WPI Executive Board of Directors: Statement on Aftermarket Super Stock and Pro Stock Engine Blocks

(Updated February 4, 2023: Bore Spacing Specifications)

The WPI Executive Board of Directors voted to clarify rules related to dimensions and configuration of aftermarket recast and billet engine blocks used in Super Stock and Pro Stock Divisions. Rules are specific to any legal block with a cylinder bore spacing of 5.875-inches or less at Grand National level only. The only change from rules printed in 2023 NTPA Pulling Rules is to allow altering of head bolt position based on engine block bore spacing.

All other divisions and competition levels where a recast or billet aftermarket block is allowed are not affected. Those divisions will continue to adhere to all stock OEM block and cylinder head specs including OEM head bolt position as found in 2023 NTPA Pulling Rules.

The following is a summary of rules currently found in 2023 NTPA Pulling Rules with the addition of allowance to alter original OEM head bolt position only.

1. GN Super Stock and Pro Stock Engines (5.875-inches or less bore spacing)
   1. Specifications for block to be based on factory OEM specs of the block chosen to be replicated.
   2. To be considered a legal block:
      1. Must swing a stock OEM crankshaft and retain stock OEM bore spacing of make and model OEM block replicated.
      2. Overall height, width, and length of block must be same dimension as stock OEM block replaced.
         1. Super Stock block not to exceed 5/8-inch total increase to OEM deck height of block measured from centerline of crankshaft. An additional .130-inch head gasket is allowed not to exceed 540 maximum cid. No deck plate or additional block deck height allowed on Super Stock engines over 540 cid.
         2. Pro Stock block not to exceed 1-inch total increase to OEM deck height measured from centerline of crankshaft to cylinder head surface including any gaskets and/or spacers.
      3. Crankshaft location must be same as stock OEM block replicated.
      4. Alteration to stock OEM head bolt pattern allowed including quantity, size, and position of head bolts attaching cylinder head to block. This is the only exception to OEM gasket requirement. All other OEM dimensions, specs, and component position must be maintained as described in previous rules.
      5. Manufacturer must submit aftermarket block for approval by NTPA Technical Services before being allowed to compete.
   3. OEM Block Cylinder Bore Spacing
      1. The following specifications are the accepted cylinder block bore spacing dimensions accepted by WPI/NTPA when replicating a stock OEM block for use where legal in NTPA competition.
         1. Big Block John Deere 619
            • 6.000 inches
            • Cylinders 1 thru 6
         2. Small Block John Deere 466
            • 5.375 inches
            • Cylinders 1 thru 6
         3. IH DT466/530
            • 5.330 inches
            • Cylinders 1 thru 6
         4. Detroit 40 Series 466/530
            • 5.330 inches
            • Cylinders 1 thru 6
         5. Case 504
            • 5.563 inches
            • Cylinders 1 thru 6
         6. Ford / New Holland 401/456
            • 5.130 inches
            • Cylinders 1 thru 6
         7. Allis Chalmers 426
            • 5.188 inches
            • Cylinders 1-3 & 4-6
            • (Center Space – 5.563 inches)
         8. White/Oliver (Hercules 478)
            • 5.313 inches
            • Cylinders 1-3 & 4-6
            • (Center Space – 5.875 inches)