Next, tap the lifter crossover galleries at the rear of the cam tunnel. These are for oil restrictors to limit the amount of oil that gets to the heads. For this application, Troutman recommends a 5/16-18 tap. These galleries can be difficult to reach without the right equipment, but it can be done.
Restrictors are simply pipe plugs with holes drilled through the center. The plugs can be bought for pocket change at your local hardware store, and drilling the holes yourself allows you to control exactly how much oil gets to the cylinder heads. For standard flat-tappet lifters, Troutman recommends a hole 0.060 inch in diameter. If you are using lifters with EDM oiling holes or grooves cut in the lifter bores to aid oiling to the lifter face, the increased oil demands usually require a 0.080-inch diameter hole. You must secure the restrictor with red thread locker. You should also tap and plug the ends of the lifter galleries (the three holes are visible underneath the cam tunnel).
You can also use commercial restrictors such as these, but they cost more money. They also screw into the back of the gallery and double as a plug. The problem with these plugs is that the O-ring seals can leak, and you have to be careful not to insert the plug/restrictor too far because it may interfere with the distributor gear.
The oil gallery openings at the back of this block have been properly plugged. All have had thread sealer applied to prevent leakage.
The same process must be repeated at the front of the block. Here, the oil galleries are being tapped with the same 1/4 pipe tap. Do not tap the center gallery (directly above the cam tunnel) too deeply because the plug can partially block the crossover galleries that feed the lifter bores.
The stock block was intended for use with a single-chain timing seat. Going this route is fine, but many racers prefer to upgrade to a double-width timing chain for extra durability. This is a great idea, but it can cause interference with the front of the block. If it isn't caught, the chain will rub against the block, severely weakening the timing chain. The mark on the front of this block is from such an occurrence. This often happens when a cam button is used to help minimize walk with a roller cam. The button pushes the cam toward the back of the block, which also pulls the timing set into the front face of the block.