Countersinking coachbolts?

[davidallen23]

Hi
I've got some wrought iron bench ends and some oak slats and I'm going to use stainless steel coach bolts to fit them all together.
My question: Will I need to countersink the holes in some way or will the oak be soft enough to allow the square section under the head to sink in when tightened?

 

[AndyT]

The normal way to use coachbolts is to just drill a round hole and tighten the bolt so it pulls the square shank into the hole. No countersinking or chiselling needed.
Clearly this is easier in soft wood. Try it on the wood you have. If you need to make it a little bit easier you could choose a slightly bigger drill bit and you could line up the coach bolt so the corners of the square are in line with the grain of the wood.

 

[Bod]

Should sink in, maybe just need a little help from a mallet.
Unless you got some very hard oak.

Bod

 

[davidallen23]

Well, it's new (green) oak so it'll be softer than old oak but I'm fairly new to this so I thought it worth asking.

 

[CHJ]

As the bolt heads are going to be in the seat slats I personally would counter bore the heads with a forstener bit prior to drilling the stem hole, just enough for the crown of the bolt to be flush with the surface.

By counter boring, if the wood swells with moisture then the surface immediately around the head wont show any stress marks/indentations/bruising from the swollen wood around the bolt head.

Some folks may not like the potential of the counter bore to hold water but I personally prefer not to have proud bolt heads.

 

[blackrodd]

As the previous posters have said, the coach bolts should tap in with the mallet, but a blunt drill bit leaving a "woolly"
hole means the bolt hole can be a bit undersized and tighter.
A good auger with sharp spurs would probably be best.
I hope you enjoy the seat build, stick some pics on if you can as most members just love a W.I.P.
Regards Rodders

 

[custard]

Will I need to countersink the holes in some way or will the oak be soft enough to allow the square section under the head to sink in when tightened?

If they're the style of coach bolts I'm thinking they are (domed head with a square sided "nut" underneath) then you you should drill a countersink hole that's the exact same diameter as across the flats of the nut, i.e. a bit too small to allow the whole nut to enter. It's only by crushing the corners of the nut into the hole that you stop the whole coach bolt rotating as you tighten.

 

[Terry - Somerset]

Roofing bolts are very similar to coach bolts, albeit without the square shank but with a screwdriver slot.

Alternatively start the hole (5 - 10mm) with a larger drill to clear the square shank. Hoping to crush oak like a softwood may be ambitious!

Rgds

Terry

 

[custard]

Hoping to crush oak like a softwood may be ambitious!

It's only the corners of the nut not the whole thing, just a couple of square millimetres, I've done it plenty of times and it's really not a problem.

 

[Max Power]

I definitely wouldn't counter bore for the bolt heads. Standing water is guaranteed to rot timber over time, unless you were to to plug the holes to hide the heads of course

 

[davidallen23]

I definitely wouldn't counter bore for the bolt heads. Standing water is guaranteed to rot timber over time, unless you were to to plug the holes to hide the heads of course

Good point! Also, as custard states, it's only a couple of sq mm so I'll give it a go.
Thanks for your suggestions.
Next issue I'm exploring is whether to treat the oak or not - a bit of a minefield from what I can tell. I'm tempted to leave it untreated

 

[Myfordman]

If you do decide to treat them, use a product that soaks in rather than attempts to form a skin or decorative coat.

The latter will fail, bits will fall off and the rest will stick like the proverbial to a fluffy item of bedding! You will then be into a cycle of scraping/sanding and re-coating for the duration of the items life time.

The former method might still need re-treatment from time to time (should be a several year cycle, longer than above)
but other than removing loose stuff and green growth, it should be little more than re-application.

Leaving it untreated, will allow it to gradually go greyish and no further work will be required other than to sit on it and enjoy your handiwork and open the odd beer or two!

 

[davidallen23]

Thanks.
So it's untreated and un-countersunk then.

 

[RogerP]

............
Some folks may not like the potential of the counter bore to hold water but I personally prefer not to have proud bolt heads.

I agree and provided the wood is properly finished there won't be a problem. I have several garden benches with bolts fitted this way. They're years old and I can't see any rot around the holes. They certainly look much neater and no bump to sit on

 

[CHJ]

Think there has been some confusion in some of the responses between Counter Boring the head (my preference) and handling the square anti rotation shank under the head, the latter as has been mentioned by AndT and others needs no relieving, the interference fit in normal stem clearance hole being needed to stop rotation.

 

[davidallen23]

...and no bump to sit on

Well, since the bolts are at the very ends of the slats, sitting on them wouldn't be an issue and I'm not convinced that counter boring the heads is visually preferable.