Bit size and type for 6.7mm timberfix screw for green oak

[JonCamo]

I’ve used these for fixing sleepers together and wondered if the timber would split or the bolts break off - used a long 3mm bit just in case and had no issues- these bolts are stronger than they look .just use a quality impact bit .

Thank you

 

[JonCamo]

There appears to be some confusion of terminology.

I have always understood that you drill a 'clearance hole' (slightly larger than the largest diameter of the screw shank) though the part that the screw is restraining and a 'pilot hole' in the part into which the threads bite (roughly equal to the minor diameter of the threads on the screw).

We need to be clear on this point as if people are suggesting numbers for a clearance hole and you use it for a pilot hole, the screw will not grip.

That would make sense , in this case then a drill bit 5 to 6mm for the 'pilot'

 

[JonCamo]

That makes sense, thank you

My rule of thumb has always been to pick a pilot hole that is the size of the shaft, as oppossed to the thread, so that the wood isn't being wedged apart, encouraging a split, and only the thread is engaging with the wood, that is what my father told me when I was a child and I've never questioned it. However a couple of weeks ago I bought some 175mm coach bolts for a field gate fixing, then realised I didn't have a pilot bit that long for them, so was doing a bit of research, found a guide, I think it was actually timco website though I wouldn't swear to it, which said for softwood your pilot should be half the diamater of the screw and for hardwood three quarters of the diamater.

Most of the time I make clearance holes in the piece I'm attaching, and tend to only pilot in areas that are likely to split (ie ends), but that is pretty much laziness, I can see no downside in piloting apart from time, so long as you don't make a hole too big for a decent grip.

My rule of thumb has always been to pick a pilot hole that is the size of the shaft, as oppossed to the thread, so that the wood isn't being wedged apart, encouraging a split, and only the thread is engaging with the wood, that is what my father told me when I was a child and I've never questioned it. However a couple of weeks ago I bought some 175mm coach bolts for a field gate fixing, then realised I didn't have a pilot bit that long for them, so was doing a bit of research, found a guide, I think it was actually timco website though I wouldn't swear to it, which said for softwood your pilot should be half the diamater of the screw and for hardwood three quarters of the diamater.

Most of the time I make clearance holes in the piece I'm attaching, and tend to only pilot in areas that are likely to split (ie ends), but that is pretty much laziness, I can see no downside in piloting apart from time, so long as you don't make a hole too big for a decent grip.

Makes sense, thank you

 

[JonCamo]

I'm in the middle of a hardwood framing project *garden bed) which is subject to a lot of twisting/racking.....
all the joints are mortice an tennon with stainless n brass thru bolts.....this lot is 8mm......
View attachment 158924
might not what u want but this'll give u an idea.....u can always bury the heads under plugs if u want.....
when I need longer bolts than I can buy I use st/steel threaded rod and dome nuts.....

If ever u need to use st/steel nuts n bolts NEVER use a nut gun or impact driver.....often they will gaul the threads....nightmare...

if ur screws are stainless be carefull as they are quite soft and will snap.....esp long ones....
ask me how I know.......

There's a hardened steel screw like that shown.....it has a green finish which normally will withstand outside use with no rust....
green Oak might still create rust tho......

If I need to use stainless steel wood screws in outside furniture I ALWAYS drill a pilot and then drive in a hardened gold screw to create the thread in the wood....The finish stainless screw wont snap when driven in and there's better than 1/2 a chance it will come out without snapping....

Cant afford brass anymore besides the orrible slotted head.....OK for antique stuff tho...

Thats informative thank you. The two I have driven in after a 6mm pulot hole went in fine but I just wondered if a smaller pilot would be better as they are for tying rafters down

Thanks

 

[Nail]

Thats informative thank you. The two I have driven in after a 6mm pulot hole went in fine but I just wondered if a smaller pilot would be better as they are for tying rafters down

Thanks

I checked with some I have here and there was no difference on the finished hole if it was piloted first or not. Definitely no need to go up to 5mm hole size. I think it just depends on how the driving in goes as to if you need to predrill. If the rafter is quite thick it’ll be a fair bit of initial friction pushing through.

 

[Spectric]

Timco don't say you need a pilot hole, a clearance hole is not needed as they have a self drilling section but this may work on softwood and softwood sleepers but I can say it needs a small pilot hole when using hardwood sleepers and a dab of wax helps as well.

 

[clogs]

Spectric.......
thanks for that I forgot about the wax.....

or Brylcream if anyone remembers.......? hahaha.......

 

[JonCamo]

Timco don't say you need a pilot hole, a clearance hole is not needed as they have a self drilling section but this may work on softwood and softwood sleepers but I can say it needs a small pilot hole when using hardwood sleepers and a dab of wax helps as well.

Sorry, to confirm, you do need a pilot hole with green oak ?

