Inspection Cover fasteners

[VicParticipant @vic]

I had to lift a plastic inspection cover on my driveway to clear a blocked drain. Unfortunately the three fasteners used to secure the cover were very badly rusted. A large screwdriver wouldn’t even touch them so I had to use an impact driver. One fastener just spun and wouldn’t come out, the other two snapped off. The countersunk screws seem to be hardened steel and they appear to have been threaded into brass bushes in the plastic casing. I’m now left with three fairly clean holes in the casing. I’m hoping to fit some rivnuts (probably M8 – I need to check the hole size) and get some new screws. I’m planning on using stainless steel screws so I don’t have this problem again. For rivnuts I have a choice of plated steel, stainless steel or aluminium alloy. I’ve heard it mentioned that you can get galling happen if you use stainless nuts and bolts but it’s not been an issue for me before. As a result I’m wondering if I should use alloy rivnuts. What do folks think?

[peak4Participant @peak4]

Stainless screws into alloy can lead to electrolytic corrosion, though there are specialised lubricants which can help; ask anyone who’s used stainless bolts to affix alloy panels on Landrovers.
Personally I’d use stainless for both, but assemble with an anti seize compound; bear in mind stainless rivnuts often take quite a lot of force to set them.

Bill.

[Robert Atkinson 2Participant @robertatkinson2]

What Bill said. Don’t mix stainless steel and light alloy.

[SillyOldDufferModerator @sillyoldduffer]

If I read Vic correctly, he’s not proposing to mix stainless with another metal,  he’s concerned a male and female stainless pair might gall.

Me too, though there’s a reasonable chance the makers of nuts and bolts avoid Stainless alloys that gall.  We hope!

In any case, galling is more likely to occur when Stainless is repeatedly rubbed, or rubbed hard. Neither seems likely when stainless is used hold down a rarely disturbed inspection plate.  Should be OK.

A potential booby trap!  Rivnuts are often sold alone and the buyer buys bolts to fit them separately.  Opens the door to  mismatching different grades of stainless, which might slowly corrode.  Low risk I feel, but bolts and nuts that come together in a pack should be compatible.

Robert and Bill rightly warn against mixing metals.  For as long as I can remember fairly common in manufactured products.  I suspect deliberately: planned obsolescence to boost trade!

Dave

[VicParticipant @vic]

Thanks, I’ll get stainless and maybe put some grease in the thread on assembly.

[Julie AnnParticipant @julieann]

Using the same grade of stainless steel for bolt and nut may well lead to galling. Using different grades makes galling far less likely. That’s one reason stainless steel fasteners come in several grades. The usual combination is A2 (304) for the bolt and A4 (316) for the nut.

Julie

[Phil SParticipant @phils66830]

These screws are there to hold down a lightweight cover in order to comply with an air tightness pressure test. Depending on the cover design you may find it remains in place satisfactorily without screws when walked over. Although may not be advisable in a public area. If on a shared sewer with houses “upstream” of your location leaving the screws out can provide an escape route for sewage in the event of a blockage “downstream”. In breach of the rules but preferable to an indoor sewage flood. Original screws likely to have been brass.

[CirclipParticipant @circlip]

Stainless/Stainless and use “Copperslip” (check spullin) to lubricate/coat.

 

Regards   Ian

[Martin KyteParticipant @martinkyte99762]

Maybe you could fit DZUS fasteners.

[noel shelleyParticipant @noelshelley55608]

Steel WILL rust, Alloy WILL corrode, stainless at least gives you a sporting chance. Fitted with lubrication and NOT over tightened will be OK. The biggest problem may be fitting rivnuts – as they expand they may well split the plastic housing. Tapping the hole and using a much larger countersunk SS bolt might work ? PLAN B use a helicoil of a size to fit the hole in the plastic and a suitable SS countersunk bolt. Good Luck. Noel

[bernard towersParticipant @bernardtowers37738]

Can you thread the rim and make Delrin screws then fit with silicone grease?

[Clive FosterParticipant @clivefoster55965]

There are brass knurled and split inserts made to go into plain holes in plastic. nesting these crew expands the knurled portion giving a very secure grip. Not sure of sizes and availability above 6 mm as I’ve never needed anything bigger.

Maybe consider the collapsing shank devices made to put a threaded fixing into plasterboard and their relatives. There seem to be a fair number of varieties of such things both for simple plasterboard use and as an alternative to rivet-nuts. I imagine the rivet-nut equivalents put vastly less expansion force on the hole when inserted so as to reduce the risk of plastic fractures.

Various all plastic thingies too. Obviously not as strong as metal but how strong do they need to be to hold a cover.

Another alternative is the expanding “rubber” sleeve devices with a thread insert moulded into the end. Tightening these crew collapsed the sleeve expanding it into a very strong grip. Popular on Japanese motorcycles for holding plastic fairing parts and screens but made in many sizes.

I’d go for the knurled, screw expanded, inserts if available in the right size as first choice followed by the rubber thingies. Plaster board fixing style are for when you can’t get anything else.

Clive

[VicParticipant @vic]

I just checked the hole size to order some nuts and I noticed one of the holes in the plastic has marks very similar to what a Rivnut might leave behind. The manufacturer has offered to send me some new screws. I won’t use them but it’s an indication of the bolt length I need to order. Rivnuts are on order, plus a mandrel as I don’t have one for the size needed.

[Phil S]

On Phil S Said:

These screws are there to hold down a lightweight cover in order to comply with an air tightness pressure test. Depending on the cover design you may find it remains in place satisfactorily without screws when walked over. Although may not be advisable in a public area. If on a shared sewer with houses “upstream” of your location leaving the screws out can provide an escape route for sewage in the event of a blockage “downstream”. In breach of the rules but preferable to an indoor sewage flood. Original screws likely to have been brass.

I have I think, four of these plastic covers on my property. The very lowest access cover we have, nearest the road is, actually an unsecured cast iron cover. When we had a blockage some years ago a small amount of water leaked out at this point.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I agree with Julie about using different grades for the nut and bolts with stainless steel. I used to maintain submersible pumps, down to component level, these spent their time in acidic water for most of their lives, and were mostly made with aluminium castings. One make had different grades, and were never any bother with being undone and done up again, time after time, the other make though, was a different matter, and had to have oil, or anti-seize compound on them. they used to go about three months between a strip-down, mainly due to the impellor wearing away, and or the seals failing, and flooding the motor coils. The wetted coils could usually be dried out, but sometimes they got fried.

Regards Nick.

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