Small Block Chevy Cylinder Head Casting Numbers

The small block Chevy head can be one of the most mis identified component of any Chevy part.Although the same basic design has been used since 1955,there are some very significant changes that are difficult to notice at first glance without taking the valve covers off. One of the biggest mis-understandings come from the external markings on the outside of the casting,which many people use for indentification.The most widly known is the "Double Hump","Camel Backs",and "Humper".This mark is found on many medium and high performance heads of the 1960'sMany times this mark has been mistakenly beleived to be the scarce "fuelie head" that was originally installed on the fuel injected Corvettes.Truth be told however,the Double Hump casting was used on three different heads,including the 275 hp 327 that was installed on thousands of passenger cars sold in North America from 1966-68.These were excellent engines,being Chevy of coures! But,they were not fuel injected nor were they considered high performance in their day.Castings were very offten used on more than one application,and they were fitted with both large and small valves depending on the application.This goes to show that the Double Hump head may have either large or small valves even after you have verified the casting numbers,depending on the performance level of the vehicle the engine was installed in. When shelling out your hard earned dollar for a set of "Humper"heads just bear in mind the info posted here.More importantly,external casting marks should by no means be used as positive ID for any small block Chevy head.

From 1955-59,the small block head used a stagered valve cover bolt pattern,meaning the holed did not line up with the ones directly across for one another.Sometime late in the production year,Chevy made them so that they were across from each other and parallel.With the 1960 and newer heads,the valve covers can be swapped end for end and still bolt up.There are bound to be some exceptions to this rule but,generally you can trust it.Once you know the difference,it's easy to spot the wrong vintage heads on a vehicle.

 In 1969,Chevy switched the accessory mounting to the front of the cylinder head.A new longer water pump was also introduced and the alternator was moved over to the passenger side of the engine.This new configuration necessitated new brakets and mounting locations.So,it's no suprise that a new casting was required for 1969.There are some of the pre 69 castings that feature the bosses for the new location of the accessories,most notably the truck head with the casting number 3917293 which have the bosses at one end of the head but,they are not drilled and tapped to accept bolts.These heads are usually reffered to as the "Transition Heads"

More Later,have to manufacture some more time.

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