Prior to engines 3A-8330, 4A-19529, 6A16283, 8A-133, 12A-1345 and 24A-140, the cylinder blocks were smooth (ungrooved) on their top surfaces, A conventional type one-piece cylinder head gasket was used. The gasket contained the compression gaskets and small water and oil seals grommets within the one assembly. A separate four-piece oil seal gasket was used to seal around the outer edge of the block and cylinder head (see picture below).
The above cylinder blocks were known as the "Low" blocks and are approximately 1/16" lower than the current "High" blocks as measured from the top surface of the block to the centerline of the crankshaft. The difference in height between the current and former blocks is a result of the change in the sealing arrangement between the cylinder head and block. Due to this difference in height, shorter camshaft intermediate bearing lock screws are used with the "Low" block (see picture below).
Cylinder liners are installed in the "Low" cylinder block with the top of the liner flange .002" to .006" above the top of the block. Shims are available to permit adjustment of the liner to these specifications. Liners are installed with the top of the liner flange below the top surface of the "High" block. Cylinder liner inserts are used for adjusting the liners in the "High" block.
Cup plugs are used in place of drive pins to plug the vertical oil gallery openings at the four corners of the cylinder block. Also 5 3/4" long cylinder head studs are used with the "Low" block whereas the studs in the holes in the "High" block are 6 3/8" long. The cylinder head stud holes in the "High" block have been drilled and tapped 5/8" deeper than the studs holes in the early design blocks to permit the use of the longer studs. The new stud holes are counterbored 1/2" deep rather than countersunk. This minimizes extrusion of the block around the studs when the cylinder head nuts are tightened to their specified toque.
Clean-out holes are provided for the air box just below the hand hole covers on the side opposite the blower. The 1/4" pipe plugs amy be removed from these tapped holes when necessary for draining cleaning solvents from the air box. These clean-out holes have been discontinued on the current cylinder blocks since redesign of the air box drain passages makes them unnecessary.
Clean-out wires (striggle pins) were used in the connecting passages between the air box and grooves in the end of the cylinder blocks on engines 371-1986, 471-3116 and 671-3679 to 3A-3710, 4A-4133 and 6A-5647. These clean-out wires must be removed whenever the end plates are removed from the block and saved for reassembly purposes.
The current "High" cylinder block and its sealing arrangement replaces the former "Low" block and its sealing arrangement for service.