When it comes to general aviation, I complain about products and services as much as the next guy. My pet peeve is the slow pace at which cutting-edge engine technology trickles into aircraft powerplants.
True, electronic ignition is making inroads but you still cant buy one and the valve train in your typical Lycoming-arguably the most wear-sensitive components of the engine-is little better than what youd find in the 1927 Massey Ferguson tractor your granddad left rusting behind the barn. Current aircraft engines are typically equipped with crude rocker arms with simple shelf or brass bearing insert material and a hardened rocker arm tip which directly contacts the valve stem and forces the valve to open.
Rocker arms are a simple fulcrum with the sole purpose of transferring the cam action-the bumpa-bumpa of cam lobes-the up and down of opening and closing valves. As automotive designers learned long ago, this type of valve train isnt the most efficient, encompassing lots of reciprocating mass and reversing motion hundreds of times a minute. Not to mention the friction of all those moving components.
Eliminating wasteful upper end energy and needless valve train friction is so central to efficient engine design that it was the driving forces in the development of modern overhead cam engines which have no pushrods and (usually) no rockers at all. The cam works directly against the valves.
This is good for cars. But is it also good for airplanes? Probably. Although true OHC aircraft engines are but a dream, the next best thing is finally coming on the market in a big way: Roller rocker arms. In OHV engines such as our TCMs, Franklins, and Lycomings, with rocker assemblies using push rods and hydraulic lifters, the pressure applied to the valve to open it isnt directly downward or exactly parallel to the valve stems centerline.
The rocker assembly operates in an arc so the stem moves inward, introducing side loading, which is thought to be a prime cause of excessive valve guide wear. Side loads on the valves also introduce friction, which robs power and efficiency.
As valve guide wear progresses, oil can pass into the combustion chamber from the top, causing ring sticking and other nastiness. Some engines seem more susceptible to this than others, to the extent that mid-run valve work is all but inevitable.
Eliminating this friction and side-loading is where roller rockers come in. Instead of just a hardened and rounded tip to push on the valve, the roller rocker has a small roller on an axle that rotates ever-so-lightly when the rocker is pushed against the valve.
This absorbs the side loads on the step and all but elimiminates the excessive friction produced by a conventional rocker arm. This type of rocker arm has been in use for years in auto racing circles, where the elimination of excess valve train friction has markedly dropped oil temperatures-by as much as 20 degrees-under race conditions. Roller rockers have another advantage: needle bearings. The plain bushing that rockers rotate on are replaced with precision needle bearings, or the sort found in high load industrial applications and, again, in racing engines. Theoretically, this also eliminated bushing wear, which dumps wear particles into the oil.
Not the First
ECIs venture into the roller rocker field is not exactly a pioneering effort. Way back in 1986, Terry Capehart introduced roller rockers, producing some 200 sets under his Performance Engineering Inc. banner. The STCs applied to a limited number of engines, mostly found in the Bonanzas which constituted Capeharts market.
Most of these rockers were produced by several sub-contractors and both the rocker and Capehart got rave reviews from aircraft owners. Subsequently, a few failed due to improper heat treating, since the roller is quite small and carries a lot of load.
After seven known failures of the rocker casting, an AD was issued requiring replacement. None of the failures resulted in serious engine damage or off airport landings. Even after the AD was issued, many aircraft owners refused to change their roller rockers, the AD notwithstanding. Interestingly, none failed past 250 hours in service.
Having withdrawn what was probably a good idea that simply needed a little tweaking, Capehart went on to other projects and, eventually, got out of the piston engine business altogether. His shop, Ultimate Engines, still does business in Mena, Arkanasas.
Enter U.S. Airparts
In 1997, after assembling a group of investors familiar with the value of roller rocker arms, U.S. Airparts was formed. U.S. Airparts purchased the assets of Performance Engineering, including their two STCs for roller rockers. While pursuing a design for another rocker arm, an AD was issued in January, 1998, calling for the removal of all PE92 exhaust rocker arms from service. U.S. Airparts stopped work on other projects and had three companies do failure analysis on the old PE design.
All three reports concluded that the failures (seven exhaust rocker arms) were due to improper heat treat at manufacturing. U.S. Airparts figured this could be addressed with quality control.
