by Coy Jacob and Danny Schultz
Choosing the right cylinders has always been a critical part of buying an overhaul, never more so than now. Since about the mid-1990s, the new and replacement cylinder market has become intensely competitive. We are constantly being asked by readers for recommendations on cylinders.
For some time, we have been fielding reports of disappointing service life for many cylinders. Although some cylinders seem to fare better than others, no brand appears entirely immune from complaints.
In the July 1999 issue of Aviation Consumer, we examined and carefully measured cylinders from various sources-including factory new ones-and found that many did not met the factory specifications for valve machining, specifically for dimensions of valve seat contact area. Further, we found that many cylinders appeared to have concentricity inaccuracies between the valve guide bores and seat.
As a means of testing quality for ourselves, we elected to do our own trial by pitting a popular new/reman cylinder supplier ECI (Engine Components Inc.) against factory new cylinders from Continental. We installed three of each on a Continental TSIO-360-MB engine. Our plan was to remove these cylinders about a third of the way towards TBO and then again closer to TBO, to check bore and valve wear.
However, we had occasion to remove the cylinders earlier than that due to a minor prop strike and we took this opportunity to examine the cylinders and valves for wear.
Frankly, the results werent encouraging. Both the TCM and ECI cylinders showed what we consider to be worrisome valve guide wear and the TCM cylinder-in only 160 hours of flight time-had lost its cylinder choke and exhibited measurably more cylinder bore wear than did the ECI jugs, which showed virtually no bore wear.
Continuing our detailed examination of new cylinders from Continental revealed that the company appears to have improved its quality control somewhat, at least in terms of valve seat dimensions. More on the specifics later, but first, lets review what specifications a cylinder ought to meet to be considered good or at least conforming to the manufacturers specifications.
Valves, Guides and Bores
Both Lycoming and Continental establish dimensional specifications for new cylinders and they also publish what are called service limits specs for used or overhauled cylinders. Service limit cylinders are allowed to exhibit more wear and looser tolerances than new ones are but are still considered airworthy.
For this discussion, were interested in several key dimensions, including cylinder choke, cylinder bore consistency, valve guide bore dimensions and how accurately the seat is machined to mate with the valve, which is important in allowing the valve to transfer combustion heat to the cylinder head and cooling fins.
Last and related to the valve-to-seat fit, the valve guide bore should be concentric with the diameter of the valve seat. If it isnt, it wont matter how accurately the guide is machined because the valve simply wont fit into the seat accurately if its off center. If this fit is bad enough, the off-center valve will eventually hammer the seat and damage it, leading to lost compression and, eventually, damaged valves.
Cylinder choke-which is a tapering of the cylinder bore toward the top of the cylinder-has become somewhat of a controversial issue. The purpose of cylinder choke is to counteract thermal expansion so the piston rings will still seal as the piston reaches the top of the cylinder. As a cylinder wears, the bore increases slightly but the choke should remain constant.
This is often not the case, as well explain later. Worth noting is that some cylinders dont have choke and some engine experts say it really isnt necessary anyway. But if its not, why bother putting it into the cylinder in the first place?
Valve guide dimensions are critical, too. If theyre too loose, the valve can wobble, causing incomplete sealing and premature wear. Second, the valve guide material must be resistant to wear for it doesnt do a bit of good to machine the guide accurately only to have it wear out of limits in a few hours because of material shortfalls.
The factories publish specifications for most of these dimensions and while were not contending that all of these are arbitrary, we do take issue with factory recommendations for measuring valve guides. We think it misses an important aspect of potential gude wear.
Our engine field test revealed what appears to us to be more valve guide wear in both TCM and ECI cylinders than we would like to see.
Bottom or Middle?
Logically, the critical area for valve guide wear is at the bottom, near the business end of the valve where it seals against its seat. In our view, if that portion of the guide becomes sloppy and worn, the valve wont seat correctly. Moreover, the wear may accelerate as the guide becomes more worn.
But Continental sees it differently on two counts. According to John Barton, Continentals chief technical officer, TCMs SB 98-2 calls for measuring wear in the middle to upper end of the valve guide bore, not the bottom end. Further, he says, Continental data suggests that guide wear is front-end loaded, meaning the rate of wear decreases with time and doesnt accelerate.
ECI vice-president of engineering Jimmy Tubbs doesnt necessarily agree. When asked about how to measure the guide wear and given the fact ECIs own guides show as much wear as the TCM guides, Tubbs opines that the guide should be measured where wear occurs first, which he agrees is at the bottom end of the guide. He says this is where most of the heat is dissipated and also its the most difficult to lubricate.
Putting some numbers on this, in our test engine, with only 160 hours on new cylinders, we found nearly all the guides had worn more than .010-inch when measured at bottom end, near the seat. One was worn .013-inch, which, in our opinion, renders them significantly worn, even though TCM says theyre in spec.
Frankly, given the fact the cylinders were only about 10 percent into their expected life and were operated according to the POH with a state-of-the-art engine monitor, we were surprised at this amount of wear.
