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Engine Rebuilding: Rods & Pistons

The most crucial step of any engine rebuild

Think about it: At 6,000 rpm, a piston accelerates, comes to a complete stop, and repeats the cycle more than a 100 times per second! We're not talking about an electrical process here where a sensor samples info 100 times per second—we're describing a mechanical process where a 3-pound piston/rod assembly is trying to launch into orbit 100 times per second. The forces on the piston, wrist pin, connecting rod and crankshaft are scary. It's amazing that an engine can keep from literally exploding at those speeds. All the more reason to get it right when installing rods and pistons in the engine YOU'RE rebuilding. The magazine articles make it look simple, but if this is your first attempt at building an engine, installing rods and pistons may seem overwhelming. We will outline a few basic steps here to guide you through the process.

Ring End-Gap

Your automotive machine shop should have installed the new pistons onto your connecting rods so your first operation is to check ring end-gap in the engine block. Ring end-gap is the amount of space left between the ends of the piston ring when it is compressed and installed in the engine block. You can purchase either "pre-gapped" piston rings or "file-fit," which are oversize with smaller gaps. This allows you to file the ends of the rings for the end-gap you desire. Most hobbyists use pre-gapped rings because it saves considerable time and effort. Racers who are always looking for that last little bit of power file-fit their rings for less blow-by through the gap. Regardless of which style of rings you buy, you must check the end-gap of the top and 2nd rings by evenly pushing them down the cylinder bore approximately one inch. Using the top of the piston to "square" them up in the bore works really well. Then, using a strip feeler gauge, measure the ring end-gap. The correct gap can be found in the instructions in the ring box or in any good engine manual. A general rule of thumb is .004-inch end-gap per 1.00-inch bore diameter. A 4.00-inch bore engine would require .016-inches of end-gap. If the gap is too small the end of the ring can be filed for a larger gap—just be sure to smooth off the edges with a fine sharpening stone after using the file. If the gap is too large, exchange the ring set for a file-fit set and custom gap each compression ring.

Ring Install

After establishing the correct end-gap on the compression rings, the next step is to actually install the rings on to the pistons. Special tools are available for this task, but most engine builders carefully "wind" the rings into the grooves. Support the piston when you are installing the rings: A bench vise with soft jaws clamping the connecting rod works well or a professional piston tray makes the job much easier. Start with the bottom oil ring and work your way to the top compression ring. First, the bottom oil rail gets slipped over the piston, then the corrugated expander and finally the top oil ring rail is sandwiched in the bottom ring groove. Space the top and bottom oil rail gaps 180 degrees apart and line them up with the wrist pin. Make certain that the ends of the expander do not overlap, but instead butt together. Next install the second compression ring by inserting one end in the groove and winding the remainder over the crown of the piston. Make certain that the inside beveled edge is pointing up. Next, do the same with the top ring. Be careful and take your time installing the rings. They are very brittle and do not tolerate too much twisting before they snap. Position the end-gaps of these compression rings 180 degrees apart, aligned with the wrist pin.

Ring Compressor

It's now time to put the pistons where they belong—in the engine. A few things need to be checked before you begin the process, however. First, make sure that you have the correct rod and piston assembly for that cylinder (#1 rod and piston for #1 cylinder). Next, make sure the arrow or dot on the piston is pointing forward and the chamfer on the rod cheek is facing the fillet radius of the crankshaft. (If your machinist installed the pistons correctly on the rods, this will be the case. It doesn't hurt to check one last time.) Slip the top half of the rod bearing into the connecting rod and coat with assembly lube. Slip some rubber tubing over the rod bolt threads to protect the crank surface. Next, coat the piston skirts and ring package with plenty of assembly lube, place the ring compressor over the rings and tighten until the rings are fully compressed. Rotate the crankshaft so that the rod journal is in the full bottom-dead-center position. Place the piston/rod into the block. After you have made certain that the ring compressor is flush against the block deck, you can tap the piston down into the bore. If it seems to hang up, do not force it. Remove the piston, reinstall the ring compressor and try again. The pistons should slide smoothly into the bore. Finally, remove the rubber tubing off the rod bolts, install the rod cap and torque to specifications. After installing each piston, rotate the engine by hand to make sure that the crank rotates smoothly and that there is no scoring on the cylinder walls. If you've made it this far, the rest of the engine assembly is a piece of cake.