Small blanking plug missing from my Willis heater based UFH

[dnb]

Now it's getting cold and I am still a regular member of house not finished anonymous I have installed a Willis heater setup for my underfloor heating using old bits of plumbing from the many boxes of spares I have. All good so far, but I find that I have lost a small blanking plug that goes in the mixer unit's return piece. I assume it is for a temperature probe or similar, but I wasn't going to bother for the moment. (The UFH is intended as a backup heat source for the house - the "real" heating is via ducted air, but I am a while away from commissioning this but it seemed silly to not put pipes in the screed when the opportunity presented itself.)

 

Here is a photo of the offending hole in the UFH assembly. I believe it might be 7/16" UNF but Google suggests this is not a common plumbing part size - mostly returning hits on brake pipe flares and similar automotive things (all my cars have metric brake lines, so no help here). It could also be M12, but this doesn't feel a particularly good match with the thread gauge and as one would expect the entire stock of M12 bolts in the house have all gone on their annual holiday to the back of the extremely full shed.

 

Does anyone know for sure? The local plumbing places don't have much that is non-standard in stock and I would prefer to not have to buy anything big to get this to work. Thanks!

 

Edited November 18, 2023 by dnb

[JohnMo]

It likely to a standard plumbing thread size, so 1/2" looks about right. Not something you get on a car.

 

[dnb]

Definitely not 1/2". Far too small. (I have loads of these in the spares box and they don't fit.)

 

And not M12 either now I have found a bolt to try, although this is much closer.

[JohnMo]

A 1/4" BSP or NPT has an outside diameter of about 1/2" (close to M12), so likely to that. Two minute job to remove that section, take to plumbing merchant, b&q, Screwfix etc, and find the plug that fits.

[dnb]

Damned keyboard batteries... Lost the post on editing to add some new measurements.

 

My thread is approx 14TPI, so close to 1/2" BSP, but the hole is less than 13.2mm, so this doesn't fit at all! 1/4" BSP thread pitch is 19TPI so even crude measurements tend to rule this out. The hole is approx 10mm to the edge of the thread, so best attempt at measurement gives jsut over  11mm as an outside diameter. It does all point to 7/16th UNC, which as you rightly say is odd. The hole has parallel threads, so a tapered plug might work, but not if the thread pitch is wildly different.

 

Screwloose and B&Q were not helpful. This has been done. I wouldn't ask here if it were too obvious!

Edited November 18, 2023 by dnb

[Conor]

It'll be 1/4 or 3/8, likely the latter as widely used on heating systems and components. I bet if you unscrew one of the pressure gauges, it'll fit. Definitely worth bringing it down to your nearest plumbers merchant and getting it right first time. Or just order a selection of plugs, will only be a couple quid.

 

A small box of random brass  bushes, reducers, straights etc is sooo handy to have.

Edited November 18, 2023 by Conor

[dnb]

2 minutes ago, Conor said:

I bet if you unscrew one of the pressure gauges,

How much? Want to bet that I haven't done this and found they don't fit already? (The gauges in the picture are temperature, and push in to holes in the manifold) I tried the pressure gauge from a couple of spare PRVs and both were too small (one was  3/8" BSP and the other was 1/8" BSP) and neither worked.  3/8" BSP OD is around 16mm so this is hardly surprising.  And I've already established that 1/4" BSP isn't right.

 

I have an extremely large box of plumbing spares from my mis-spent youth and from various parent's estate clearances. I am most surprised there is nothing in the magic box that works.

[dnb]

From left to right we have 1/8 bspt, JIC -6AN with M12, 1/4bpst to npt, 1/4bppp and 1/4bspt to 1/8 npt. None fit.

 

On the bottom we have a random banjo bolt from the brakes of one of my cars. I can't recall which. This fits well but doesn't help block the hole up!

[dnb]

Got some better measurements now I have a long bolt that fits. Seems I mis-counted the threads in the hole (off-by-one error I think... ) so it's very close to 20TPI. Major diameter is still as before as between 11 and 12mm.

 

So I am left with 2 likely alternatives: M12 1.25mm pitch or 7/16" UNF.

The banjo bolt appears to be M12 and is a little slack in the manifold, but nothing PTFE tape can't fix, but the 7/16" UNF is has a slightly bigger diameter and over 5 threads 20 TPI vs 20.3 TPI makes little difference.

 

Why would anyone make plumbing parts with unusual threads? It's just irritating. I found a 1/4" BSPT thread tap in one of my old tool boxes. I think I know where this is going. 🤪

[SteamyTea]

8 hours ago, dnb said:

Why would anyone make plumbing parts with unusual threads

Metric Fine and UNF are not that odd.

They are often used as they are self locking.

 

 

 

And just for a laugh, ACME, as we all like a cartoon.

 

 

Looks like something that is found in @Pocster's dungeon.

[dnb]

3 hours ago, SteamyTea said:

Metric Fine and UNF are not that odd.

I know they aren't odd per se. But it is odd to design them in to a part that uses common plumbing sizes everywhere else with no obvious need to use something different.