What's the difference between ...291 and ...462 head castings? - NCRS Discussion Boards

What's the difference between ...291 and ...462 head castings?

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  • Duke W.
    Beyond Control Poster
    • January 1, 1993
    • 15610

    What's the difference between ...291 and ...462 head castings?

    I'm working with a guy on a small block engine configuration. He has a set of 3917291 head castings, which according to the NCRS Spec Guide are one year only use on '68 300 and 350 HP engines. I thought they might have accessory bracket bolt holes, but he said no.

    So my question is: What's the physical difference, if any, between these ...291 castings and the prior 3890462 castings used on '67 300 and 350 HP engines?

    One clue might be that the '68 300 HP advertised CR dropped to 10:1. It was 10.5 thru '66, then dropped to 10.25 for '67 because of the elimination of the small spark plug side quench zone on 462 heads, which increased head chamber volume by about 2 cc to 62-63 cc with the standard 1.94/1.5" valves, and I thought that chamber volume with the standard valve sizes carried over to later big port head castings through '70.

    Duke
  • Edward B.
    Expired
    • March 29, 2013
    • 691

    #2
    Re: What's the difference between ...291 and ...462 head castings?

    Duke, according to CBTN, the 291 is identical with the 462 except it has the temperature sending unit boss and the valve guide boss diameter was increased to .410 inch.

    Ed

    Comment

    • Duke W.
      Beyond Control Poster
      • January 1, 1993
      • 15610

      #3
      Re: What's the difference between ...291 and ...462 head castings?

      Great... thanks for the info.

      Duke

      Comment

      • Joe L.
        Beyond Control Poster
        • February 1, 1988
        • 43193

        #4
        Re: What's the difference between ...291 and ...462 head castings?

        Originally posted by Duke Williams (22045)
        Great... thanks for the info.

        Duke
        Duke-----

        Small block accessory bolt holes on the head ends with attendant casting flats were first added for the 1969 model year. The strange thing is that the casting numbers for these heads imply a 1968 model year release but they were not used for any 1968 application I am aware of.
        In Appreciation of John Hinckley

        Comment

        • Mark E.
          Extremely Frequent Poster
          • April 1, 1993
          • 4498

          #5
          Re: What's the difference between ...291 and ...462 head castings?

          Originally posted by Duke Williams (22045)
          ...with the standard 1.94/1.5" valves...

          Duke
          Don't L79, L46, L82 engines have 2.02/1.60 valves?
          Mark Edmondson
          Dallas, Texas
          Texas Chapter

          1970 Coupe, Donnybrooke Green, Light Saddle LS5 M20 A31 C60 G81 N37 N40 UA6 U79
          1993 Coupe, 40th Anniversary, 6-speed, PEG 1, FX3, CD, Bronze Top

          Comment

          • Duke W.
            Beyond Control Poster
            • January 1, 1993
            • 15610

            #6
            Re: What's the difference between ...291 and ...462 head castings?

            Yes, but I was talking about a 300 HP engine; '64 461 and later head castings used on Corvette engines could be processed for either the "standard" 1.94/1.5" or 2.02/1.6" set used on SHP/FI engines, so the finished machined heads had different part numbers due to the different valve sizes.

            The larger valve heads also have a chamber relief machined using a 2.34 or 2.40" diameter cutter centered on the inlet valve guide to unshroud the larger inlet valve.

            These castings, if originally machined for the standard valve size, can be opening up in the field for the larger set, however I don't recommend this. Heads processed for the larger valves have more tendency to develop a crack between the valve seats. The best flowing massaged heads I have in my database actually still have the standard valve size. The owner went through three iterations of massaging and flow testing. He did a great job.

            Also, if standard valve size heads are processed in the field for the larger valve set without the unshrouding cut, they actually flow worse than with the standard valve size.

            Depending on piston and camshaft choice, it can be a good idea to do the unshrouding cut on standard valve size heads to keep the CR from being to high. I have guidelines for each cam for 93 PON fuel and recommend dropping the target maximum 0.1 point for each point in octane below 93, so for a "California" configuration I recommend a target maximum 0.2 point below the recommended max for 93 because 91 is the highest commonly available octane in California

            Duke

            Comment

            • Joe L.
              Beyond Control Poster
              • February 1, 1988
              • 43193

              #7
              Re: What's the difference between ...291 and ...462 head castings?

