Removing a Piston and Rod Assembly

Prior to removal of the piston and rod assembly, check all connecting rods and main bearing caps for correct position and numbering. Rod caps are not interchangeable and must be reinstalled on their respective connecting rods.

  • If the numbers are not visible, use a center punch or number stamp to number them.
Mark the connecting rod caps prior to disassembling the engine. Courtesy of General Motors Corporation - Service Operations.
  • Caps and rods should be stamped on the external flat surface.
    • Remember, the caps and rods must remain as a set.
    • Be certain to inspect connecting rods previously marked (at the factory or during a previous rebuild) for correct location.
    • When marking rods on a V-type engine, mark them according to the cylinder's number. The only side of the rod that is easily accessible is the side that faces the outside of the engine (away from the cam on a pushrod engine).
Do not file notches on the rod beam. This can cause stress raisers, which weaken the rod.
  • When marking rods on a V-type engine, mark them according to the cylinder's number. The only side of the rod that is easily accessible is the side that faces the outside of the engine (away from the cam on a pushrod engine).
Mark the connecting rods on the correct side.
  • To remove the piston and rod assemblies, position the crankshaft throw at the bottom of its stroke.
  • Remove the connecting rod nuts and cap.
  • Tap the cap lightly with a soft hammer or wood block to aid in cap removal.
  • Cover the rod bolts with protectors to avoid damage to the crankshaft journals.
Install rod bolt protectors before removing the rod and piston assembly from the engine block. Courtesy of Dana Corporation.
  • Carefully push the piston and rod assembly out with the wooden hammer handle or wooden drift and support the piston by hand as it comes out of the cylinder.
  • Be sure the connecting rod does not damage the cylinder during removal. With the bearing inserts in the rod and cap, replace the cap (numbers on same side) and install the nuts.
  • Repeat the procedure for all other piston and rod assemblies.

Disassembling a Piston and Rod Assembly

Usually the old pistons are reused unless the cylinder bores are worn badly enough to require reboring.
  • When original pistons are to be reused during an overhaul, it is best to simply leave the pistons and rods assembled. Pressing them apart serves no useful purpose and risks ruining an otherwise good piston.
  • Do not glass bead blast pistons while pistons are assembled to the connecting rods.
  • Do not soak the piston and rod assembly in carburetor cleaner. The pin could seize on the piston.
  • Be sure to keep track of the direction the connecting rod faces in relation to the top of the piston. Pistons have a notch on the side of the piston head that faces the front of the engine.

The small end or piston pin end of the connecting rod is made to accept the piston pin, or wrist pin, which connects the piston to the connecting rod.

Piston Pins are either pressed into the rod or they pivot in both the piston and the rod. Courtesy of Sunnen Products Company, St. Louis, Missouri.
  • The piston pin can be pressed fit in the piston and free fit in the rod. When this is the case, the small end of the rod will be fitted with a bushing.
  • The pin can also be a free fit in the piston and pressed fit in the rod. In this case no bushings are used. The pin simply moves in the piston using the piston hole as a bearing surface.
  • You can separate pressed-fit pins and pistons from their connecting rods using a machine shop tool called a pin press.
Using a piston pin press to remove a pin from a pressed-fit connecting rod. Courtesy of Sunnen Products Company, St. Louis, Missouri.
  • A third mounting allows the pin to move freely in both the piston and the rod. This design requires the use of clips or or caps to prevent the pin from moving out against the cylinder walls.

Assembling a Piston and Rod Assembly

As each piston is attached to the connecting rod, double check the oil clearance of the rod bearing.

  • If the engine is not going to be balanced, the pistons and connecting rods can be assembled at this time.
  • If the engine is going to be balanced, assemble the pistons and connecting rods after each component is weighed separately and matched.
  • Remember to keep all pistons, rods, and pins identified to the cylinder bore to which they will be installed.
  • In addition, check the service manual to determine if the pistons are offset and the proper orientation of the rod to the piston.
  • Also, the connecting rod may have spit holes or jet valves that must be installed in the proper direction. The spit hole is used to squirt oil onto the cylinder walls and under the piston head to keep it cool.
  • V-type engines typically use offset connecting rods. If these are installed backward, the engine will lock up and not rotate.
  • Before beginning the reassembly procedure, double-check the rods and pistons. They must be absolutely clean.
  • As each piston is fitted to the rod, double-check the clearances.

Piston-to-connecting rod installation methods depend on whether the pin is full-floating or press-fit.

Full-Floating Pins

  • Installing piston with full-floating pins does not require any special tools. These types of pins can generally be pushed in using your hands.
  • Coat the pin, piston bore, and small-end bore with light oil or assembly lube prior to installation.
  • Fit the piston pin into one side of the piston bore until it enters into the center of the piston about 1/16 inch.
  • Next, locate the rod into the piston, making sure it is facing the proper direction.
  • Align the small-end bore with the protruding piston pin and push the piston pin through the connecting rod bore and into the other piston bore.
  • If necessary, tap the pin in with light hits with a plastic hammer.
  • Check to be sure the pin is free to rotate in both the piston bosses and connecting rod.
  • Finally, install the snap rings.
    • It is very important to install the retaining clip properly.
    • If it is installed improperly, the pin can come out and damage the cylinder bore.
    • Handle the snap rings carefully.
    • Do not overstress them by compressing them more than necessary.
    • The open end of the snap ring should face toward the bottom of the piston. This places the strongest part of the snap ring in the portion of the pin boss receiving the highest stress.
    • In addition, some snap rings are tapered or chamfered and must be reinstalled in the proper direction to prevent the pin from coming loose.

