[TowerTalk] Pinning mast?

TexasRF at aol.com TexasRF at aol.com
Tue Nov 8 05:58:50 PST 2011

Don't take this as a recommendation but rather as a data point: I have had  
great success with pinning masts using 4 X 3/8" grade 8 bolts. Two pinning 
holes  are drilled completely through the mast to size for a 3/8" coarse 
threaded tap.  Then the four holes are tapped as far as the tap will go. Some 
of them tap  completely through a 3" mast. The four bolt lengths are about 
half the mast  diameter and are screwed into the threaded holes and torqed as 
tight as possible  with a standard end wrench.
The mast adapter fits inside the mast and is made of solid material. The  
fit is pretty close, maybe a couple .001" clearance. The wind loads fall 
between  30 and 200 sq ft for the eme antennas. So far, there have been no 
failures,  thank goodness! The 200 sq ft antenna has been in service since 1992 
and the  others from 1999 and 2011.
This is a non reversible procedure. The 2011 antenna (32ft dish for 70cm)  
uses a recycled mast and rotator from a previous antenna. I had to cut the 
mast  at the rotator and have the remaining material machined off to reuse 
it. Stuck  like glue as they say. The rotators are all homebrew worm drive 
designs and  quite large/heavy, but that is another story.
Gerald K5GW
In a message dated 11/7/2011 10:25:35 P.M. Central Standard Time,  
grants2 at pacbell.net writes:

A good  question - even a pair of "slim" shims at 90 degrees apart should 
work  with the compression clamp.  These can be ~2x the thickness of a 
360  deg shim and easier to drive in.  Can the clamp be spread with a  
drift or N x chisels to help get the shims in?  Or maybe several  
threaded studs with slim nuts can be backed against the inside of the  
clamp flanges although these will need to be much smaller than the clamp  
bolts in order to get the nuts to fit.

The "problem" with pins is  getting the pin to hole clearance essentially 
zero - as in rivets in a  Boeing airplane.  Otherwise the mast and clamp 
work against each  other enlarging the hole.  The idea of a taper pin is 
reasonable  since they have zero clearance, but I wouldn't want to try 
that up the  tower with a #9 or 10 reamer ($$ and hard to do, step drill  

Since the K7NV mast clamp can be deformed in one plane, if  shims can't 
be placed the best bet is to add a couple of through bolts 90  deg to 
this plane.  Drill the holes a bit (1/16") undersized and use  a core 
drill exactly the size of the bolts.  A core drill will  essentially ream 
the hole to round and very close to diameter, even hand  held.  (but I 
think any pin solution is not as good as making the  clamp work +/- as 

Grant KZ1W

On 11/7/2011 4:46  PM, john at kk9a.com wrote:
> You didn't say what kind of mast clamp you  are using.  While pinning a 
> is good thing in many cases, I  think you need to correct the fit of the 
> clamp before going any  further.  My prop pitch has a 3" K7NV mast clamp 
> my mast is 2  7/8 diameter so I purchased as 3" diameter stainless steel 
> with a  1/16 wall and made a shim out of it.  The clamp then tightened to 
> mast securely.   Why won't yours shim  properly?
> John KK9A
> To:  towertalk at contesting.com
> Subject: [TowerTalk] Pinning mast?
>  From: K7LXC at aol.com
> Date: Mon, 7 Nov 2011 14:50:44 -0500 (EST)
>  List-post:<towertalk at contesting.com">mailto:towertalk at contesting.com>
>  Howdy --
>      I've got a sloppy fit between a  mast clamp and a  mast - both 
steel. It
> won't shim properly so I  think my only option is to pin  the mast. I've
> done it a bunch of  times and mostly they hold butr  sometimes they don't 
>  I'm
> proposing to use 2 in one direction and  another two 90  degrees from the
> first two with good-sized high strength  bolts.  Other than the challenge 
> doing it at altitude, is there anything  else  I should be considering? 
Or is
> there another way to do  it?
>      This is a prop pitch with big antennas  on it.
> Cheers,
> Steve     K7LXC
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