Charles I do not show any kind of an answer for your question so I'll "... go for it". The first series auto 383, 413 and 426 wedge heads all had only four bolts/studs to hold on the valve covers. The covers leaked on the front and rear lower corners, the valve chambers were smaller and the rocker assembly was held on in a different manner. These heads were used from 1959 to the first series 1963 engines. >From the marine engine division they started in 1963 or the first of 1964 engine production using the different style rocker arm assembly and six bolt/stud valve cover heads on the auto engines. The extra two attachments were placed at the corners of the valve covers to address the leaking corner problem. ( Even though the leaking valve cover problem was addressed it was never completely fixed. ) This new set of heads was used into the 1968 year on all 383/413/426 and 440 engines. The new heads could also hold 6,500 rpm's. During this time period no Imperial was ever rolled out with the 383 or 426 engine. ( The 426 engine in either wedge or Hemi form was never used in the Chrysler and Imperial Divisions. ) During this period of time the lifters from all of the above engines used the same part number. The 413, 426 wedge and all 440's used the same push rods. The first series intake manifold from any 413, 426 wedge or 440 was used until the new style Carter came out. In short order form any 1959 - 1965 Imperial with the 413 or the 1966 - '67 with the first series 440 will perform better with the 1968 and newer heads and any factory intake manifold that you have. Rodger & Gabby Colo Spgs '47 De Soto Custom w/Fluid Drive & passenger door mounted swamp cooler '63 Le Baron w/F-Sweep trunk lid & std A/C '66 Le Baron w/dual A/C & reverbed AM/FM '70 Overlander - InterNational Dbl '77 D150 w/360, NP 4 Spd, 12 Bolt Axle, OEM Buckets & etc ----------------- http://www.imperialclub.com ----------------- This message was sent to you by the Imperial Mailing List. Please reply to mailing-list@xxxxxxxxxxxxxxxx and your response will be shared with everyone. Private messages (and attachments) for the Administrators should be sent to webmaster@xxxxxxxxxxxxxxxx To UN-SUBSCRIBE, go to http://imperialclub.com/unsubscribe.htm