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

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JonCamo

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Dear all


What drill bit diameter and bit type would you recommend for 6.7mm timberfix screws. They are 150mm long. I have a 6mm Auger bit which seems ok but wonder if I need a smaller pilot hole
Screenshot_20230510-210330_Chrome.jpg
20230510_135939.jpg
as they are for tying down green oak rafters for the roof of a gazebo project (images attached)

Thanks in advance

Jon
 
Do they even need a pilot hole? They’re self cutting. Is the pilot to stop future splits?
 
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 .
 

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I’ve thought about it more and if the rafters are quite deep it’ll limit that initial friction to drive them through. It certainly won’t hurt to pilot hole.

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.
 
My understanding is that with green oak fixings should be avoided as tannins react to the metal. However I have seen a video where the Spax screws were used by Robin Clevett as they are coated. He didn't drill a pilot hole.
 
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.
 
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......
IMG_9505.jpeg

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...
 
I don't have the Timco brand however the Timbascrew of similar size 6.7 x 100 measures -
5mm across the unthreaded shank
6.6mm across the crests of the thread
4.46mm across the root of the thread

so 5mm pilot would give 0,8mm of thread or a 4.5mm would give full thread penetration but some binding of the shank
 
I don't have the Timco brand however the Timbascrew of similar size 6.7 x 100 measures -
5mm across the unthreaded shank
6.6mm across the crests of the thread
4.46mm across the root of the thread

so 5mm pilot would give 0,8mm of thread or a 4.5mm would give full thread penetration but some binding of the shank
root of thread?
 
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.
Yep, but if you use a pilot hole with one of these it’ll turn into a clearance hole for the shank anyway.
 
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