Press-Fit Piston Pins

Press-fit piston pins can be installed using a hydraulic press or by heating. Be sure to use the proper tools and equipment when pressing the pin. The heating method expands the small-end bore of the connecting rod to allow the piston pin to be slip fit. The small end of the connecting rod is raised to a temperature of about 425°F (204°C) using a special rod oven.

A deluxe rod furnace. Courtesy of Sunnen Products Company, St. Louis, Missouri.

Following is a typical procedure for installing press-fit pins using a pin press:

Pressing the pin in can distort the piston if the proper tools and equipment are not used. The bottom adapter must support the piston in a reinforced area. The driver shaft must be slightly smaller than the outside diameter of the pin. Some adapter kits provide a pilot collar to align the connecting rod bore to the bore in the piston. This helps prevent cocking the pin during installation.

Do not use excessive pressure to install the pin. The piston may explode under this pressure, resulting in injury. Always wear eye protection when operating a press.

  • Chamfer the piston bin bore of the connecting rod.
  • Coat the pin, piston bosses, and inside of the connecting rod bore with light oil or assembly lube.
  • Select the proper press adapters to support the piston.
  • Set up the base adapters onto the press. Slide the piston pin into the top piston boss.
  • Push it in by hand until it contacts the connecting rod bore.
  • Be sure the rod is properly aligned with the pin.
  • Place the piston assembly onto the adapter base.
  • Install the guide collar (if required).
  • Insert the proper size driver into the piston pin.
  • Tap the driver with a soft-faced hammer to start it into the connecting rod.
  • Press the pin in until it contacts the stop.

Following is a typical procedure for installing press-fit pins by heating the small-end bore:

The entire rod will become very hot during this operation. Use good heat-resistant gloves when handling the connecting rods.

  • Place the connecting rod into the rod heater and turn on the heater. Adjust the temperature to about 425°F (204°C).
If the end of the rod begins to turn blue, it is too hot. Remove the rod immediately and readjust the temperature of the oven.
  • While the rod is in the heater unit, place the piston in the vise and adjust the limiting fixture.
  • Fit the piston pin into the installation tool.
  • When the rod is at temperature, remove it from the oven and locate it into the piston.
  • Make sure to properly orient the rod to the piston.
  • Use the installation tool to push the pin through the connecting rod bore.
  • Push the pin until it contacts the limiting fixture. The fixture prevents the pin from being installed too far into the connecting rod bore.

Installing a Piston and Rod Assembly

  • Insert a new piston ring into the cylinder.
  • Use the head of the piston to position the ring so that it is square with the cylinder wall.
  • Use a feeler gauge to check the end gap. Compare end-gap specifications with the measured gap.
  • Correct as needed.
  • Normally, the gaps of the piston rings are staggered to prevent them from being in line with each other.
  • Piston rings are installed easily with a ring expander.
  • Before attempting to install the piston and rod assembly into the cylinder bore, place rubber or aluminum protectors or boots over the threaded section of the rod bolts.
  • This will help to prevent bore and crankpin damage. Nicking the crank could also result in a damaged bearing insert.
  • Lightly coat the piston, rings, rod bearings, cylinder wall, and crankpin with an approved assembly lubricant or a light engine oil.
  • Some technicians submerge the piston in a large can of clean engine oil before it is installed.
  • Align the ring end gaps and compress the rings with the ring compressor. This tool is expanded to fit around the piston rings. It is tightened to compress the piston rings.
  • When the rings are fully compressed, the tool will not compress any further. The piston will fit snugly, but not tightly.
  • Rotate the crankshaft until the crankpin is at its lowest level (bottom dead center or BDC).
  • Then place the piston/rod assembly into the cylinder bore until the ring compressor contacts the cylinder block deck.
  • Make sure that the piston reference mark is in correct relation to the front of the engine.
  • Also, when installing the assembly, make certain that the rod threads do not touch or damage the crankpin.
  • Lightly tap on the head of the piston with a mallet handle or block of wood until the piston enters the cylinder bore.
    • Be certain that the connecting rod faces in the right direction.
    • On a V-type engine, with the notches on the piston facing forward, the left cylinder bank's rod should face the opposite direction from the right bank's.
    • If they're facing the wrong way, the rods might have been improperly installed on the pistons.
    • Do not continue with assembly until the rods are correctly installed.
  • Push the piston down the bore while making sure the connecting rod fits into place on the crankpin. Remove the protective covering from the rod bolts.
  • Assemble the connecting rod on the crankpin.
    • Rod nuts are square and flat on the bottom side and often are curved on the top.
    • Be sure that the flat side of each rod nut is faced against the cap.
  • A drop of anaerobic thread-locking compound is good insurance against a loose rod cap nut.
    • Ideally, nuts should not be reused. Repeated reuse of nuts is asking for trouble.
    • Use new nuts, because they are self-locking.
Art courtesy of Delmar - Thomson Learning.
  • Position the matching connecting rod cap and finger tighten the rod nuts.
  • Make sure the connecting rod blade and cap markings are on the same side.
    • The numbers on each rod and cap should be on the same side with the lock tab facing each other.
  • Gently tap each cap with a plastic mallet as it is being installed to properly position and seat it.
  • Torque the rod nuts to the specifications given in the service manual.
  • Repeat the piston/rod assembly procedure for each assembly.
  • When all the pistons and rods have been installed, connecting rod side clearance can be measured. side clearance is the amount of clearance between the crankshaft and the side of the connecting rod. side clearance is measured with a feeler gauge. if the clearance is not correct, the rods may need to be machined or replaced.