M8 to M6 Reducer

[cwhaley]

I have a small lamppost fixed to the ground with threaded bar (set deep into 2' of concrete). Unfortunately the M8-sized bars aren't long enough to go entirely through the base of the lamppost.

My plan is to use an M8 to M6 reducer as I have a long length of M6 threaded bar which can extend the bar up.

Unfortunately there's only 15-20mm of room underneath the base of the post so I don't have a lot of room to extend the thread. I need a reducer from M8 to M6 with an overall length of no more than 20mm. I can't find them in the UK or even at the local fastenings shop -- the only source is from a seller based in Honk Kong and the delivery time is up to 4 weeks. Something like this:



Does anybody know if this is something available in the UK? Maybe I am looking at the wrong solution?

 

[JohnD]

consider sleeve-nuts

You didn't say how far the bars project above the surface of the concrete. M8 sleeve-nuts will typically be 15mm or 20mm long.


is the lamp-post aluminium? I'd be inclined to put a thick C-form stainless washer or two under the head.

You will need nickel grease to prevent the stainless from seizing.

 

[Tigercubrider]

http://vi.raptor.ebaydesc.com/ws/eB...ategory=42905&pm=1&ds=0&t=1550780484000&ver=0

 

[cwhaley]

consider sleeve-nuts

You didn't say how far the bars project above the surface of the concrete. M8 sleeve-nuts will typically be 15mm or 20mm long.

is the lamp-post aluminium? I'd be inclined to put a thick C-form stainless washer or two under the head.

You will need nickel grease to prevent the stainless from seizing.

They may work -- I just need to see if there is enough room within the holes/tunnels on the base for the sleeve nuts to go through if you know what I mean. I can drill out further but I'm afraid of removing the layer of protection on the oxidised metal. The picture below shows the lamppost base and the pre-drilled holes for the bar:



The bars project exactly 30mm beyond the surface of the concrete which brings the ends of them almost completely flush with the surface of the lamppost base, which is obviously stopping me from fastening the capping nuts on as there's no accessible thread.

It is aluminium yes so I will pick up some washers from the old man's shed, luckily I do have a tub of nickel grease thanks for the heads up. Cheers.

 

[cwhaley]

http://vi.raptor.ebaydesc.com/ws/eB...ategory=42905&pm=1&ds=0&t=1550780484000&ver=0

Unfortunately they are 30mm long which is too long

 

[JohnD]

If it's aluminium you can drill out the holes to fit, or maybe file them. Leave a loose enough fit for thermal movement. You will have to use an anti-corrosive primer and a protective paint on the exposed metal. Dissimilar metals mean the aluminium will corrode away if not separated.

Don't use Copaslip as coppr accelerates corrosion of aluminium.
https://www.bssa.org.uk/topics.php?article=89

There are a few specialist aluminium primers, you might find one in a Yottie shop or a place that sells paint for Jags, or the lamp-makers may have a touch-up kit.

 

[SammyInnit]

Unfortunately they are 30mm long which is too long

You'll find they'll be 50/50 threaded so you would be able to cut off each end to suit. They won't be hardened steel so you can vice them up and use a hack saw if not an angle grinder.

A deburr or chamfer bit might be of help later but you may not need it.

 

[mattylad]

I would not want to be using M6 to support a lamppost, if you have M8 then use M8 fittings.
How about showing a picture of the situation and the bottom of the lamppost?

 

[cwhaley]

Thanks all for the very useful information -- appreciate the replies.

I'm going to try @SammyInnit 's suggestion first. I presumed they would be 50/50 threaded but didn't want to ruin the thread when sawing. I'll give them a go and if this doesn't work I think it'll be @JohnD 's advice on widening the thread holes in the base and then protecting with a primer and paint.

@mattylad - I only have M8 because the bars I had included a large head on the end which I thought would give extra support in the concrete. M6 is adequate to support this lamppost. It is only 6' tall and made of very light aluminium. If it was a cast iron or thick aluminum post I'd be going M8 or M10 and setting the concrete another 6" deeper.

 

[SammyInnit]

I would not want to be using M6 to support a lamppost, if you have M8 then use M8 fittings.
How about showing a picture of the situation and the bottom of the lamppost?

While it wouldn't be my first choice either the tensile strength of mild steel M6 will be sufficient.

 

[Tigercubrider]

Unfortunately they are 30mm long which is too long

so saw a bit off?

 

[JohnD]

What diameter are the existing holes?

 

[Ian H]

Buy some M8 bar, it’s only £2/mtr

https://www.toolstation.com/threade...E8GBu41pa6G8cpvmvcDfRmAXZXetkHq4aAgvVEALw_wcB

 

[SammyInnit]

Buy some M8 bar, it’s only £2/mtr

https://www.toolstation.com/threade...E8GBu41pa6G8cpvmvcDfRmAXZXetkHq4aAgvVEALw_wcB

It's if the M8 rod is set in concrete as he suggests or if they're screwed into anchors which are set into concrete such as Hilti HKD.

If they are threaded anchors I'd do this and replace the rod. In fact, go to any commercial looking job and ask the sparks or plumbers, they'll probably give you a length or ask if you can fish a bit out the skip.

 

[mattylad]

the tensile strength of mild steel M6 will be sufficient.

That completely depends upon the quality of it, I find that much of modern materials available are total crap and will not have a lot of tensile strength unless you go and find good quality one, certainly nothing from a shed!

Even a 6 foot lamppost can sway about in the wind a fair bit, how many of these threaded have you got in the ground?

Again - I still would not want to use M6 bar/fittings to retain something like a 6' lamppost, unless there was something like 6-8+ of them.
Stick to the M8 and it should be there in a few years time - just as strong as day one.
How about put some extensions on it and use a plinth (slab with holes in) to allow you to raise it up so you have rooms for any additional fittings required.