While it is true that our test engine still had acceptable compression ranging from 69 to 72 PSI, if the guide wear continued at the same rate, we wonder how well the compression would hold up in another 200 hours.
Would better guide material reduce wear? Tubbs says yes and Lycoming has been quietly using guides with higher chrome content for a while. TCMs Barton told us his company has nothing in the works on revised guide materials.
Measuring the bore wear of a cylinder is also one means of judging how well the jug is holding up. Both factories publish new limits and service limits for cylinder bore diameter. For the TSIO-360-as well as other cylinders-Continental recommends measuring the bores at three points along the length of the cylinder, as shown in the diagram at left.
The table shows that the ECI cylinders exhibited hardly any wear at all but the Continental version showed some bore wear that already placed them in the service limits category, a finding that we think is consistent with numerous reader complaints weve heard about premature barrel wear in Continental cylinders.
Although we cant put our finger on why, ECIs barrel finish seems clearly more wear resistant than does Continentals, based on this confined test.
Further, the Continental cylinders showed noticeable step wear at the tops of the cylinders while the ECIs did not. This wear pattern was significant enough that we wouldnt re-install the cylinders without some attention to the bores. Other than the guide wear and some guttering on the intake valves, the valve train on both the Continental and ECI cylinders was in acceptable shape, as it should be after just 160 hours.
When we described our findings to TCMs John Barton, he says the company reserves the right to examine the cylinders before rendering a definitive opinion. (We sent the cylinders to TCM for inspection.) As with valve guides, Barton says cylinder wear is not linear but plateaus with time before resuming a graceful wear pattern. While this may be true, it does nothing to explain the many reports we receive of apparent premature cylinder wear.
As we did during our last cylinder comparison, we looked at some new cylinders, specifically three each of Continentals TSIO-360 and O-520 series cylinders. Last time, we found that the factory wasnt building these cylinders to meet its own stated specifications and we wanted to see if they have improved. In a word, the answer is yes, they have.
But in our view, there are still problems. We found that all of the cylinders had accurate barrel bores and valve guide bores; no problems there.
Valve seats were a different story. As shown in the chart above, valve seat dimensions were somewhat inconsistent. We measured these by removing the valves in all of the cylinders and measuring the width of the seating surface at four clock positions, 12, 3, 6 and 9 oclock. This yielded 24 dimensions for each set of three cylinders.
Rather than average these, we have presented the maximum and minimum dimensions in the results table. As explained in the charts, the TSIO-360 cylinders were mostly well within Continentals stated specifications. All but two of seat dimensions were within the required range.
This is a considerable improvement over our last examination of Continental cylinders, at which time we found that most of the seat dimensions were out of spec. Our only real complaint is that one of the seats had a step in it, which, in our view, is indicative of a faulty machine set-up or off-spec tooling. The findings werent quite so rosy for the O-520 series cylinders. We found these seat widths to be substantially in variance with Continentals own stated specifications.
We were disappointed to find that only three of the 24 data points met the required specification from Continentals published cylinder documentation.
There are several issues at hand here and were not sure anyone has a complete understanding of them, including the manufacturers. If a cylinder is to make it to TBO or, at the least, deliver acceptable service, it seems to us that it should at least meet factory specifications. It seems logical that this is a good starting place.
To the issue of barrel wear, it seems clear to us that the ECI cylinders wore noticeably less than did the Continental versions, flown for exactly the same number of hours and in exactly the same way. We cant honestly say the Continental jugs were headed for low compression and the scrap heap but given the choice of a little wear or more wear, we think we know which one to pick.
We repeatedly hear reports of low compression on Continental cylinders at relatively low hours and we think our test tends to give credence to these reports. In our view, the ECI barrel finish is simply more wear resistant, all other factors aside.
As ECIs Tubbs tells us, valve guide wear continues to be a worry and we think both companies need to address this with a more wear-resistant guide material. We have found that many cylinders which have serviceable bores have worn guides that require rework. We think this shortcoming should be corrected.
Continental has clearly improved its valve seat quality control on at least the 360 cylinders. We are still troubled that the 520 cylinders did not meet specifications for valve seat dimensions.
And while its true that an out-of-spec seat may not cause a cylinder to tank prematurely-or at all, for that matter-its also true that a too-narrow mating surface reduces the valves ability to shed heat and that provides less margin against thermal valve damage.
In other words, it may not hurt to have too wide a mating surface but it can definitely hurt to have one thats too narrow, at least for exhaust valves. In any case, the seats should meet the stated specification. In a future article, well examine cylinders from Superior and ECI, for both Continental and Lycoming engines.
Also With This Article
Click here to view “Checklist.”
Click here to view “Cylinder/Guide Wear Findings.”
Click here to view “New Valve Seat Findings.”
Click here to view “IRAN: The Cylinder, Not the Country.”
Coy Jacob owns The Mooney Mart in Venice, Florida. Danny Schultz is a manufacturing engineer and A&P.