Twist Bolt Drill bit sizing (2024)

Matthew Jaggers · Oct 15, 2021 · Red River Gorge · Joined Sep 2017 · Points: 695

Calling Jim Titt!

So the 6mm twist works great in a 1/2" hole, and has just a bit of snugness once tapped in. On the TT website, the 8mm twist calls for a 3/4" bit, and when eyeing it up with a fresh bit, it looks like there won't be any interference fit at all. my plan is to use the big 8mm x 150mm in horizontal roofs, and would hope to have a bit of initial holding power while the epoxy dries.

I know an 11/16th bit is uncommon and a little harder to find, but would I get the advantage of some initial holding strength by using the 11/16th instead of 3/4 bit? Also, do I need the larger space for glue with the 3/4" or would the tighter hole be fine with 11/16" bit? Just from eyeing them up, it looks like 11/16" would be a more similar fit to the 6mm with a 1/2" hole. Or, am I wrong and a 3/4 bit with get some intial grab with the 8mm?

Bobby Hutton · Oct 15, 2021 · Grizzly Flat, CA · Joined Aug 2013 · Points: 1,153

I think that might be an error on team tough's part? On Jim's site it says 16mm which is roughly 5/8".

I have installed a few hundred of the 8mm twist bolts in 5/8" holes. Interference is fairly light. Very similar to the 6mm variety in 1/2" holes. We did a dozen or so tests with the 8mm twists in 5/8" holes. It was really hard to figure out how to make the bolts fail. All failed at at least 50 kn iirc.

timothy fisher · Oct 15, 2021 · CHARLOTTE · Joined Nov 2017 · Points: 30

Yes there are a few discrepancies on TT site

Use a 5/8 bit for 8mm twist. It is vurtually the same as 16mm.

mattm · Oct 16, 2021 · TX · Joined Jun 2006 · Points: 1,885

In my experience, the 8mm twists are much less tolerant of tight holes, especially in harder rock. The 6mm can conform a bit but the 8s are just stiff. Using an undersized bit like 5/8th is not recommended. Bobby seems to have had a different experience though... I bought a 16mm thinking that would be perfect and instead had all sorts of headaches with tight holes. NOT the same as the 1/2" bit with 6mm twist. I ended up getting 11/16th bits which work a lot better for me. Zoro sells them for not-too-bad pricing but you can find name brand 11/16th SDS on eBay for CHEAP.

Bobby Hutton · Oct 16, 2021 · Grizzly Flat, CA · Joined Aug 2013 · Points: 1,153

I will admit that I generally wallow the 5/8" holes out a bit. I generally don't need the interference fit. Even without the wallow I don't have much trouble driving them in with a rubber mallet. I don't install them in hard rock tho.

Keel Elan · Oct 16, 2021 · Berkeley, CA · Joined Jan 2020 · Points: 241

Posting a question to this thread because it seems to have some knowledgeable participants.

According to Jim's website, the 6mm twist bolts call for a 12mm (=.47") hole. A friend is of the opinion that a 1/2" hole does NOT allow for enough glue to coat the bolt and bond with the rock. He prefers a 9/16" hole, which lets in a bit more glue.

Does anyone have any experiences that permit comparison? Is a 1/2" hole good (enough)? Is 9/16" better?

rocknice2 · Oct 16, 2021 · Montreal, QC · Joined Nov 2006 · Points: 3,847

Bolt Drill Hole Size

Please use the following table as an outline for bolt hole drill size.

Twist Bolts Drill Hole Dia. Min. Drill Hole Depth

6mmx80mm 1/2” 3-1/2″

6mmx100mm 1/2″ 4″

6mmx150mm 1/2″ 6″

8mmx100mm 5/8″ 4″

8mmx150mm 5/8″ 6″

https://team-tough.com/bolt-drill-hole-size

mattm · Oct 16, 2021 · TX · Joined Jun 2006 · Points: 1,885

Keel Elan wrote:

Posting a question to this thread because it seems to have some knowledgeable participants.

According to Jim's website, the 6mm twist bolts call for a 12mm (=.47") hole. A friend is of the opinion that a 1/2" hole does NOT allow for enough glue to coat the bolt and bond with the rock. He prefers a 9/16" hole, which lets in a bit more glue.

Does anyone have any experiences that permit comparison? Is a 1/2" hole good (enough)? Is 9/16" better?

