Shown in the pics below are three pump assemblies - the first one is the Buick Olds style pump stamped with the "R" code for Buick - Olds and the second style pump is the Chevrolet pump stamped "S" for Chevrolet - the third assembly is for Chevrolet pump with relief valve and is coded "Y" and was
used on 1969 427 Impalas with the "RED" Pressure Relief Valve
Note the pump stamped "R" for Buick and Olds is threaded on the bottom mounting position without the metal spacer between the front housing and rear backing plate and is not threaded at the top mounting position - short bolt is used at bottom threaded position and the long bolt is used on the non threaded top position - I removed this pump from a 1968 Buick grandma type car at the junk yard
The Chevrolet pump stamped "S" for Chevrolet is just the opposite - The bottom position is not threaded and uses a metal spacer between the front pump housing and rear backing plate and uses the long bolt - the top position is threaded and uses the short style bolt
ALL 1968 Chevrolet and some special application 1969 Chevrolet pump assemblies with the Pressure relief Valve on top of housing are stamped with a different code such as "Y" used on the 1969 427 impala dated smog pump below
There some codes such as "Z" and a few others stamped on housings that I have found but were already removed from the vehicle and I don't know what applicaiton they are for
USER - 3965175
