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Engine Assembly, Part 3: Fitting Rods and Pistons
By Wayne Scraba/
Printable version
Degree of DifficultyModerate
Estimated Time300 minutes
300 minutes
Wrist Pins and Retainers
Keeping the wrist pin in check within the piston is critical. If the pin moves sideways, then the cylinder wall will ultimately be destroyed. There are several means to keep a pin in check, the most common being a pressed pin (in the rod) or a full floating arrangement with some form of lock ring.
Pressed pins are regularly found in passenger car applications. Here, the pin is press fit into the small end of the connecting rod. The pin movement takes place between the wrist pin and the pin bore of the piston. Engines with press-fit pins must be assembled by an engine (machine) shop with special equipment designed for the task at hand. The process involves heating the small end of the rod. Next the piston is set in place over the rod small end and the wrist pin is pressed into place. It’s not a job for a home engine-builder.
Floating pins, on the other hand, are often found in high-performance engines. Here, the pin is allowed to float within the connecting rod small end. Instead of being pressed in place, the wrist pin is held in check with one of three types of locking devices: Spiral Locs, Round Wire Locks or Snap Rings (regularly called “Tru Arcs”). Pistons are machined with retainer grooves (some for double retainers per side) that accept the locking device. The most effective device of the bunch is the Spiral Loc (essentially a flat coil of hardened steel), but it’s also the most difficult to install.
The conventional way to install a set of Spiral Locs is to use a pair of small screwdrivers to literally spin the lock wire into place (into the groove machined in each end of the piston pin bore).
Basically, you start at one end (without the wrist pin or rod in place), turning in both Spiral Locs (one after the other). Next, the wrist pin and piston are assembled on the connecting rod. At that point, the remaining pair of Spiral Locs are worked into place. Most folks will tell you to keep a box of bandages handy with this job. Good advice.
Tru Arcs are essentially heavy-duty snap rings and they install by way of snap-ring pliers. Round wire retainers aren’t that common, but there are tools available to install them (more in the photo guide). In any case, never re-use a pin retainer of any sort. Once used, they can become distorted (particularly from the removal process). Tension is lost and you run the risk of the retainer being damaged or destroyed during engine operation.
Installing Piston Rings
Once the piston-pin-rod combination is assembled, it is time to install the piston rings. Here, it’s a matter of winding the rings onto the pistons. In times past, piston rings were thick, higher tension affairs than we commonly see today. Some folks installed the rings on the piston by way of a special set of pliers. Today, with lighter tension (and regularly thinner cross section) piston rings, it’s easy to assemble them on the piston.
Start with the oil ring expander. It doesn’t matter which direction it faces. Simply wrap it over the piston and insert in the oil ring groove. Next, carefully wind on the oil ring scraper rails. One goes to the bottom of the oil ring groove; another to the top. Each gap on the oil ring (expander and the pair of upper and lower scraper rails) should be positioned 120º away from each other. You have to be careful with the expander since they have a tendency for the ends to overlap. Once installed, the oil ring package (expander and the pair of scraper rails) should be able to easily float from side to side in the groove. Double check the expander before the piston is “pushed” into the bore (more on this later).
Next, the second ring is installed, followed by the top ring. If you examine the ring package, you’ll find a set of “pip” marks on both the top and the second ring. These pip marks are always installed facing up. Once installed, rotate the gaps so that they don’t coincide. Keep in mind, however that rings actually spin within the piston groove. Keeping the gaps separated minimizes the amount of oil fouling that can occur during initial engine fire-up.
Installing the Pistons and Rods
Wipe the cylinder bores with a clean, lint-free towel and apply a small amount of conventional (non-synthetic) oil to the walls. We always apply a liberal amount of oil to the wrist pins and piston rings as well. There’s a caveat here though: You don’t need to drench the piston in oil. You only need sufficient oil to lubricate the rings as they pass through the piston ring compressor.
There are two common types of ring compressors on the market: Expander types where the tool is clamped over the piston and you tighten it in place or a tapered job where the rings are progressively tightened as the piston is pushed through it into the bore. In either case, liberally coat the compressor with oil (again, it doesn’t have to be dripping wet).
Rotate the crankshaft so that the rod journal is at Top Dead Center for the piston you’re installing. Place the piston/ring compressor combination over the cylinder bore. You have to be positive the piston is correctly oriented. For example, if the piston is domed, the dome is on the outside of the block or the valve notches are at the top (nearest the lifter valley). Additionally, be sure you’ve assembled the piston-rod combination correctly so that the chamfer on the connecting rod (and rod bearing) faces the fillet radius on the crankshaft. Finally, (and as mentioned previously) double check the oil ring(s) to ensure the expander ring end gaps haven’t overlapped.
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