The 1970 Mack RS731LST
Second Unit Rubber Duck truck.
This project initially started in 2003 as an attempt to build a spot-on replica.  The
project started with a 1977 RSK700L ("K" means Factory Glider Kit) and a 1977
RS786LST.  I had spent the prior 5 years locating and assembling various trucks
and parts.  It was crucial to get every detail correct to match the Rubber Duck's
Mack RS700L.  From the longer driver's side Grover air horn, to the removal of
the bulldog from the mirrors, we had exhausted every resource to match detail for
detail the as-filmed Mack truck.

The problem is that with four different complete Mack's used as the Duck's truck
during filming, it is impossible to build a single exact replica.  To the untrained
eye, it may look as though there is a single truck, but as this website shows, there
are significant differences between the different RS700L's used and significant
variances on a single truck depending on when it was used in the sequence.  My
original intention was to produce a replica that has the look of the 1977 used for
many of the exterior shots.  After our purchase of the wrecked 1970 Mack
RS731LST that was used as the second unit truck, our plans changed somewhat.
 Using the parts gathered to build a replica as well as what new parts we could
get from Mack, we rebuilt the 1970 Mack RS731LST.  While the truck had
suffered severe damage when it barrel rolled as a mixer, we have incorporated
as many of the original parts back into the project as possible.  In doing so,
however, I felt it was important to more closely match the 1970 RS731LST to the
purpose-built 1977 Mack RS712LST that was built by Mack for EMI Films.  In
taking that charge, we made several updates to the effort, including using a
tip-turbine style Donaldson dual tube air cleaner and '73 & up style cab.  After
consultation with the state officials, the 1970 Mack VIN RS731LST6646 has been
declared rebuilt and passed inspection for licensing.  The rebuilt truck now
proudly bears Missouri Personalized Historical Plate RBR-DUK.

This Second Unit truck and its twin (VIN 6648) both came from the factory with
spoke wheels.  When they were purchased in April 1977 to replicate the new
1977 RS712LST, the spoke wheels and hubs were replaced with disc wheels.  In
the case of this truck, a set of Rockwell SQHD axles on White-brand Velvet Ride
suspension were used to replace the factory Mack Camelback suspension and
axles.  The swap was likely made in the interest of least-effort.  This truck retains
a Velvet Ride suspension which itself remains a source of inquiry and study for
old truck fans.  

Although the 1977 truck had a Mack Tip Turbine engine, the second unit truck
had a Cummins 855 block engine.  The truck now currently has an updated
Cummins NTC400 BCIII.  After careful consideration, I am working towards
eventually replacing the Cummins with a fresh Detroit Diesel 8V71T so that the
replica will also have the sound of the truck in the movie.  I know the purist will
have issues, but the Detroit sound is such a pronounced part of the experience
of the movie, I believe that it is important to try to present a truck that looks AND
sounds like it just drove off of the screen.

Now that we have purchased the original 1st Unit Truck and brought it back to St.
Louis, the question comes up about what our plans are for this truck.  Is it for
sale?  Everything's for sale for the right price, but the right price is not cheap.  
We are debating on selling the tractor to some lucky collector or turning it into the
the 2nd Unit truck in its post-jailhouse crash appearance.  We'll wait on that for a
while, but its fun to think about.