Corrective machining, which is covered in Chapter 3, can relocate the cylinder bores to achieve exact centerline location and cylinder wall angle. As a result, removing material in order to accomplish this means you use oversize pistons and rings. But because most performance builds involve increasing displacement anyway, this is a moot point. The same holds true for lifter bores in overhead valve engines. The lifter bores may also be corrected, relocating their centerlines and angles. Oversizing the lifter bores in order to make these corrections simply means that bronze lifter bore liners are then installed and honed to the required diameter.
Machining decks to the proper height and angles equalizes and angle-corrects the base for cylinder volume. But you also equalize crank stroke, rod length, piston compression height, piston dome volume, and cylinder head chamber volume. By correcting cylinder bore centerline (and bore spacing), you place the bores and pistons in a no-compromise, as-designed, on-center travel path, which reduces operating friction and stresses. By correcting lifter bore centerline location and bore angles, you improve valvetrain efficiency. It all boils down to reducing friction and wasted energy, which translates into better performance and longer engine life.
Core shift commonly occurs during the casting process at the manufacturing level. It’s not uncommon for cylinder bores to shift slightly beyond engineering specification. This is one of the primary reasons for thoroughly accurizing the block by determining where these shifts have occurred and machining to correct these out-of-spec centerlines. Out-of-spec tolerances may pass the manufacturer’s acceptance range for common street applications, but for high performance and racing you want to correct these issues in order to achieve a high level of precision, both to accommodate increased horsepower and to aid in engine longevity.
The purpose of oil restrictors is to reduce the amount of oil at the top end of the engine, so more oil is delivered to the rod and main bearings. These restrictors are threaded plugs that are installed in the oil passages, which feed oil to the lifters, valves, springs, and rockers. Windage drag, which is drainback oil accumulating on the crank counterweights, is also reduced. Opinions vary with regard to restrictors, and the need for them also varies depending on the type of engine.
In general, restrictors may be used with solid (mechanical) roller lifters, but should not be used with hydraulic lifters or with flat-tappet lifters, since they both need more lubrication. Also, while restricting oil to the upper end may be fine with full-roller rockers, stock OEM ball/pivot rockers need more oil delivery, so don’t install restrictors if you’re using ball-type rockers.
Depending on the type of engine, restrictors may be installed at the rear of the block. These are installed deep inside the lifter-valley oil gallery holes or in the lifter valley. In either case, thread tapping is required, so if restrictors are planned, the tapping and subsequent cleaning must be done prior to final wash and block assembly. The size of the oil holes in restrictors varies from about .040 to .065 inch. Again, this depends upon the application and is quite often based on the opinion and experience of the block machinist/engine builder.
The large oil drainback holes in the lifter valley of a V-type block obviously allow oil that was delivered to the upper end to drain back to the sump. However, if something goes awry at the top end, such as valvespring failure, valve failure, keepers falling loose, etc., the resulting fragments can travel across the lifter valley and drop into the oil sump. To avoid this, it’s common for race engine builders to epoxy a piece of screen over these drainback holes. If you decide to do this, make sure that the block surface is clean and dry to provide good adhesion for the epoxy. If the screen pops loose, it can cause damage by interfering with lifters or by working its way down to the cam.
Many OEM blocks have ragged, unfinished drainback holes and/or slots that have jagged casting flash edges. It’s always a good idea to grind these edges smooth, not for the sake of appearance, but to ensure that no flashing pieces break loose in the future.
Residual oil that has done its job at the upper end needs to return to the sump in a timely manner. Because older cast-iron blocks commonly had fairly rough, raw casting surfaces in the valley, it was/is common practice to smooth out the surfaces or coat the valley with a high-build paint to fill the casting pores and promote faster drainback. Today’s castings, especially quality aftermarket blocks, tend to have finely finished surfaces. In the case of the GM LS engine, there is no lifter valley.
If you decide to apply a coating to your lifter valley, the block must be absolutely clean and dry (serious hot-tank wash). The most commonly used coatings include Glyptal, an electrical armature winding coating, which is highly resistant to oils and acids. All machined surfaces, threaded holes, lifter bores, etc., must be plugged or masked to avoid paint contamination. In my and many others’ opinion, the time and effort required as well as the risk of dried paint particles breaking loose just isn’t worth it, considering the small increased rate of oil return. You’re better off by simply grinding/smoothing any noticeable surface defects/lumps, sharp edges, etc., and ignore the paint idea.
Whenever you deviate from “stock,” in crankshaft and rod selection, you must check for rotating and reciprocating clearance at the block. Specifically, if you choose a longer crankshaft stroke and/or beefier aftermarket connecting rods, clearance checking is an absolute must. The common clearance issues occur at the bottom of the cylinders and at the oil pan rail areas. Mock-install the crankshaft to the block with clean and lubricated main bearings, and using the same main caps that you plan to use in the final build. For interference checking, there’s really no reason to fully torque the main caps at this time. Simply snug them to about 20 ft-lbs.
Slowly rotate the crank and, starting from the front, watch each crank counterweight as it moves close to the block (near the pan rails). You should have about .060 inch clearance between the counterweights and the block at the tightest locations. If you see component contact or too tight of a clearance, mark the block location using an ink or paint marker. After all counterweight clearances have been checked, remove the main caps, crank, and bearings; then remove block material as needed with a high-speed die grinder. Clean off the metal particles and re-install the crank and perform the clearance check again to verify.
When test fitting the crankshaft (with main bearings installed and lubed), check crank counterweight-to-block clearance, especially when using a stroker crank. This increased-stroke crank in a Pontiac 455 block had a fairly tight clearance between the forward area of the front counterweight and the block, just above the oil-pan rail. Once the crank was removed, hand grinding removed a small bit of material from the block to achieve a comfy .050-inch clearance at this tight spot.
During this test fit, you can see that the rod bolt is contacting the block. The area to be relieved was marked as a reference. The interference area of the block must be relieved to obtain a bare minimum of about .060-inch clearance.