the 17 years the shower's been in service (not sure what plastic it is, though).

However, three of these screws had corroded, and I had to saw through the heads to get them off.  I've acquired four lovely stainless steel self-tappers to replace them, and was wondering if a bit of lubrication might help me to get them out again in another 17 years' time!

Any suggestions?

[mrcharly-YHT]

 Put them in gently. SS screws shear off quite easily. BTDTGTTS

[Martin109]

That's also something I was hoping some suitable lube might help with, as well as getting them out again in 2029!

[fred the great]

You could use some threadlock which will stop the screws from coming loose and make it easy to unscrew them when you are a pensioner

If made of poor quality stainless the screws could in fact corrode also.   Note that there are many grades of stainless steel and it is unlikely that your screws are coated or made  with the best quality. In fact chromium plated brass screws would, I think, be a better idea.

[Martin109]

OK, I'll see if Apex Fasteners in Slough can to those (they're very good).

[Kim]

I may be being thick, but what's a 'cartridge' in the context of a shower and lasting 17 years?  Seems a bit long for a filter...

[Martin109]

Kim, it's Part No. 5   on this diagram.  I originally installed the whole shower as new in about 1996, and this is the only thing that has needed attention since then!

[rafletcher]

I assume these are machine screws (parallel thread, presumably metric) or are they tapered self tappers?  If the first a bit of vaseline may help, but the thread (in the plastic) is pre-formed (if only by the origianal screw, or perhaps in a metal insert), and the SS shouldn't corrode so no issue later. If self tappers (ire like wood screws) I wouldn't bother with grease.

[Martin109]

They are (slightly) tapered self-tappers.  They should be OK, as they're 0.2mm narrower than the ones that came out.

[rafletcher]

In which case just screw 'm in. The taper acts as a "quick release" as you'll appreciate. You should have to apply much force to turn until nearly fully home. I'm assuming, again, that they're either slightly longer, or a different thread pitch, to get some grip - but you'll soon find out when you put them in!

[Jurek]

You could use some threadlock which will stop the screws from coming loose and make it easy to unscrew them when you are a pensioner

If made of poor quality stainless the screws could in fact corrode also.   Note that there are many grades of stainless steel and it is unlikely that your screws are coated or made  with the best quality. In fact chromium plated brass screws would, I think, be a better idea.

No, no, no, no, no.
Threadlock will eat many plastics - and certainly the acrylic based ones.
At which point you will be royally stuffed.

[fred the great]

There are various types of threadlock available. I was making a general comment and not recommending a specific product. More important is to use screws that are very similar threadwise to those that have been removed otherwise the plastic could be damaged very easily