As some 3000 owners suffer through troubles with TCM crankshaft inspections, yet another quality-control related engine issue may be about to emerge: Accelerated wear on connecting rod bearings.
As we reported in last months issue, our survey of shops conducting the massive TCM crankshaft inspection turned up an incident of unusually high rod bearing wear on a low-time engine. Were seeing evidence that this may not be isolated.
As of early June, in an afternoon of phone calling, we found at least seven engines built by three overhaulers which appear to have prematurely worn rod bearings, some of which have come from Superior Air Parts but others that may have been supplied by other sources.
Babbitt Background
The problem first came light when a reader phoned to report that Superior authorized him to pull all 12 cylinders off a 25-hour-since-major-overhaul Baron simply because he found some funny looking connecting rod bearings.
The suspect bearings are Superior part number SA630826 with a date code of 10-98. They were found when the engine was opened for the TCM crank inspection, which requires the removal of only two cylinders.
Despite the low time, the bearings were in various stages of losing babbitt. Some of the bearings exhibited only superficial pitting and flaking of the wear metal. Others showed extensive and wholesale loss of all babbitt and copper layers, leaving only the steel shell behind.
The plain bearings used in the connecting rod and main journal areas are referred to as tri-metal bearings. Theyre constructed of steel upon which multiple layers of various metals are applied. The cast steel bearing shell is treated with a sintered or electroplated layer of copper-lead or copper-tin, often referred to as bronze.
The steel shell and copper-lead or bronze layer is used for strength while providing good heat transfer. The top layer is lead-tin, referred to as babbitt, a soft material that soaks up minor inconsistencies in the fit between a bearing and its journal.
Babbitt provides a malleable media in which the crankshaft can mold itself while suspended on a film of oil. Babbitt accepts a measure of oil-suspended contaminant by capturing the debris and burying the stuff in its soft layers, protecting the crankshaft from scoring when a contaminant is large enough to compromise the fit.
Babbitt wears slightly during the break-in process but this wear is minimal and normal. In fact, measuring it with conventional shop equipment is impossible, which is why plain bearings are usually scrutinized visually.
Mums the Word
When we contacted Superior to query about the bearing problem, we were told that they werent aware of it. Yet as more complaints surfaced and we followed up with more calls, Superior declined to return them.
Evidently something is up, however. As we continued to call around, we learned that Superior was hearing more complaints and was telling its customers that babbitt loss is of no consequence and that the copper-lead layer beneath is the important bearing surface.
Superiors John Lauer told customers that the manufacturer of its bearings considers the babbitt to be sacrificial and its naturally worn away during the seating process. (Thats a view not shared by Lycoming and Continental, by the way.)
Superiors official position is that if the babbitt is worn through exposing the copper-bronze layer, the bearing is considered airworthy. If the wear has progressed into the copper, the bearing should be replaced.
This doesnt square with experience in the field, however. Engines reaching TBO seldom have bearings that show any copper or bronze layers at all. In general, the babbitt remains on the bearing through the life of the engine with only a minimal loss in the loaded surface of the bearing. (In the aforementioned Baron, all of the bearings showed excess babbitt loss and in a couple, the wear had exposed the steel bearing shell itself. )
When such widespread bearing distress is found in one engine, its usually indicative of oil starvation or an assembly error. However, the two Baron engines showed no indication that heat was a problem nor does whats left of the bearings show the classic smearing and pulling typically associated with oil starvation. A bore check of the rods indicated that the fit to the journal was within specifications.
The loss of the babbitt and the base layers of copper seems to be one of bond separation rather than a wearing away of material. The babbitt loss left sharp edges in areas that had flaked off and chunks of material were missing, leaving large pits and low spots in the bearing wear surface. Some bearings in the set had blisters in the wear surfaces, indicating poor adhesion and bearing shell contamination prior to plating.
TCM, Too
Bearing wear may not be just a Superior problem. Continental is also getting bearings back for warranty replacement. The crankshaft inspection turned up factory reman connecting rod bearings fractured in a way that leaves a spiderweb look on the babbitt surface.
This cracked look isnt new and in fact, Federal Mogal, a major babbitt bearing supplier, maintains that its the result of fatigue that has no deleterious effects on bearing operation. Engine rebuilders have seen this kind of thing before so it may or may not be a normal wear pattern.
At issue here is where the bearings are coming from. Superior declined to name their supplier but the bearing industry is so small that its not hard to find out. Calling around, we learned that Glacier-Vandervell of Atlantic, Iowa is currently one supplier of rod, main and thrust bearings to Superior Air Parts. We dont know if this company is the sole supplier.
Also, its conceivable that one or several companies build bearings for both Superior and the OEMs. If the bearing problem is a manufacturing flaw-and the blistered and delamination of the babbitt surface that weve seen suggests that it very well could be-expect to see premature wear mushroom.
By early June, we learned of a Beechcraft Duke equipped with two very expensive Lycoming TIO-541 engines is on its way back to the rebuilder for metal contamination caused by, you guessed it, bearing distress.
Frankly, were not sure how widespread this bearing problem is but our sneaking suspicion is that its not going to go away. (We wish Superior were more forthcoming with information.)
Meanwhile, all owners of recently overhauled or remanufactured engines can do is inspect the oil filter carefully for excess babbitt flakes.
Unfortunately, this is very much an eye-of-the-beholder judgment call and further investigation requires pulling at least a couple of cylinders to have a look at the bearing surfaces. But the inspection is one worth pondering, given that bearing failures are likely to be catastrophic.
-by Paul Brevard
Paul Brevard is editor of Light Plane Maintenance.
