eyell -
I assume that the outside mirrors on your '51 GMC mount the same as on Chevy trucks. Here is info on mounting outside mirrors on both sides:
A reinforcement plate was installed at the factory on the drivers side and the passenger side to mount the mirror arms. The reinforcements are above a ledge on the left and right inner cowls and difficult to see without a mirror or sticking your head far up behind the instrument panel. Remove the glovebox case to see the passenger side. They are predrilled to accept the bolts for the mirror arms. You may see nuts attached to the reinforcements; or the holes may be threaded; or the bolts may require nuts and lock washers - I'm not sure.
I feel that it is difficult to drill through the outside of the fender to hit the holes in the reinforcements. Other AD truck owners suggest inserting a
spring punch through the correct two holes in the reinforcements and snap it to make bumps on the outside of the fender. Then set the mirror bracket over the impressions to fine tune the location to drill the holes in the fender. I agree with the suggestion.
A previous owner drilled from the outside of the drivers fender on my '54 and missed one of the holes in the reinforcement. As a result I have a third hole in the bracket. Also, it looks like the previous owner missed both "factory" holes in the reinforcement bracket when he drilled holes for the passenger mirror. You can see the factory holes in the passenger reinforcement in the photo below and you can see the bolts that were inserted into the incorrectly drilled holes.
> Passenger side reinforcement seen through the glovebox> Closeup view of the passenger mirror reinforcement Bolts are in holes that were incorrectly drilled. "Factory" holes are visible.
> Drivers side mirror reinforcement with mirror screws removed(Tim Lederman)
> Incorrectly bored hole though drivers renforcement> FAM mirror installationMake sure that you have the correct mirror arms for your '51. The passenger side arm (RPO 210G) was the same for all years of AD trucks '47-'55. It has a screwy bend so that you can see the mirror through the lower corner of the windshield. The drivers side outside mirror arm for '47-'50 trucks is different than the arm for '51-'55 AD trucks because a vent window was added to the doors in 1951. The arm and mirror head were painted black.
> '47-'55 AD passenger mirror arm> Passenger mirror mounted> '47-'50 drivers mirror arm> '51-'55 AD drivers mirror arm> Correct angle of '51-'55AD drivers mirrorBetween you and me, the factory outside mirrors are pretty bad. The passenger mirror doesn't provide much coverage. If you have a side mounted spare tire, it hides the view from the drivers side mirror. An inside mirror is necessary in modern traffic.
EDIT:If you are referring to mounting an extended arm mirror, instructions are in the 1951 Accessories Installation Manual.
"5. For Left-Hand Installation: Remove existing mirror bracket and use existing holes in cowl"So, you are back to drilling the holes as I described above. I believe that Tim Lederman or Brad Allen has the template for drilling holes for the extended arm on the passenger side.
> #986440 MIRROR - Outside Rear View - Extension, TruckGood luck with your installation,
- Lonnie
