Taper Pins
For Wing Spar Fitting Connections
(Mystery Removed)

[ Return to Home Page ] [ Created 6/25/2009 ... updated 11/17/2009 ]

Click on Drawing and Photo for Larger and Complete  Image

Photo: Courtesy of Aircraft Spruce

Photo: Courtesy of Matronics Wiki  a Wikipedia for Aviation
One final point, my friend Chris Bobka points out that taper pins are a handy solution to fastening problems.  He will be using taper pins to repair a planer.  Chris recommends having a few extra taper pins on hand.
Shorly after joining the UltraCruiser Yahoo discussion group, one of the discussion topics was about using Taper Pins for connecting the wing fittings.  Being a glider pilot and having owned a Schweitzer 1-35c, I have great respect for taper pins over bolts and close tolerance bolts.  After much consideration, I decided to use taper pins in place of bolts.  Since I didn't know how to proceed, I contacted friends with more knowledge and researched taper pins on the web. Advantages:  The taper pins are very easy to insert (important when trying to align a heavy 22 foot glider wing) and easy to remove (a slight tap, and they pop out).

Disadvantages of bolts:  If tight, very hard to insert.  Inserting can cause wear.  If slightly loose, the hole will eventually elongate 
RV builders/owners are replacing their nose gear bolts with taper pins LINK due to elongation of the bolt hole.
Before making the choice to use tapered pins, you must determine:

- Does the taper pin you select have the required tensile strength?

- After reaming the tapered hole, do you have sufficient edge distance?
S. Steve Adkins  Email

As always, click on photo for larger and more complete image
I made a test run on scrap aluminum which exactly matched the composition of all the connecting pieces in the main spar fittings as shown on the plans, sheet 6 (even including a piece of .020 sheet).  Here is how I did it ... For the fittings on the main spar:  
Clamp the fittings in place ... block up to ensure clear space below the hole.
Drill a pilot hole, then ....
For the #3 taper pin, drill a 5/16 inch hole (which happens to be the size on the plans)
... if you have already drilled your fittings, the previous steps are complete.
Replace the drill bit with a #3 reamer.
Carefully advance the reamer until you can feel it at the bottom of the hole.
Then advance the reamer another 1/8 inch.
Remove reamer, turn off power, and slide fitting to one side.
... insert taper pin and count the threads sticking out the bottom.
Calculate how much further you must advance the reamer.
Make advances in small steps ... a bit too far ... and your fittings will be ruined.
As a final note, the taper reaming of the hole is very sensitive.  If you take off just a smidge too much, then the pin will drop into the hole a long way....  You might practice once or twice....on scraps. (Chris Bobka)
When the desired fit of  the taper pin is achieved ... stop.
This image shows the finished hole
with the drilled taper pin special
washer and castle nut.

Note the indented portion of the washer.
Threaded end of taper pin extending
through joint with a small shoulder.

The special taper pin washer will fit
over the shoulder.

Taper pin with special washer and castle nut (shown finger tight).  A one-notch turn with wrench snugs down the bolt and aligns the castle nut with the bolt hole.

Be sure to install a cotter pin or "safety pin".

Notes: Do not over tighten.  If over tightened, the taper may cause undue stresses in the fitting material.  
Chris Bobka says:
Remember that any taper pinned joint has the potential to put an axial load on the hole that can be quite large if you tighten up the securing nut too much.  The idea is to have a perfect fit with minimum nut torque.
Taper pins are to be used only in "shear" situations.  All layers pinned together must fit tightly together.
The UltraCruiser wing fitting design is double shear with the single fitting on the center wing section (Sheet 6 of the plans).
AN386-3-14 ...
Nomenclature for the B&S taper pin
3-x is for a 1/4 nut.  An -x-6 is for 3/4" length which would be used to join a total thickness (grip) of 3/4 - 1/8 = 5/8".  The length you select should be 1/8" longer than the total thickness (grip) you are joining.  In other words, L = grip + 1/8". ... The length and the grip do not include the threaded part for the nut.....just like when you size regular AN bolts.....(from Chris Bobka)

Note:  The #3 pin is .317 inches at the narrowest end (wider than a 5/16 hole)
           The #3 reamer is .3099 inches at the narrowest end (narrower than a 5/16 hole)
Thus, thus a 5/16 inch hole ( .0312) would be the appropriate pilot hole 
   B&S #3 Taper Pin Reamer... Cat. No. 1-017-030
   B&S #1 or #2 Taper Pin Reamer ... Cat. No. 1-017-020
    JTS Machinery & Supply Co ... Reamers  
Reamer; Finishing; Brown & Sharp Taper; Straight Shank; HSS;
3 B & S Taper;   $26.35 

   four  AN386-3-14 Taper Pin ... note two sizes larger than my plan
   four  AN975-4 Taper pin washers
   four  AN310-4  Castle nut

   two    AN386-2-13  Taper Pins
   two   AN975-3         Taper Pin Washers

     From Aircraft Spruce  
    Taper pin  Taper Pin Washers  Castle Nuts  
My Costs:
$26.35       B&S #3 Taper Pin Reamer (one time only)
$  0       B&S #2 Reamer  (I will borrow from Chris for the rear spar.)
$35.20  Four Taper pins for main spar
$  9.90  Two taper pins for rear spar
$71.45   Total Cost plus shipping .... minus peace of mind!

Note: Pin costs can vary without logic with longer pins costing less
than shorter pins.  Select taper pin length to obtain the lowest cost.
Oversize taper pins are not a problem since there is plenty of room
for the taper pin to stick out on the front side of the spar fittings.  Also,
the extra length gives you something to hang on to.

B&S or Brown & Sharp Standard Taper ...
         (1/2 inch taper to the foot)
         (0.0416" taper per inch of length).
While drilling close tolerance holes, I discovered that my drill press had significant runout or wobble.  A check on the web found several complaints about the Ryobi Drill Press in this regard after drilling many holes (and the UltraCruiser requires a lot of holes!).  Testing with a dial indicator showed that the arbor is dead-on, so a new chuck was ordered.  While I was at it, I bought a 1/2 inch keyless chuck.

Dale Johnson loaned me his dial indicator.

Video on using Dial Indicators  another link for same video  

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Address of this webpage:  http://quid.us/hummel/taperpin.html
Copyright 2009, by S. Steve Adkins, all rights reserved.