Audi Cars 1999-05

Pistons & Connecting Rods

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Only pistons with the wrist pin retained by C-clips are serviceable by the home-mechanic. Press fit pistons require special presses and/or heaters to remove/install the connecting rod and should only be performed by a machine shop.

All pistons will have a mark indicating the direction to the front of the engine and the must be installed into the engine in that manner. Usually it is a notch or arrow on the top of the piston, or it may be the letter 'F' cast or stamped into the piston.



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Fig. Most pistons are marked to indicate positioning in the engine (usually a mark means the side facing the front)

Cleaning & Inspection



Connecting Rods

You should have the connecting rod checked for straightness at a machine shop. If the connecting rod is bent, it will unevenly wear the bearing and piston, as well as place greater stress on these components. Any bent or twisted connecting rods must be replaced. If the rods are straight and the wrist pin clearance is within specifications, then only the bearing end of the rod need be checked.

  1. Place the connecting rod into a vice, with the bearing inserts in place, install the cap to the rod and torque the fasteners to specifications.
  2.  
  3. Use a telescoping gauge and carefully measure the inside diameter of the bearings. Compare this reading to the rods original crankshaft journal diameter measurement. The difference is the oil clearance.
  4.  
  5. If the oil clearance is not within specifications, install new bearings in the rod and take another measurement. If the clearance is still out of specifications, and the crankshaft is not, the rod will need to be reconditioned by a machine shop.
  6.  


NOTE
You can also use Plastigage® to check the bearing clearances. The assembling section has complete instructions on its use.

Pistons


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Fig. Exploded view of the piston and its components-VR6 engine



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Fig. Place rubber caps over the connecting rod studs before you push the piston up and out of the block



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Fig. Removing the piston from the block



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Fig. Removing the bearing from the connecting rod



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Fig. Visually inspect the piston for cracking



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Fig. Also check for any signs of wear



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Fig. Removing the ring from the piston

The piston should be visually inspected for any signs of cracking or burning (caused by hot spots or detonation), and scuffing or excessive wear on the skirts. The wrist pin attaches the piston to the connecting rod. The piston should move freely on the wrist pin, both sliding and pivoting.

Grasp the connecting rod securely, or mount it in a vise, and try to rock the piston back and forth along the centerline of the wrist pin. There should not be any excessive play evident between the piston and the pin. If there are C-clips retaining the pin in the piston then you have wrist pin bushings in the rods.

There should not be any excessive play between the wrist pin and the rod bushing. Normal clearance for the wrist pin is approx. 0.001-0.002 inch (0.025-0.051mm).



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Fig. Measure the piston's outer diameter, perpendicular to the wrist pin, with a micrometer



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Fig. Notice the ring of carbon at the top to the cylinder



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Fig. A ridge reamer can be used to remove any residual carbon stuck at the top of the cylinders. This will ease piston removal



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Fig. Fit the piston ring into the cylinder as shown.



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Fig. Use a feeler gauge to measure end gap



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Fig. Soak the piston and rings in oil before reinstalling them into the engine



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Fig. A piston ring installer is needed to reinsert the piston

Use a micrometer and measure the diameter of the piston, perpendicular to the wrist pin, on the skirt. Compare the reading to its original cylinder measurement obtained earlier. The difference between the two readings is the piston-to-wall clearance. If the clearance is within specifications, the piston may be used as is.

If the piston is out of specification, but the bore is not, you will need a new piston. If both are out of specification, you will need the cylinder rebored and oversize pistons installed. Generally if two or more pistons/bores are out of specification, it is best to rebore the entire block and purchase a complete set of oversize pistons.

 
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