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Powers 5-piece 3/8 inch vs 1/2 inch

Original Post
Taylor Krosbakken · · Duluth, MN · Joined Nov 2008 · Points: 673

Some questions on the differences between 1/2 in and 3/8 in 5 piece expansion bolts.
 
So here is what I have concluded, tell me if I am wrong.
 
1/2 inch bolt advantages:
Stronger
Does better in softer rock because of a less concentrated force.
 
3/8 inch bolt advantages:
Cheaper $7 (compared to $14 for 1/2 in) still about the same as a glue in.
Smaller hole to drill (faster)
Strong enough
 
So looking at the strengths of these bolts in different strength concretes, the 3/8 in bolts lose almost half their strength going from 41 MPa compressive strength concrete to 14mpa, while the 1/2 in only goes down 20% in strength. Concluding the 1/2 in do better in softer rock.
 
Now the only rock I could find even as low as 20 MPa compressive strength is mudstone, with most sandstone, Dolomite, Limestone being at least 40 MPa compressive strength. So in most rock you are comparing 38.9 kN to 55.4 kN and 3/8 in are plenty strong.


So are all of my assumptions correct? Is the only good reason to use the more expensive 1/2 in bolts if you are bolting really soft rock? I think I have also seen using 1/2 inch bolts on a crux bolt that sees repetitive whips. Is that even necessary?

What am I missing? The funny thing is, I got some 3/8 in 5 -piece bolts in the mail, and I thought they looked teeny tiny, but the published data sure makes em sound plenty strong.

Thanks!
Bruce Hildenbrand · · Unknown Hometown · Joined Apr 2003 · Points: 1,220

3/8" 5-pieces are plenty strong.  We use the 3.5" ones at Pinnacles National Park for rebolting.  The rock quality there is a bit suspect and we have to hand drill so that is a good bolt for us.

Taylor Krosbakken · · Duluth, MN · Joined Nov 2008 · Points: 673

Ya I assume hand drilling 1/2 in would be terrible. That would be a good reason to not use them, but luckily I do not have to hand drill anywhere I am bolting. 

Matt Z · · Bozeman, MT · Joined Mar 2012 · Points: 170

You probably already know this, it's worth repeating for the masses:

The new Powers PB+ plated bolt in 3/8 is not suitable for climbing applications. The 304SS Powerbolt in 3/8 is suitable. The inner bolt on the new PB+ is only a 1/4" bolt.

Besides that minor detail, SS 3/8" sleeve bolts are fine. Most (certainly not all) 1/2" bolts are used for replacement of 3/8" and it makes sense to drill the hole out to 1/2" after spinning the old bolt and wallowing out the hole a bit. Bolt replacement is also another reason to consider using a 3/8" initially as the hole can get widened to 1/2" when the bolt reaches the end of its lifespan to take either a glue-in or a 1/2" sleeve.

Taylor Krosbakken · · Duluth, MN · Joined Nov 2008 · Points: 673

Yup. I am talking all stainless here. But a good reminder M Z.

Looks like the PB+ plated bolt in 3/8" are rated to 17.7 kN. Which isn't insta-death, but for a modern bolt is pretty poor.

And good thoughts on replacement.

Brandon Fields · · Boulder · Joined Apr 2016 · Points: 500

If you’re working with decent stone and are putting in a new route, I’d reach for the 3/8in almost every time. I’m a proponent of starting with a smaller hole to extend the life of the hole if it will be rebolted or to have as little impact on the rock as possible if it is never climbed again. The benefit of saving tons of money is just icing.
I’d save the 1/2 bolts for rebolting, bad stone and high traffic cruxes.

Timothy Fisher · · Unknown Hometown · Joined Nov 2017 · Points: 0

I think the 25kn standard is a good one. High quality 3/8 bolts are right there. I only use the Hilti KB3 wedge and of course the Powers 5 piece in 3/8. Goes without saying SS.

Ordinary stainless wedges in 3/8 do not satisfy my idea of a good permanent anchor.

For softer rock bigger is the right thing to do.

Matthew Fienup · · Ventura, CA · Joined Feb 2006 · Points: 7,296

In 2007, Fire Crags in Santa Barbara, California, suffered its second failure of a 3/8" x 3.5" five-piece sleeve bolt. The bolt pulled out of the wall during a routine lead fall. The sandstone at this particular crag is among the softest on the Central Coast, but the bolt failure was still surprising. 1/2" sleeve bolts are the standard for sandstone routes on the Central Coast.

I would encourage you to treat the numbers in the Performance Data table as a best case scenario. The density of rock at many climbing formations, especially sandstone ones, varies dramatically over space. Moving a bolt four feet might yield a placement in rock that is 50% more dense. Also, the quality of the holes drilled in the field is likely to be much lower than in a testing environment, resulting in very different pull out strengths.

Ward Smith · · Wendell MA · Joined Apr 2009 · Points: 295

If you are using a power drill then there is no reason not to use a 1/2 inch bolt other than cost.  I'm using mostly 1/2 inch wave bolt glue ins in hard rock (gneiss) and 1/2 inch power bolts only rarely.  Been bolting for 30 years, and the 3/8 stainless I put in long ago are still mostly good in hard rock but they do end up spinning and eventually need to be replaced.

Nick Drake · · Newcastle, WA · Joined Jan 2015 · Points: 637
Ward Smith wrote: If you are using a power drill then there is no reason not to use a 1/2 inch bolt other than cost.  I'm using mostly 1/2 inch wave bolt glue ins in hard rock (gneiss) and 1/2 inch power bolts only rarely.  Been bolting for 30 years, and the 3/8 stainless I put in long ago are still mostly good in hard rock but they do end up spinning and eventually need to be replaced.

I agree with your sentiment and think that's best practice. One exception I can see to not go right to a 1/2" glue in is if you're not 100% sure of the best location for a clip it's easier to remove and enlarge or patch the 3/8" hole later. For something that's steep enough that running it on TR is tough for some reason that might be a consideration. 

Guideline #1: Don't be a jerk.

Fixed Hardware: Bolts & Anchors
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