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Rocker arms  casting number located on side of rocker. If there is no number the rocker arm is an aftermarket  replacement and is 1.43:1 ratio

  

EAN rocker is 1954-1955 1.43:1 ratio

5751066 is 1958 - 1964 1.43:1  ratio

1.54:1 HIGH RATIO ROCKERS ARE ECG 6564

PART#         ROCKER ARM RATIO                  VALVE DIAMETER                    PUSH ROD LENGTH (Effective)

EAN                54-55 jamb nut     1.43:1                EXHAUST all 1.51"                1954-55 3/16" radius ball 8.11"

ECG 6564     56 jamb nut            1.54:1                INTAKE 1954 1.64"              1956-59  3/16" radius ball 7.98"

ECG 6564    57 self locking      1.54:1                 INTAKE 1955-56 1.78"        1960-64  3/16" radius ball 8.11"

5751066      58-64 self locking 1.43:1                INTAKE 1957-59 1.93"

                                                                                    INTAKE 1960-64 1.64"

VALVE STEM DIAMETER                      VALVE SPRING INSTALLED HEIGHT                           VALVE LENGTH

 11/32" single groove keeper.                                        1.75" to 1.78"                                                              5.090" O.A.L.

.3410"-.3415"

OIL PUMP DRIVE             FIRING ORDER         CYLINDER NUMBERING    DISTRIBUTOR ROTATION 

                                                                                                  #5 in front of #1

" Hex X 8.060"                  1-5-4-8-6-3-7-2             front        1 2 3 4                                CCW                                                                                                              5 6 7 8