Author Topic: Correct finish on brake booster bracket-to-firewall nuts and lock washers?  (Read 2978 times)


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There are 4 hex nuts (size 3/8"-16) with lock washers that mount the power brake booster to the firewall.  On UPC J50/A2 of the1968 FAIM, the bottom two nuts/washers are listed as "PRODUCTION NUT & L. WASHER."  The upper two are GM part numbers 9418931 (nut) and 103321 (lock washer).  Are the bottom two nuts/lock washers different than the top two?  What finish are these fasteners, phosphate (gray - black) or zinc (dark silver)?  The nut is not in the AMK Products catalog and the lock washer is available in both finishes. 

My gut says phosphate, but then why is the screw (3848408) zinc plated on the zinc dichromate bracket for the proportioning valve (UPC J52/A2 of the 1968 FAIM)?  Is it because it contacts the zinc dichromate plating?  If so, then shouldn't the 4 nuts/lock washers be zinc plated?

Any help would be appreciated.  Thanks in advance!
68 NOR SS/RS L48/M35 Convertible
Cordovan Maroon - Special Paint Exterior (black stripes)
Black Custom Interior
Silver Concours (Legends Class) at 2012 Camaro Nationals, Gold Concours at 2014 MCACN.


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I know this may cause controversy - but my late 69Z's both have gold iridite finish just like the booster on the nut and lock washer......

I have seem others phosphated as well......
Eddie  Montini