After several design changes, including the addition of radii in stress-concentrated areas, and a 25 percent increase in tensile strength in the castings, U.S. Airpart had a new product.
They instituted a quality control program which includes three x-ray inspections, fluorescent penetrant inspection and testing for proper heat treat on each rocker produced. This re-certified design is now in production.
U.S. Airparts has partnered with a major player in the engine aftermarket, San Antonio-based Engine Components Inc. ECI will produce and perform quality control checks on the roller rockers they produce for U.S Airparts, including a heat treat test on every unit.
Whats in it for ECI? They plan to add roller rockers as an option for their line of new Classic Cast Cylinder assemblies to improve reliability of the valve and guide system. Gary Garvin, ECIs CEO, told us ECI would like to eventually extend the warranty on Classic Cast cylinders treated with the Cerminil process.
U.S. Airparts also promises a new companion product, an adjustable push rod available this month for most TCM engines after which the adjustable rods for Lycomings will follow. These push rods will eliminate the need for shops to stock multiple sized push rods and will also allow the mechanic to blueprint the valve lash (dry tappet clearance) in increments of .003 inch.
What The OEMs Say
According to Lycomings VP of Engineering, Rick Moffett, theres a significant amount of sliding friction which accompanies the usual OEM non-roller rocker design.
Still, he says, the prime source of valve guide wear appears to be the bottom of the guide area where the majority of the heat transfer takes place, not at the top where the roller would help. Moffett says one persistent problem with stock rockers is the shape of the tip, which is occasionally field dressed inappropriately on a belt sander during overhauls.
Lycoming has examined roller rocker arms but believes the primary benefit of roller bearings in the valve train would be for roller cams, which theyre considering. One downside of roller rockers is extra parts count, says Moffett. (The roller and needle bearings.)
But, says U.S. Airparts, the needle bearings are fully encased assemblies and that it would be impossible for them to escape given the side clearances on the rocker shafts.
Teledyne-Continental says roller rockers arent under development in Mobile, since theres no data showing them to be beneficial for the TCM line. Worth noting is that Continental said the same thing about GAMIjectors but now has its own line of precision flowed injectors.
What to Do
Problems with valves and valve guides are legion in the aircraft industry. Both Lycoming and Continental have had their turns in the barrel on this issue. Given how the best engine improvements seem to come from the aftermarket based on focused research areas the OEMs ignore, maybe roller rockers are a good fix for valve wear.
In any case, its obvious that in the typical aircraft engine, the valve train and cylinders are the weakest link. Roller rockers seem to address part of this problem head on.
When GM introduced roller rockers and cams in their well0regarded small block 350s for Corvette, they claimed increased smoothness, efficiency and a 20 HP increase in power. Capeharts Performance Engineering said that a claimed increase of 6 to 9 HP for aircraft engines was documented on dynamometer runs.
However, U.S. Airparts makes no claims about increased power. As for smoothness, we agree with Lycomings Moffett: It may be in the eye of the beholder.
At press time, STCd kits were available for TCM 470/520/550 and O-200/300 series engines only. For a 470/520/550 TCM engine, the kit cost is $1795. In addition to the 12 rockers, these kits also include new rocker shafts, thrust washers and shaft bushings.
For an O-200 Continental, cost is $695 complete while an O-300 kit sells for $975. An STC is in the works for the TCM six-clyinder O-360 series projected costs are in the $1395 range.
At press time, an STC was in progress for Lycoming parallel-valve 360s, to be followed by angle-valve 360s for both four- and six-cylinder Lycs. Deliveries expected this summer on some Lycoming models with pricing similar to TCM.
We know from experience that for various reasons many engines dont make TBO. Some dont even get close. For that reason, we think roller rockers are worth considering as a hedge against valve wear. They arent especially expensive and can be installed without having to remove cylinders.
Since the labor is relatively minor-about four hours- roller rockers look promising to us, although were not willing or able to say these devices will work miracles. Well do a more in-depth report when weve seen some dyno data and field tested an engine with roller rockers.
Also With This Article
Click here to view the basic roller rocker theory.
Click here to view Addresses.
Click here to view the Checklist.
by Coy Jacob
Coy Jacob owns and operates the Mooney Mart in Venice, Florida. Hes a long-time contributor to Aviation Consumer.