Surprising Discovery - Failed Piston Ring/Land
Posted: Mon Jun 19, 2023 2:21 pm
I am in the middle of Annual Inspection which will be due July 1, and Jamie and I are planning to fly from our central Texas strip to New York to visit our 2 yr-old granddaughter,…and of course our daughter and son-in-law. (Placed those in order of importance)
After a few days in NY, we plan to fly to the 2023 Convention in Michigan, after-which we plan to attend Airventure/Oshkosh featuring the 75th Anniversary of the Cessna 170. (I am particularly interested in making that as I’ve never been to Oshkosh, and our ‘53 B-model was produced by Cessna as the 50th Anniversary of Flight Model. We took N146YS to the 100th Anniversary of Flight Convention and flew it to Kill Devil Hills…. so bringing her to the 75th Anniversary is something to which we look forward.
This engine has 1079 hours Since Major. This (#3) cylinder has never been off since that time in the early ‘90s. Running the compression test portion of the 100 hr inspection provided readings of 72 psi or greater (over 80) … except for Cylinder #3…which gave only 40 psi. Considerable “hissing” was heard at the oil filler cap. This initially caused me to suspect stuck rings (although it is often said that aligned piston-ring gaps can cause serious loss of compression, I’ve never witnessed that.)
Having drained the oil and removed the oil filter (Tempest filter)…I cut open the filter and removed the pleated filter-media and discovered a tiny chip or two…almost invisible…not much to draw attention… However, washing the filter-media in mineral spirits and then dragging a magnet thru the solvent resulted in this :
I had considerable difficulty getting #3 cylinder removed as the cylinder base-nut of the lower/aft thru-bolt refused to come off the bolt without spinning the thru-bolt, possibly loosening the nut on the opposite #2 cylinder base nut.
After almost 3 hours of frustration attempting to put a jam-nut over the #2 cyl base-nut… success was finally achieved stopping that thru-bolt from spinning and getting the #3 cylinder base nut removed. Pulling the cylinder and revealing the piston demonstrated the piston-rings were NOT stuck…they rotated quite freely.
I was beginning to fear I’d pulled a cylinder needlessly in believing it might be source of the metal filings. It wasn’t until I got the piston pin drifted aside and removed the piston that the following,… very surprising conditon was found. I have never seen or heard of this type of ring-failure…. The ring was not broken….(surprisingly)….despite the piston ring-land having “disappeared” Below the ring (between the top and 2nd ring).
But even MORE Surprising…. is the area Above ring #1 is hammered by the #1 ring creating a “recessed” area toward the piston crown. This #1 ring had to flex SIGNIFICANTLY for this to occur…. many times each minute…. Yet, despite being a brittle item, it did not break!
The cylinder bore itself has no detectable damage from 5 o’clock to 7 o’clock … but at the 6 o’clock region near the bottom of the piston-stroke range is a smooth “ridge” which can be felt with the naked finger. Inside calipers show the ridge to be about .003” … I believe this is where the top ring must have flexed during piston-travel reversal. It cannot be noticed by eyeball. It can barely be detected with a finger.
LESSONS LEARNED :
1) TEMPEST filters, which have a magnet incorporated in their media-base (near the by=pass spring)…. the Tempest magnet has no special benefit. Zero metal was caught by the Tempest magnet….Yet washing the media and then putting my magnet into the solvent resulted in Significant Metal Filings attaching to my magnet, which prompted me to investigate further.
2) Rings can contribute to cylinder-wall damage without breaking, especially when piston-lands are damaged or missing.
3) Never Fail to cut open filters and WASH the media in Mineral Spirits and Check with a Magnet for ferrous metal. Run the media-wash thru a coffee filter to discover other types of metal or contaminates.
I am ordering a new cylinder assy and will install it later this week.
I hope to bring this piston to convention for “show and tell” at the maintenance seminar, and hope to see you guys/gals there!
After a few days in NY, we plan to fly to the 2023 Convention in Michigan, after-which we plan to attend Airventure/Oshkosh featuring the 75th Anniversary of the Cessna 170. (I am particularly interested in making that as I’ve never been to Oshkosh, and our ‘53 B-model was produced by Cessna as the 50th Anniversary of Flight Model. We took N146YS to the 100th Anniversary of Flight Convention and flew it to Kill Devil Hills…. so bringing her to the 75th Anniversary is something to which we look forward.
This engine has 1079 hours Since Major. This (#3) cylinder has never been off since that time in the early ‘90s. Running the compression test portion of the 100 hr inspection provided readings of 72 psi or greater (over 80) … except for Cylinder #3…which gave only 40 psi. Considerable “hissing” was heard at the oil filler cap. This initially caused me to suspect stuck rings (although it is often said that aligned piston-ring gaps can cause serious loss of compression, I’ve never witnessed that.)
Having drained the oil and removed the oil filter (Tempest filter)…I cut open the filter and removed the pleated filter-media and discovered a tiny chip or two…almost invisible…not much to draw attention… However, washing the filter-media in mineral spirits and then dragging a magnet thru the solvent resulted in this :
I had considerable difficulty getting #3 cylinder removed as the cylinder base-nut of the lower/aft thru-bolt refused to come off the bolt without spinning the thru-bolt, possibly loosening the nut on the opposite #2 cylinder base nut.
After almost 3 hours of frustration attempting to put a jam-nut over the #2 cyl base-nut… success was finally achieved stopping that thru-bolt from spinning and getting the #3 cylinder base nut removed. Pulling the cylinder and revealing the piston demonstrated the piston-rings were NOT stuck…they rotated quite freely.
I was beginning to fear I’d pulled a cylinder needlessly in believing it might be source of the metal filings. It wasn’t until I got the piston pin drifted aside and removed the piston that the following,… very surprising conditon was found. I have never seen or heard of this type of ring-failure…. The ring was not broken….(surprisingly)….despite the piston ring-land having “disappeared” Below the ring (between the top and 2nd ring).
But even MORE Surprising…. is the area Above ring #1 is hammered by the #1 ring creating a “recessed” area toward the piston crown. This #1 ring had to flex SIGNIFICANTLY for this to occur…. many times each minute…. Yet, despite being a brittle item, it did not break!
The cylinder bore itself has no detectable damage from 5 o’clock to 7 o’clock … but at the 6 o’clock region near the bottom of the piston-stroke range is a smooth “ridge” which can be felt with the naked finger. Inside calipers show the ridge to be about .003” … I believe this is where the top ring must have flexed during piston-travel reversal. It cannot be noticed by eyeball. It can barely be detected with a finger.
LESSONS LEARNED :
1) TEMPEST filters, which have a magnet incorporated in their media-base (near the by=pass spring)…. the Tempest magnet has no special benefit. Zero metal was caught by the Tempest magnet….Yet washing the media and then putting my magnet into the solvent resulted in Significant Metal Filings attaching to my magnet, which prompted me to investigate further.
2) Rings can contribute to cylinder-wall damage without breaking, especially when piston-lands are damaged or missing.
3) Never Fail to cut open filters and WASH the media in Mineral Spirits and Check with a Magnet for ferrous metal. Run the media-wash thru a coffee filter to discover other types of metal or contaminates.
I am ordering a new cylinder assy and will install it later this week.
I hope to bring this piston to convention for “show and tell” at the maintenance seminar, and hope to see you guys/gals there!