Regards

Jon

 

[macca]

Pilot hole is advisable in oak

More specific to these fixings I'd consider using the wafer head type, same fixing essentially but a much broader head and a torx driver. hex heads are great but you will either find the head will not pull in, in soft wood this can be an issue with knots so may occur in oak, if it doesn't pull in and you spin the fastener it can drill out the hole and you will not get a fixing. If they dont pull in you will be left with a noticable head sticking up on the rafters. Also if the head pulls in it makes a mess of the timber and will (i know oak is hardy but still..) be succeptible to damage from water over time
a wafer head probably wont pull in on oak but it has a lower profile head and the flange is double the size of that on a hex head thus IMO protecting the hole youve made from water over time.

I've used thousands of these fixing both hex and wafer framing houses and I must say they are cracking bits of kit, they really pull well but can be a pain in the arse if the thread ends up in end grain.

 

[Jake]

In green oak these will be fine without a pilot, that slash in the point of the thread makes it basically a drill bit. I would think more about dealing with shrinkage over time in the green oak with metal fixings than difficulty getting them in to start with. Depends how structural the fixing needs to be I guess.

 

[Spectric]

you do need a pilot hole with green oak

I would in any hardwoods, if nothing else it gives your drill / driver an easier time.

 

[MikeJhn]

A pilot hole is what it says, just a guide hole for a larger bit, but the difference between parting the fibres of the timber which can cause splitting too breaking the fibres and prevent splitting is very different, remember being told to blunt a nail before driving it in?

 

[JonCamo]

Pilot hole is advisable in oak

More specific to these fixings I'd consider using the wafer head type, same fixing essentially but a much broader head and a torx driver. hex heads are great but you will either find the head will not pull in, in soft wood this can be an issue with knots so may occur in oak, if it doesn't pull in and you spin the fastener it can drill out the hole and you will not get a fixing. If they dont pull in you will be left with a noticable head sticking up on the rafters. Also if the head pulls in it makes a mess of the timber and will (i know oak is hardy but still..) be succeptible to damage from water over time
a wafer head probably wont pull in on oak but it has a lower profile head and the flange is double the size of that on a hex head thus IMO protecting the hole youve made from water over time.

I've used thousands of these fixing both hex and wafer framing houses and I must say they are cracking bits of kit, they really pull well but can be a pain in the buttocks if the thread ends up in end grain.

Will take a look. Thanks

 

[Sgian Dubh]

The need for a pilot hole decreases dependent upon the wood species and the wetter the wood. In this case, as I understand it, the plan is to screw into green oak, which suggests it's relatively wet. i.e., ~25% MC and greater. In this state the wood is relatively soft and flexible so the chances are that no pilot hole is required because the fibres will distort around the penetrating screw. If the oak is dry, ≤~12%MC, the fibres are stiffer and much more likely to separate (a tension splitting force) without a pilot hole. Wood species such as maple, when dry, are very likely to split if screws are inserted without a pilot hole, whereas softer species of wood such as fir, for example, are less likely to split. Similar splitting characteristics apply to driving nails into different wood species: more brittle species tend to split and if that's the case it's often beneficial to pre-drill a reliving hole prior to driving the nail.

In the OP's case no pilot hole may be needed to drive the fixings into the oak, but if that's too difficult for whatever tool he's driving the screws with, or splits develop, it might be all that's needed is a 4 or 5 mm diameter pilot hole. I suggest experimenting to see what works and go from there. I also suggest a clearance hole 0.5mm greater than the screw shank diameter through the piece that's being affixed is probably a good idea just save effort and to alleviate any possibility of bridging, i.e., a gap between the two parts being fixed together. Slainte.

 

[JonCamo]

The need for a pilot hole decreases dependent upon the wood species and the wetter the wood. In this case, as I understand it, the plan is to screw into green oak, which suggests it's relatively wet. i.e., ~25% MC and greater. In this state the wood is relatively soft and flexible so the chances are that no pilot hole is required because the fibres will distort around the penetrating screw. If the oak is dry, ≤~12%MC, the fibres are stiffer and much more likely to separate (a tension splitting force) without a pilot hole. Wood species such as maple, when dry, are very likely to split if screws are inserted without a pilot hole, whereas softer species of wood such as fir, for example, are less likely to split. Similar splitting characteristics apply to driving nails into different wood species: more brittle species tend to split and if that's the case it's often beneficial to pre-drill a reliving hole prior to driving the nail.

In the OP's case no pilot hole may be needed to drive the fixings into the oak, but if that's too difficult for whatever tool he's driving the screws with, or splits develop, it might be all that's needed is a 4 or 5 mm diameter pilot hole. I suggest experimenting to see what works and go from there. I also suggest a clearance hole 0.5mm greater than the screw shank diameter through the piece that's being affixed is probably a good idea just save effort and to alleviate any possibility of bridging, i.e., a gap between the two parts being fixed together. Slainte.

Thank you for the detailed reply. I have smalled bits now so will use a narrower pilot hole

Thanks