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 identification.The most widely 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 believed 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 course! But,they were not fuel injected nor were they considered high performance in their day.Castings were very often 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 cash 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 staggered 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 brackets 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 refered to as the "Transition Heads". Keep in mind that threaded holes were changed at times through out the years to work with various applications. Often with different bolt holes sizes,mostly 3/8" sometimes 7/16" holes.
There were some small block Chevy heads that featured an extra bolt hole on the exhaust flange,this hole was used for power steering pumps and such. Something to keep in mind when selecting cylinder heads for your application is combustion chamber size. Chambers are measured in cubic centimeters (cc's). The smaller the chamber size,the more the compression ratio.You may run into a set of heads that have been "milled" to increase compression. It is always a good idea to check cc's or have them checked by your friendly neighborhood machine shop. The 67 chevy head had no threaded hole for the temperature sending unit. On these engines the sending unit was installed on the intake manifold water jacket. In 68 the boss was cast into the head for the sending unit but,was generally not drilled out or tapped.
Some important notes:
Small block Chevy heads had a staggered bolt pattern for the valve cover until some time during the 1959 model year.
The 1969 head was drilled and tapped to accommodate for new accessories as a result of the new style of water pump.
External casting marks can be very mis-leading. Humper heads the 461,462 and 291 all bore the Humper casting.
Small block Chevy heads began using tapered seat spark plugs in 1971 reducing the wrench hex size from 13/16" to 5/8"
During the 68 model year they started drilling and tapping the temperature sending unit into the cylinder head.
