The International Cessna^® 170 Association

Last Friday while attempting to start the engine, the starter shaft failed just in front of the case (C-145 w/ Delco Remy starter). The first start was uneventfull and I taxied to the pumps to get gas. When I tried to restart the engine, the prop barely twitched, there was an audible pop and I could hear the starter motor running but the prop would not turn. When I pulled the starter, the splined shaft that turns the clutch was missing! You guessed it, it had fallen into the engine. Thank God I didn't turn the prop! The shaft end was resting on top of the crankshaft gear and I was able to retrieve it with a magnet on a stick. Had this shaft failed while the engine was turning the damage could have been extensive.

Closer examination revealed that fatigue cracking had been present for some time. The shaft is 3/8" in diameter where it failed, and fatigue cracking was evident around the entire shaft. The area that finally failed in shear was only a 7/32" by 3/16" oval.

The starter was overhauled by Aerotech of Louisville in 1997 and has 520 hours since. If any of you have similar time or higher starters it may be worth your while to inspect this shaft. I got off easy but it could have been much worse!

Bruce

You might also check the clutch assembly, it could also have failed internally and caused the failure of the starter drive.

4-Shipp wrote:Last Friday while attempting to start the engine, the starter shaft failed just in front of the case (C-145 w/ Delco Remy starter). The first start was uneventfull and I taxied to the pumps to get gas. When I tried to restart the engine, the prop barely twitched, there was an audible pop and I could hear the starter motor running but the prop would not turn. When I pulled the starter, the splined shaft that turns the clutch was missing! You guessed it, it had fallen into the engine. Thank God I didn't turn the prop! The shaft end was resting on top of the crankshaft gear and I was able to retrieve it with a magnet on a stick. Had this shaft failed while the engine was turning the damage could have been extensive.

Closer examination revealed that fatigue cracking had been present for some time. The shaft is 3/8" in diameter where it failed, and fatigue cracking was evident around the entire shaft. The area that finally failed in shear was only a 7/32" by 3/16" oval.

The starter was overhauled by Aerotech of Louisville in 1997 and has 520 hours since. If any of you have similar time or higher starters it may be worth your while to inspect this shaft. I got off easy but it could have been much worse!

Bruce

Sorry to hear of the failure, Bruce. The good news is that this sort of failure will not likely occur "while the engine was turning." The only time any stress is on the part is during start. A likely cause of this failure is an engine backfiring during start (while the starter is still engaged), but sometimes the shaft just shears because it's tired (or stressed from a previous backfire.) That is usually caused by improper magneto timing, failed impulse coupling, or arcing within a dirty distributor block. It's a good reason to be certain that regular magneto maintenance is done (usually during annual.)
It WAS good that you didn't turn the prop or the other gears could have been damaged. (AeroTech [800/634-0190] is a good company for starters/generators.)