Advice: replacing vintage hardware with modern

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#1

Hello lovely people. Looking for some advice. I’m working on some old chairs that have visible M8 coach bolts, which need to be replaced.

The M8s I’m finding for sale have a 20mm head which look too big in situ. The M6s available have a 15mm head but I’d need to redrill.

Would you a) just drill out a wider diameter head recess and accept that the heads look a bit big.

Or b) fill the holes and re-drill for an M6 bolt. Is there a way to do this which won’t lead to the filler deteriorating over time? The bolts are load-bearing so it’s important they stay firm.

Thanks!

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#2

To re fill and redrill hole, you could try drilling it out to the diameter of a dowel, glue the dowel in, then drill your new diameter hole into the patched bit, should be plenty strong

#3

Can you not grind down the head of the coach bolt to give you a 15mm diameter? You could probably do it in the woodshop with a cordless drill and some sandpaper. Or if you’re feeling particularly adventurous, do it on the edge of the belt sander (not the nice one) . :open_mouth:

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#4

The legs look curved so might not lay flat and would need to be properly aligned somehow if drilling dowels.

Another way would be to use a sleeve for the M6 bolts. You could 3D print those. A quick alternative would be to use fibreglass tape or similar wrapped around the bolts. Wrap a couple of turns with non sticky side around the bolt so it’s not gummed in, then wrap tape around that to the right thickness.

You could also use Paraloid B72 (thermoplastic resin) with the fibreglass tape. It’s reversible and can be removed with acetone. Getting a properly centred and aligned hole that way may be easier than drilling, but that depends on your level of skill.

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#5

Thanks @mbg, @Ryanf. Not sure I trust myself to make a decorative job of the grinding! The dowel could possibly work - I might attempt just one and see if it’s possible with the shape. As you say, it is slightly curved so drilling will need to be on-point.

@dannz I’m intrigued by the possibility of making an M8 sleeve. The slight complication is that coach bolts have a square grip just under the head that you hammer in. I imagine it compensates for the lack of head notch when screwing on the nut.

#6

These kinds of things exist and may help

#7

Re the square grip - the sleeve needn’t be around that, just the round part. The M6 square grip will be smaller than the M8, but when the nut is being tightened, the bolt’s rotation will be limited- it couldn’t spin fully in the slightly larger recess. Or you could glue on some shims if you wanted, or 3D print a part for the square grip so it fits snugly in the existing square recess.

There’s also plenty of community acrylic and ply scraps which you could laser cut, making collars and also a piece as a square grip holder. Ply could be sanded down if needed. It would be like making a dowel that fits the M8 hole which has an M6 hole already in it and perfectly centred. You might need 5-8 collars using 20mm ply - glue those together on the bolt so they are properly aligned. (insert the square grip part after, so it’s easier to fit). I like that approach best of all. It’s closest to the original using wood, it’s accurate, pretty easy with no tricky drilling and zero cost.