RC Crawler How-To
1 x 6 wood
One piece at 14-1/2"  You could narrow that a little
Two pieces at 4-1/2"
Here the side pieces are slotted for the arm holes
I used a 3/8" slot for mine at 1-5/8" long
Depending on the width and thickness of the arm
material, the slot size may need adjusted
Now to bevel the slot.
Set the depth so about an
1/8" is left un-beveled.
For later on, I made my swivel joint to arm
adapters out of 1/2" x 1/2" x 1" aluminum.

For most of my aluminum, I get it at
or else http://www.yarde.com/cgi-bin/dropzone.pl
On my early ones, I scribed the center.  I later on made a jig piece to it much easier and faster.  The ends are tapped for a 5/16" bolt thread and the side
for a 1/4" bolt thread (mainly due to using a 1/4" x 1/2" long bolt, couldn't find a shorter 5/16" bolt less than 3/4" and didn't want to cut them down.
I made special motor mounts out of 1-1/4" x 1/8"
square aluminum tubing for the 150 rpm 12 volt
motors found at http://www.surpluscenter.com

The motors had metric bolts, so I drilled them out
bigger to slip 8-32 x 1" machine screws thru
Here, the 5/16" swivel joints
from http://www.mcmaster.com
item 6072K63 are welded to
the motor shafts.
Parts list:

For either Mk II or Mk III version

Transmitter - search on ebay for Vex transmitter, All
 Electronics is sold out
Receiver - I used a GWS 75mhz receiver on channel 89
Motors - SurplusCenter.com items 5-1581-L and R
2 ESCs - Traxxas XL-1, these seemed the most quiet for the
Battery - 7.2 volt RC car type
Battery Y splitter
swivels - McMaster.com item 60645K131
aluminum metal - got mine from onlinemetalsupply.com as
the yarde.com dropzone minimum has gotten too high.
For the upper arms I used 5/16" x 1-3/8" last time I
ordered.  The motor mounts was 1/8"x1-1/4"x1-1/4"
square tubing.  The swivel to arm adapters were 1/2"
square aluminum bar stock.  You may need to adjust.
1 x 6 wood
some 8-32 machine screw and nylok nuts

Mk III version
additional parts only
forearm swivels - McMaster.com item 60645K431
5/16 - 24 x 2 feet threaded rod 90032A030        
This was the last of the pics I
took when I was building some
the last time.