              Originally posted by Duke Williams (22045)
              Yes, but I was talking about a 300 HP engine; '64 461 and later head castings used on Corvette engines could be processed for either the "standard" 1.94/1.5" or 2.02/1.6" set used on SHP/FI engines, so the finished machined heads had different part numbers due to the different valve sizes.

              The larger valve heads also have a chamber relief machined using a 2.34 or 2.40" diameter cutter centered on the inlet valve guide to unshroud the larger inlet valve.

              These castings, if originally machined for the standard valve size, can be opening up in the field for the larger set, however I don't recommend this. Heads processed for the larger valves have more tendency to develop a crack between the valve seats. The best flowing massaged heads I have in my database actually still have the standard valve size. The owner went through three iterations of massaging and flow testing. He did a great job.

              Also, if standard valve size heads are processed in the field for the larger valve set without the unshrouding cut, they actually flow worse than with the standard valve size.

              Depending on piston and camshaft choice, it can be a good idea to do the unshrouding cut on standard valve size heads to keep the CR from being to high. I have guidelines for each cam for 93 PON fuel and recommend dropping the target maximum 0.1 point for each point in octane below 93, so for a "California" configuration I recommend a target maximum 0.2 point below the recommended max for 93 because 91 is the highest commonly available octane in California

              Duke
              Duke------


              One other thing that's often forgotten is that 1962-63 FI and L-76 engines used the 1.94/1.50 "standard" size valves.
              In Appreciation of John Hinckley

              Comment

              • Duke W.
                Beyond Control Poster
                • January 1, 1993
                • 15610

                #8
                Re: What's the difference between ...291 and ...462 head castings?

                Yes, the 461X heads were never processed for the larger valve set. They didn't show up until '64 when the 461 replaced the 461X. I recall that Bill Clupper recommended against increasing valve sizes along with the unshrouding cut on 461X heads, so apparently the 461s had more material behind the seats and chamber to accommodate the larger valves and unshrouding cut.

                Duke

                Comment

                • Joe R.
                  Extremely Frequent Poster
                  • March 1, 2002
                  • 1356

                  #9
                  Re: What's the difference between ...291 and ...462 head castings?

                  Hi Duke:

                  I'm away from home and don't have my Judging Guide available, but I'm pretty sure that the '291 head started to appear on late production '67 Corvettes.

                  Another post mentioned the addition of the temperature sensor boss under the exhaust manifold. Interestingly, that boss (un-drilled) first appeared on the '462 casting around March 1967. I have some March '67 '462 heads without the boss, and some other March '67 '462 heads with the boss. I find it interesting that the casting number was not changed when the boss was added.

                  Comment

                  • Nick C.
                    Very Frequent User
                    • August 31, 1998
                    • 542

                    #10
                    Re: What's the difference between ...291 and ...462 head castings?

                    Originally posted by Joe Randolph (37610)
                    Hi Duke:

                    I'm away from home and don't have my Judging Guide available, but I'm pretty sure that the '291 head started to appear on late production '67 Corvettes.

                    Another post mentioned the addition of the temperature sensor boss under the exhaust manifold. Interestingly, that boss (un-drilled) first appeared on the '462 casting around March 1967. I have some March '67 '462 heads without the boss, and some other March '67 '462 heads with the boss. I find it interesting that the casting number was not changed when the boss was added.
                    Have you found any coincidence with "temperature sensor boss" on square port BB heads, i.e. 840 used in late production?

                    Comment

                    • Joe R.
                      Extremely Frequent Poster
                      • March 1, 2002
                      • 1356

                      #11
                      Re: What's the difference between ...291 and ...462 head castings?

                      Hi Nick:

                      I am not familiar with the evolution of big block heads, so I can't comment on that. Regarding small block heads, can you clarify whether the 1967 JG mentions the possibility of '291 heads appearing on late production small blocks? Somehow I have seen or heard that somewhere, but I'm not sure where.

                      Comment

                      • Nick C.
                        Very Frequent User
                        • August 31, 1998
                        • 542

                        #12
                        Re: What's the difference between ...291 and ...462 head castings?

                        Originally posted by Joe Randolph (37610)
                        Hi Nick:

                        I am not familiar with the evolution of big block heads, so I can't comment on that. Regarding small block heads, can you clarify whether the 1967 JG mentions the possibility of '291 heads appearing on late production small blocks? Somehow I have seen or heard that somewhere, but I'm not sure where.
                        Yes, page 101 of the 7th edition TIMJG and also page 88 of the Spec Guide.

                        Comment

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