Jim covers this in a lot more detail on his Bolt Products website under “Glue Line” but essentially, your friend is not correct. With twists, the glue line is perfectly fine with 12mm or 1/2” and numerous testing done by multiple parties has shown this to be more than true. How much north of 40kN does one need?

Even single leg bolts don’t require all that much oversizing. Raumer notes their 8mm SLBs can be used in a 3/8” or 9.5mm hole. Bolt Buster testing with Fixe Helys also supports this.

timothy fisher · Oct 16, 2021 · CHARLOTTE · Joined Nov 2017 · Points: 30

Jim started making his 6mm rod twist bolts a bit differently for the US maket at some point so that there is slight interference in a 1/2 inch hole.

You could drill a 9/16 hole for a 12mm twist bolt but it actually might reduce the holding power.

Read everything on the Bolt-Products page.

Jim Titt · Oct 16, 2021 · Germany · Joined Nov 2009 · Points: 490

Keel Elan wrote:

Posting a question to this thread because it seems to have some knowledgeable participants.

According to Jim's website, the 6mm twist bolts call for a 12mm (=.47") hole. A friend is of the opinion that a 1/2" hole does NOT allow for enough glue to coat the bolt and bond with the rock. He prefers a 9/16" hole, which lets in a bit more glue.

Does anyone have any experiences that permit comparison? Is a 1/2" hole good (enough)? Is 9/16" better?

Your friend is wrong, increasing the hole size makes them weaker.

Matthew Jaggers · Oct 18, 2021 · Red River Gorge · Joined Sep 2017 · Points: 695

Jim Titt wrote:

Your friend is wrong, increasing the hole size makes them weaker.

So an 8mm x 150mm twist in a 3/4" hole in a horizontal would still be plenty strong enough? There wasn't any interference fit, but the glue I use is viscous enough to hold everything in place while it was setting up.

Jim Titt · Oct 18, 2021 · Germany · Joined Nov 2009 · Points: 490

Sure, maybe 70 or 80kN.

Matthew Jaggers · Oct 18, 2021 · Red River Gorge · Joined Sep 2017 · Points: 695

Jim Titt wrote:

Sure, maybe 70 or 80kN.

Thanks for confirming. Ill be whipping this weekend, so ill let you all know if 70 is enough!

timothy fisher · Oct 20, 2021 · CHARLOTTE · Joined Nov 2017 · Points: 30

mattm wrote:

In my experience, the 8mm twists are much less tolerant of tight holes, especially in harder rock. The 6mm can conform a bit but the 8s are just stiff. Using an undersized bit like 5/8th is not recommended. Bobby seems to have had a different experience though... I bought a 16mm thinking that would be perfect and instead had all sorts of headaches with tight holes. NOT the same as the 1/2" bit with 6mm twist. I ended up getting 11/16th bits which work a lot better for me. Zoro sells them for not-too-bad pricing but you can find name brand 11/16th SDS on eBay for CHEAP.

I am curious how long a 8mm rod twist you were using in harder rock.

I always cut them down to 80 to 85 mm in hard rock. That could explain why I have had no trouble with the 5/8 bit even in the super hard Stokes County quartzite. In that rock I am only using those bolts for replacement after core drilling or removing a large self drive bolt.

I do usually ream the first 1/2" of all holes to eliminate ovality from hard hitting drill and or 2 tooth bit.

So I have to ask everyone how soft does rock have to be that the 12mm twist x 150 mm isn't good enough?

Would angling the bolt upward like an ice screw help with surface problems in the super soft?

I have placed lots of 12 x 100 mm in softer sandstone without issues. Maybe I just havent experienced or wanted too, that kind of softness?

Nikhil Jain · Oct 26, 2021 · Unknown Hometown · Joined Oct 2021 · Points: 0

I have a customer asking if we can do Ultra Sonic testing on bolts, I have no idea on what size these would be, or the methods or limitations. Any ideas? Please, any information would be helpful.

Matthew Jaggers · Oct 27, 2021 · Red River Gorge · Joined Sep 2017 · Points: 695

70kn is plenty, btw!

Nikhil Jain · Dec 30, 2021 · Unknown Hometown · Joined Oct 2021 · Points: 0

Matthew Jaggers wrote:

70kn is plenty, btw!

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