An engineering solution to cover fixing.

Owners and operators of Powerspout turbines, especially the pelton sort, will know that the front and back covers are held on by self-tapping screws which screw into the plastic of the turbine carcass.

The method has something of the wood-worker about it.

Repeated screwing and unscrewing of these fixtures inevitably causes them to de-thread eventually, whereupon one solution is to drill a new hole and continue to use a self tapping screw in the new hole.

Although the holes have not yet de-threaded for me, they're beginning to feel that way, and since that is after nearly 7 years of fairly frequent removal of both front and back covers, the self tapping screw method does have merit in lasting for quite a while.

But I am not enthusiastic about drilling new holes and continuing with self tappers so for some time I have been looking around for a better solution, - some way of fixing that comes more from the toolbox of an engineer than a wood-worker.

During this past week of continuing Covid lockdown, I have received the parts I'd decided on and have put them in: this post tells the story.

I've opted to use M5 stainless steel flanged screws and have these screwing into brass inserts fixed into the holes previously used by the self tapping screws.

Links to where I purchased the bits are at the bottom and are correct at the time of writing.

Brass inserts with M5 internal thread to accept M5 threaded stainless steel, flanged, screw.

Close up of brass insert to show coarse outer thread with longitudinal tracks to aid it to lock into the plastic and not unscrew.

Method used for placing insert by mounting it on an M5 socket headed screw with two lock nuts so it can be screwed into the plastic; a socket headed screw was used for placement because quite a bit of axial force is needed to get the coarse outer threads to bite.

Screwing the insert into place taking care to keep perpendicular to the face.

The insert in place with its surface flush with the plastic; no prior drilling out of the hole was done; it was found that the size of the hole after it had been used by self-tapping screws was about right to get a really tight fit of the brass insert into the plastic; a round file used for sharpening a chain saw (7/32", 5.5 mm size) was used to tidy up the hole beforehand but the hole should not be enlarged too much by the use of the file.

To finish off, a new neoprene self adhesive strip was placed; to make holes in it for the screws, a hot nail poked through does the job neatly.

The completed job, - no leaks ! The turbine here is running on the bottom jet only, rotating at 940 rpm, generating 322 W into the grid at a water-to-grid efficiency of exactly 50%.

And to finish off, a neat tool for dealing with the 8 mm hex-headed screws.

- M5, flanged, A2 stainless steel screws, 16 mm long were purchased here (£2.90 for 20, Free postage to UK) 
- Threaded brass, double ended, self tapped, screw fit inserts, M5 internal thread were purchased here (£7.30 for 25, Free postage to UK)
- Black Neoprene self-adhesive sponge, 6 mm thick x 15 mm wide x 5 m long purchased here (£5.50 Free postage in UK)
- Britool 8 mm nut-spinner, available while stocks last,here (£3.85 Free postage in UK)