Roller Tappets

[Guest]

4 hours ago, LANCECASPER said:

Must be at least two if @M20Doc has one in his junk drawer.

The one I have came from our local engine shop.  Ron was doing a prop strike inspection and had to replace them all per Lycoming.

Clarence

[ArtVandelay]

The one I have came from our local engine shop.  Ron was doing a prop strike inspection and had to replace them all per Lycoming.
Clarence

I assume Lycoming doesn’t require the push rods or lifters to be replaced? If true, makes you wonder if they’re not a weak link.

[jetdriven]

The needle bearings can’t be inspected since the unit is not able to be disassembled. 

[jetdriven]

Again, from thousands of engines that have been in service for 15 years now, it looks like the failure rate is very low. One or two during that time frame that is public knowledge. There have been plenty of dual mags that have fallen off since Then as  well. And probably more connecting rod and propeller failures than roller tappet failures also. 

[PT20J]

Statistically, one occurrence is meaningless. If we start seeing a rash of similar failures, then that would be concerning. The Lycoming lab couldn’t determine a root cause. From the Lycoming lab report in the docket:

3.0 Summary
Severe spalling and galling damage was observed on the #4 exhaust camshaft lobe along with the outside diameter of the associated roller tappet roller. Additionally, spalling was observed within the #4 exhaust roller tappet body ears supporting the roller shaft. Evidence of fatigue, in the form of fatigue striations, was observed on one of the #4 exhaust roller tappet body ear fracture surfaces. Witness marks on the inside of the crankcase bore holding the #4 exhaust roller tappet along with the uneven wear on the #4 exhaust cam lobe might indicate that slight rocking of the roller tappet occurred in the case at some point.

The root cause of the fractured roller tappet was not determined but was not due to a material nonconformance of the roller tappet assembly or the camshaft. The severe wear between the roller and camshaft likely led to non-standard loading conditions, perhaps worsened by or caused by, slight rocking of the roller tappet in the crankcase bore, ultimately resulting in failure of the roller tappet body and the release of the roller, roller shaft, and needle bearings.

 

[carusoam]

I did not really see large craters that can often occur in cam lobe surfaces...

I was expecting a dirt road, replete with pot holes, given their description...
 

Mixed memory of craters in both cams and flat cam followers...
 

PP thoughts only...

Best regards,

-a-

[jaylw314]

I've said before that I don't believe the numbers are not sufficient yet to tell us that roller tappets are better than flat lifters.  Conversely, I'd caution against falling for the "but sometimes!" fallacy, where we overemphasize a rare type of failure.  Kind of like how some people decide to not wear seat belts because it might trap them in a burning car, even though their expected loss is less while wearing them.

Either way, I don't believe the data is sufficient yet for either conclusion.

[larrynimmo]

From my perspective roller tappers are much less likely to fail...but when they do fail, it can cause catastrophic consequences.  It add tremendous potential to get over 3,000 hours on the lower part of a Lycomming engine.

solid tappets die a slow death as they wear out and take the cam with them....engine performance diminishes but almost  never takes a plane out of the sky...once the wear starts, engine must be pulled, split and replace cam and lifters even if nothing else is wrong.

wear is promoted with corrosion which is not a friend of the infrequent aviator.