The heart of any engine is its rotating assembly, consisting of the crankshaft, connecting rods, pistons, vibration damper, and flywheel. All of these pieces will need cleaning, careful inspection, and in some cases machine work during the rebuilding process.
Starting at the front and working toward the back, you should first take a careful look at the vibration damper for signs of deterioration, particularly the rubber insulator ring located between the inner and outer portions of the damper. Check the keyway and key for any wear and the sleeve that extends through the timing cover seal for keyway damage, pitting, or excessive rust in the machined area that contacts the seal.
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The crankshaft itself may reveal signs of wear upon visual inspection of the main and rod bearing journals, but will need to be checked thoroughly by a machinist in order to determine how best to proceed. In some cases, if the crankshaft has already had its journals turned down, or is in some other way damaged, it may be more economical to purchase a new one instead of opting for extensive machine work.
I make it a practice during any engine rebuild to have the connecting rods reconditioned. During this process, the big ends of the rods are trued to factory specification through machining and the piston pin ends are honed and fitted with new wrist pin bushings. I also recommend replacing the rod bolts and nuts as added insurance against premature failure due to the tremendous load forces these parts will endure over the life of an engine.
If you decide to reuse your original pistons, they will need a careful visual inspection, particularly for any signs of collapse around the ring lands and scuffing or deep scratches around the skirts. There will normally be carbon deposits on the piston tops, which should be cleaned off carefully, as castor forged-aluminum pistons are rather soft and not resistant to aggressive methods of cleaning such as the wire brush. Consult your machine shop for tips if you are unsure of how to get them clean. Machining a crosshatch design into the piston skirts (also called knurling), is an old-school method of reducing engine oil consumption while saving the cost of cylinder boring and replacement pistons. If you have decided to rebuild your engine, there are probably a number of good reasons for doing so, and this would not be a time to cut corners. I would recommend against knurling your pistons.
The flywheel, also referred to as a flex plate in automatic transmission applications, comes at the back of the rotating assembly, but it’s just as important as the other components. If your car is equipped with a manual transmission, there are three distinct areas to examine in relation to the flywheel. The pilot bushing, which is press-fit into a recess at the rear of the crankshaft, should be checked for any signs of wear or damage. The friction face of the flywheel (where it is contacted by the clutch disc) may show signs of heat checking, discoloration, or cracks. This does not necessarily mean that the flywheel is no longer serviceable, since machining may restore the surface if not too badly damaged. Finally, the ring gear is prone to wear from repeated contact with the starter drive. Carefully examine the teeth of the flywheel ring gear, along with those on the starter drive. If either show wear, now is the time to replace them, rather than later when the engine is back in the car. This spot is usually the only spot to check on automatic-equipped vehicles.
If any of the major components in the rotating assembly are being replaced as part of the rebuild, you should have the assembly balanced to avoid unwanted vibrations and possible damage to the engine once you have it back in the car and running.
Step-1: Prepare for Balancing
This crankshaft throw shows evidence of material being removed during the balancing process. Balancing is relatively inexpensive and highly recommended by most machine shops as part of the rebuilding process. If the flywheel or vibration damper are being replaced as a part of your rebuild, then balancing is a must.
Step-2: Record Rod Weight
The connecting rods are balanced by first obtaining a weight on the big and small ends of each individual rod using a gram scale and this fixture. The rods will then be matched to the weight of the lightest rod.
Step-3: Remove Weight
Weight is carefully removed by grinding the pads forged into each end of the rod. This process can be quite tedious as only small amounts of material are removed between checks until the weights on all the rods match.
Step-4: Match Overall Weight
The overall weights of each connecting rod are also carefully matched during the balancing process.
Step-5: Balance Pistons
Piston balancing is achieved in a similar fashion. Each piston is weighed with the wrist pin installed and all are matched to the lightest.
Step-6: Remove Weight from Pistons
Material is removed from the bottom of the pin boss in the piston using a drill press. Again, only small amounts of material are removed between each reweighing until the lightest weight has been matched.
Installing Pistons on Connecting Rods Step by Step
Step-1: Determine Proper Orientation
Determine the proper orientation of the pistons and rods prior to installing the pistons on the connecting rods. Lima series connecting rods have a raised spot on the side of the rod that faces the crankshaft journal. The crankshaft side of the rod has the larger chamfer, while the stamped numbers on the rod and cap face out. We marked the front side of the rods with a yellow indelible marker so that no mistakes are made when installing the pistons. Note: Never mark a connecting rod with a file or other means that removes material or creates a sharp edge that may result in a stress riser and eventual failure of the rod.
Step-2: Heat Wrist Pin
The piston is placed into the Sunnen rod heater with the wrist pin started into it. This machine heats the area that will accept the wrist pin end of the connecting rod. Lima series engines use fixed wrist pins. This means that piston pivots on the wrist pin, which is stationary in the small end of the rod. If you hear the term “full-floating pistons,” it refers to wrist pins that are slightly smaller in diameter than the small end of the connecting rod and thus will float, or move, both rotationally and side-to-side. Since the wrist pin in this case is not fixed to the connecting rod and floats in the piston bosses as well, grooves are machined in the piston bosses to house retainers that keep the pins from sliding out and contacting the cylinder wall. While full-floating pins produce less friction, they’re also more costly to produce due to additional machining steps and parts required. The downside of a full-floating design is that the retainers holding the wrist pin can fail, causing havoc inside the engine. Most manufacturers have used the fixed wrist pin design in their engines for many years, the only downside being the specialized equipment required to mate the piston and connecting rod.
Step-3: Heat Connecting Rod
The connecting rod is then placed into the heating portion of the machine to raise its temperature, thus expanding the bushing at the small end.
Step-4: Slide Pin Through Rod
The wrist pin is slid through the rod and into the wrist pin bore on the opposite side of the piston using a driver sized to fit the pin. When the rod cools, the rod and piston assembly will be complete.
Written by Charles R. Morris and Republished with Permission of CarTech Inc