The AERA Technical Committee offers the following information on checking piston protrusion on diesel engines. For those who have spent most of their time building gasoline engines, the importance of checking and setting piston protrusion heights is a new, critical skill to be learned and maintained.
Diesel engines normally run compression ratios of 15 – 12:1. These engines typically have “interference“ valve-train design and with approximately .020-.030” (cold) clearance between the piston crown and the cylinder head. It is therefore critical that the piston protrusion height of each piston and rod assembly be measured in the bore they will be running in. There is always a piston protrusion tolerance specification available for each diesel engine.
It is always suggested as a matter of “BEST PRACTICE” to measure the existing piston protrusion during the disassembly procedure directly after removing the cylinder head. After removing the cylinder head remove all gasket material from the block deck and clean the deck of oil, grime or burrs.
Checking piston protrusion heights is best done using a dial indicator and measurement bridge like the one shown in the photo below. After zeroing out the gauge on the block deck, locate the dial indicator as shown in the photo. Turn the engine over slowly and observe the highest value on the gauge. Make two measurements on each piston directly above the piston pin. Record your readings and average those figures for each cylinder. Doing so will give you a reference when in your final assembly of the engine after all machine work has been done. That reference may also affect the limits of machining of the block deck if there are limited head gasket thicknesses for that particular engine.
Like the article mentions, measuring piston protrusion is a “Best Practice.” It can help in finding bent rods, mismatched or wrong parts, or incorrect assembly etc. It is also a great aid in diagnosing problems if you have an issue with a newly assembled engine. If piston protrusion specs are needed, please call us and we will be happy to supply you with assembly specs.
