Ly-Con O-Ring Trick

Ly-Con makes a case for O-ringing engine crankcases leak-free.


No one likes oil leaks, and after decades of snotty-nosed flat motors California engine shop Ly-Con is doing something about it. Taking a cue from everything from various round motor pieces to numerous flat-engine cover plates and accessory parts, Ly-Con has developed a system for O-ringing Lycoming and Continental crankcases.

Probably the most appreciative will be Ly-Cons helicopter customers. Ly-Con owner Ken Tunnell says no matter how well his shop seals the vertically mounted engines, about half of them end up leaking along the case parting line. The problem is so well-known by the chopper crowd that most commercial operators slather JB Weld or other Band-Aids around the case parting line before installing an overhauled engine in the airframe. “Its going to start leaking in 300 to 500 hours,” they explain as they spread the unsightly stuff.

Fixed-wing fliers also commonly suffer from engines whose case halves perpetually weep between the starter and alternator, but at a lesser rate than the helo drivers. Still, at least 10% of the flat engines out there are busy oiling everything firewall forward-and, eventually, several items aft-and wouldn’t we love to get rid of that mess? Come overhaul time, everyone would appreciate a guaranteed leak-free job, hence the new O-ring procedure.

Thread of a Chance

Both Continental and Lycoming seal their cases using a combination of Hylomar brand sealant and a length of silk thread laid between the case parting flanges. Its a time-honored system typical of pre-war powerplants, and it does keep the cases from oozing oil faster than the operator can pour it in. But it isn’t forgiving of imperfections along the case parting line and definitely isn’t foolproof. Furthermore, in this age of seamless perfection in airframes, even a wisping discoloration along the case parting lines isn’t to be tolerated as it was back in granddads day.

Ly-Cons Cure

Ly-Cons solution involves grooving one of the crankcase flanges and fitting it with a length of O-ring material. The depth and shape of the O-ring groove have been carefully developed to provide the right amount of O-ring crush, along with a dovetail shape to hold the O-ring in place during engine assembly. No adhesive is used, installation is generally foolproof, and the finished engine doesn’t leak.

At the time of our visit, Ly-Con had fitted several of its high-visibility airshow customers engines with the O-ring cases and accumulated hundreds of hard-core aerobatic and ferry hours of leak-free operation. “Not a drop,” Tunnell said with a smile when we asked if there had been any unwanted oil.

While a long overdue freedom from leaks, or even alleviating the fear of leaks, are the O-rings most immediate advantages, there are others. Engine assembly is also eased, as fitting the O-ring is less demanding of the technicians skill than deciding on how much sealant to apply to the case halves, remembering if one or both sides of the case get glued and laying the thread without accidentally getting it too close to the bolt holes.

Even better, the O-ring could very well save the four-digit expense and consumption of precious case life by avoiding align boring at the next overhaul. The traditional string-sealing method demands flat, imperfection-free mating flanges-the same flanges that often lightly fret during use. Such distress to the case mating flanges, however shallow, requires case machining to restore an effective sealing area with the string method. This means milling the case flanges, which in turn typically requires align boring the crankshaft and cam bores to maintain round bearing saddles. This is a large price to pay for simply sealing the case, and can be completely avoided with the O-ring, because light fretting distress along the case parting line (not to be confused with fretting around the main bearing saddles) will not compromise the O-rings sealing capability. The case may be re-used without milling and subsequent align boring.

Why So Long in the Making?

If O-ringing the case halves is so beneficial, and seemingly easy to execute, how come it hasnt been around for years? At the OEM level, the answer likely involves the staggering traditionalism of the aviation world, along with the costs associated with making such a change in the certified arena.

In the rebuild aftermarket, the answer again resides in the certification paper storm, plus the need to accommodate a huge number of Lycoming and Continental case variations. After 60 years of production, who knows how many times the case flange shape or bolt-hole position have changed in the flat engine universe. Given manual machining, it would be impossible to develop techniques or tooling to accelerate the grooving process, thus relegating the O-ring grooving to a tedious custom process with every case.

With CNC machining, however, the ability to automate the process via software, plus the incredible speed afforded by todays powerful CNC machining centers, make handling the numerous case variations possible. The trick is having the financial stones to step up to a CNC machining center. Its a big, six-figure step for aviation engine shops, but one Ly-Con recently made to compete with exclusives such as case O-ringing, along with building speed into its burgeoning cylinder porting business (of which more in a later article).

At press time the case O-ring modification was available for Experimental engines, but by the time you read this Ly-Con calculates it will have FAA approval for certificated engines via a Process Specification; that is a format available to licensed repair stations such as Ly-Con. The company is also working on an STC for the O-ring, though that will take a bit longer. In the meantime, Ly-Con already has a process patent pending on the procedure.

Ly-Con has developed the O-ringing for all of the popular Continental and Lycoming engines. Pricing was preliminarily set at $648 for a four-cylinder case and $972 for a six-cylinder case. It is worth noting that once the case has been grooved for the O-ring, the only expense at future overhauls will be the negligible cost of the new O-ring material.

Of course, the standard glue-and-string sealing method is still available at no extra charge, so we now have a choice in case sealing. Were also looking forward to other machining improvements as Ly-Con explores the capabilities of its new CNC equipment. The company is already working toward running the majority of its cylinder head porting jobs through the machine. Well just have to wait and see what surprises they come up with after that. 

For more information call Ly-Con at 559/651-1070 or visit the web at Direct links can be found at


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