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Timing Radial Engine Valves Timing Radial Engine Valves From "Trade School" episode DTRS-108

Figure A

In this segment, we follow Sean to his Airplane Power Plant I class, where he performs a complete overhaul on a radial airplane engine. Seen today on crop-dusters (figure A), the powerful radial engine was prevalent during WWII, but with the advent of the lighter, more efficient turbine engine, the radial engine has become more and more rare. Aviation technology students still study the complex radial engine to serve as a springboard to more modern engines ....

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Figure B

A radial airplane engine works much like the engines found in most automobiles. The primary difference is the configuration of the cylinders and pistons, which are arranged in a circle around the crankshaft (figure B). Just like a car's engine, the radial engine utilizes the four-stroke cycle. The four strokes of the cycle are intake, compression, power, and exhaust. Inside the cylinder, intake and exhaust valves open and close at specific times, allowing an air/fuel mixture to enter and exhaust to exit the combustion chamber. We join Sean for one of the last critical stages of the rebuild: timing the valves. The opening and closing of these valves must be timed right to ensure an efficient four-stroke cycle.

Basic Tools:

Spring Compressor
Wrenches
Time-Rite Indicator
Turning bar
Feeler gauge

• By following an overhaul manual to the letter, Sean and his class partner Jamie completely disassemble the engine, measure and inspect the various parts, and then rebuild.
  

• Before securing the cylinders to the assembled engine case, Sean must seat the intake and exhaust valves within each cylinder (figure C). Sean uses a special compressing tool to compact the springs. The springs keep the valves in a closed position. He holds the compacted springs in place using a steel lock ring. With the valves seated, Sean assembles the pistons and cylinders around the engine case (figure D).

  Figure C

  Figure D

  
  

• Next, Sean installs pushrods and rocker arms in each cylinder head (figure E). The cam ring inside the engine case rotates and moves the pushrods up and down (figure F). The pushrods connect to the rocker arms, which open and close the valves (figure G).

  Figure E

  Figure F

  Figure G

  
  

• Then Sean begins adjusting the timing of cylinder #1. He installs a time right indicator into the spark plug hole in cylinder one (figure H). This tool indicates the position of the piston within the cylinder. Then Sean inserts a large turning bar to the nose end of the propeller shaft (figure I). He aligns the zero mark on the bar with the zero mark on the shaft.

• To time the cam ring to the valves, Sean rotates the turning bar until the time right indicator reads that the piston in cylinder one is at top dead center of its cycle (figure J). Jamie notes the reading on the turning bar and marks it on a worksheet.

  Figure H

  Figure I

  Figure J

  
  

• Then, to time the exhaust valve on cylinder one, Sean slowly rotates the propeller shaft while Jamie manipulates the roller on the rocker arm. When the valve begins to open, it engages the roller causing it to lock. Jamie marks this setting on the worksheet and secures the rocker arm position by tightening the adjusting nut (figure K). Then Sean reverses the propeller shaft 3/4 turn and measures the clearance between the rocker arm and the valve for tolerance using a feeler gauge (figure L). Then they rotate the propeller shaft until the valve just closes and note the position on the worksheet. Then they time the intake valve on cylinder #1 using the same process.

• Next Sean and Jamie must time the cam ring to the propeller shaft. Jamie disengages the cam gear by loosening the rear propeller shaft nut on the back of the engine (figure M). This keeps the valve settings for cylinder #1 in place. Once the cam ring is disengaged, Sean rotates the propeller shaft forward to the original zero degree setting. Then Jamie re-engages the cam ring.

  Figure K

  Figure L

  Figure M

  
  

• Sean and Jamie verify their work by turning the propeller shaft and noting the position of the valves when opened and closed. Again, they record these positions on the worksheet. They compare all the position records to the table of limits located in the overhaul manual and find everything to be within acceptable tolerance.

• With cylinder one properly timed, Sean need only to adjust the rocker arms on the remaining cylinders to finish timing. Starting with cylinder two, he turns the propeller shaft until the piston is at top dead center. Then using a feeler gauge, he adjusts the clearance between the valves and the rocker arms. Once in tolerance, he tightens the lock nut on the rocker arm and continues the process on the remaining cylinders until finished.

School Contact Information:

Westwood College of Denver
10851 West 120th Avenue
Broomfield, CO 80021
Phone: 303-464-2300
Website: www.westwood.edu