Little Big Chief - Part I: Basic Machining - Pontiac Tech

Sometimes, the cylinder block-deck surface develops irregularities that can cause compression and coolant leaks. The flatness of the block can be checked with a straight-edge and a feeler gauge. The term "decking" is used in the trade for the machine process that trues the head gasket surface of the block. Decking is also used to adjust the amount of distance the crown of the piston is from the deck when at TDC. A production engine has the piston down in the bore approximately 0.020 inch (replacement pistons may be down 0.035 inch), while a performance application may be machined for a zero deck. This means it's flush with the deck at TDC. Our 350 was machined as a zero-deck engine.

Next, the 350 was subjected to an align hone procedure. This is a precision hone that is used to correct main bearing tunnel misalignment. A drive and feed mechanism operates a mandrel with separate stones. There's a pair of guide shoes for each stone, which provide a long, rigid alignment average. This procedure will assure that the crankshaft rides true in the engine block.

At every step in the machining procedure, measuring tools are used to "sneak up" on the desired dimension and to confirm results. Here, a bore is being checked for dimensional accuracy.

The next step is to polish the crankshaft journals on a polishing machine using emery-cloth-like material to make them as good, or even better than, new.

The stock crankshaft was cleaned, stroke indexed and reground on the mains and rods (0.010 inch) to correct for wear and in preparation for undersize bearings.

It's hard to see in this picture since the connecting rod must be held firmly, but the small end was measured in preparation of being honed for the proper piston pin clearance. The stock 6.625-inch long Pontiac rod was reused. The press-fit clearance was set at 0.0015 inch.

To recondition the big end of the rods for concentricity, each one is taken apart and ground at the parting faces, which results in a smaller vertical dimension. It's then placed in a rod hone to bring it back to the same factory dimension. In addition, the bore must be round, straight, and have the proper finish.

During machining, the rod is constantly checked on a special measuring fixture that's part of the rod hone.

With the weight of the rod, piston, pin, bearing, rings, and an allotment for oil determined, this information is used to create bob weights that mount on the crankshaft to simulate the reciprocating forces of the engine. The crankshaft is then mounted on a balancing machine. This engine was balanced at 50 percent reciprocating weight, which is a typical procedure for a production powerplant. Total bob weight value is 2,459 grams.

The crankshaft is spun and the computerized balance machine identifies where either weight is needed or must be removed. This can be thought of as a very sophisticated tire balancer.

With the rods now to dimension, all of the rotating and reciprocating components are weighed on a highly accurate scale to prepare for dynamic balancing.

The 350's crankshaft only required a small amount of material to be added to a counterweight. This was accomplished by placing a weld in one of the existing balance holes.