jacobs engine exhaust valves: problems and remedies

The Jacobs R755 series of aircraft engines was introduced to the aviation world at the Cleveland Aviation Expo in 1934.  The engine was well received and was immediately fitted to the Cabin Wacos and Beech Staggerwings then just entering production.  During WWII tens of thousands of R755-9 (245 hp) engines were built to power the Cessna “Bamboo Bomber” and some important improvements came to the engine through its association with the military.  After the war, when Cessna introduced the 195 “Businessliner”, the R755 was the natural choice for a powerplant.  Forty years later when Classic Aircraft began looking for an engine to power their much-updated Waco YMF-5, the Jacobs R755B2 came out the winner.

For all its long and dependable service, the R755 engines have had one quirk that has antagonized and perplexed both owners and mechanics since its introduction:  exhaust valve leaks.  The problem is so widespread that there was even a shade tree procedure developed for cleaning the valves without removing the cylinder, AKA the Jacobs Rope Trick.  By a conservative estimate, 95% of the Jacobs cylinders sent in to us for repair are removed from service due to exhaust valve leaks.  

Some of the recent blame can be laid on the fuel that we currently have at our disposal.  The R755 engines were designed for 80 octane aviation fuel with .5 ml of Tetraethyl lead per gallon added.  The 100LL that most Jacobs owners now burn is compounded with 2 ml of lead, four times the amount for which the engine was designed.  The result is lead build-up on valves, seats, and guides.  We had one newly overhauled engine stick five of its exhaust valves with only 17 hrs. since overhaul.  When we attempted to disassemble the cylinders it was necessary to drive the valves from the guides due to the lead build-up on both the valve stem and inside of the guide.  We cleaned the valves, honed out the guides, reassembled the cylinders, and the engine is running fine to the present.  Those operating their engines on auto fuel have no problems with lead, since there is no lead in unleaded auto fuel.  However, there are other additives, known only to God and the refineries, which also can cause deposits resulting in exhaust valve leaks.  Since the Jacobs exhaust problem has been going on for seventy or more years though, fuel cannot be the only culprit.

The geometry between the valve, the rocker arm, and the push rod is laid out in all radial engine designs to cause the valves to rotate slightly with each actuation of the rocker arm.  This rotation of the valve gives a different orientation of the valve to the valve seat with each stroke, resists the build up of deposits and keeps the valves from burning.  A properly rotating valve will have a shiny round spot on the tip of the valve where the rocker arm contacts it at many angles.  A valve that is not rotating will have a square shiny spot (or sometimes will wear a depression) on the tip.  Unfortunately, the Jacobs rocker arm geometry is not quite right and the valves do not rotate as well as they should.  The geometry on the Jacobs L6 is so radical that the sides of the push rods often contact the cylinder as they operate and deep grooves are machined into the push rods.  Continental W670 valves rotate, Lycoming R680 valves rotate, Jacobs exhaust valves build up deposits and leak*. 

In 1970 Page Industries developed an exhaust valve rotator to replace the upper exhaust spring washer on all late Jacobs engines models.  Over the years the rotator has helped with the problem, but the rotators sometimes fail internally and are difficult to accurately test to see whether they are actually working or not.  Once the rotators stop rotating under a load, we are back to the original problem of deposit build-ups and leaking exhaust valves.

According to Bill Hampton, who served as the last Service Manager of Jacobs in Pottstown, PA, in the mid 1950s the factory experimented with hardened steel valve seats.  A set were installed in the Company’s Cessna 195 and flown for several hundred hours with no exhaust leaks.  Later, the airplane was sold and continued to give good service until someone at Jacobs remembered that there were experimental (read “illegal”) steel seats in the airplane that they sold.  A quick recall was made and a free top overhaul was given to the lucky customer.  Unfortunately the customer also got original aluminum bronze exhaust seats in the trade and went back to having periodic exhaust valve leaks.

In 1997, at Radial Engines Ltd we began to experiment with the same design of hardened exhaust valve seat as Jacobs used in Pottstown so many years ago.  The seats worked well as a direct replacement for the original seats and after a short time we were granted FAA approval to install them in customers’ engines.  Sometime later Air Repair incorporated hardened valve seats into Jacobs Service Bulletin #68.  

Since beginning to install the hardened valve seats the leaking exhaust valve problem has virtually disappeared.  It seems that lead and the other fuel deposits just will not stick to the steel seats.  The combination of functional valve rotators and hardened steel valve seats appears to have cured a problem that has plagued Jacobs operators for years, and has breathed new life into one of the truly great radial engines

* Roughly 80% of the cylinder repairs that we do are to Jacobs cylinders; other manufacturer’s exhaust valves rarely leak.  W670 cylinders even came from the factory with hardened steel seats.  That is not to say that the other engines are superior designs and do not have their own set of quirks—each different engine model has its unique set of design issues and problems.