jacobs baffle modifications

Over the years one of the overlooked maintenance items with the Jacobs engine has been the required cylinder base nut retorque at one hundred hour intervals.  The Jacobs R755A2/B2 Operators Manual actually says “Check at first 25 hour inspection after installation and at 100 hour inspections thereafter”.  The reason that the nuts no longer hold the specified 250-300 inch lbs. of torque after 100 hours has more to do with the different expansion rates of the steel cylinder base, the magnesium rear section of the power case, and the aluminum front section of the power case rather than the type of cylinder base nut used.  

We have overhauled and maintained literally hundreds of the R755 engines over the past twenty-five years and have seen no evidence that either the original AN365-624A cylinder base nuts (fiberlock) or the later Pratt & Whitney cylinder base nuts with pal nuts are actually moving or backing off.  Neither style of nut rotates on its own, but after a few hours of engine time you will find some of both styles of nut no longer holding the required torque. Both styles of nut require retorque because the dynamics involved have to do with the heating and cooling cycles of the engine and its effect on the three dissimilar metals held together by a stud and nut.

This project of retorquing cylinder base nuts has always been anticipated by owners and mechanics with some sense of dread because it typically involves removing the engine cowling, swinging the engine, removing the exhaust system and engine baffling, and finally getting down to the place where the cylinder base nuts can be reached and retorqued.  As a result, the job has often been delayed for “just a few more hours” (often to the point that some engines never have this maintenance done), sometimes with dire consequences.  

Once the nuts are no longer holding the original torque, each combustion event causes the cylinder to move slightly up and down, inducing fatigue in the stud.  Several thousand of these cycles will ultimately break studs and redistribute the load from the full number of studs to only the remaining studs.  This results in overload to those studs and further stud failure.

We have a cylinder base stud drill jig that we loan out to customers repairing their engines in the field after having cylinder base studs break off or pull from the case.  In extreme cases all the cylinder base studs break on a given cylinder and the cylinder departs the power case.  Fortunately, the 195 cowling catches the errant cylinder before it goes through the windshield, wing, or landing gear. Nevertheless, this scenario is one to be avoided.

In order to facilitate easier cylinder base nut maintenance (and to encourage this maintenance to be done at all!) Radial Engines, Ltd. obtained approval earlier this year to modify the existing Jacobs engine baffles to enable the retorque job to be done without removing the baffles. The modification consists of cutting away the bottom one inch of aluminum from each baffle and replacing it with rubberized baffle seal. 

The rubber seal is riveted to the baffle and when the baffle is reinstalled the seal tucks in between the cylinder base stud and the cylinder base.  This seal provides the same air dam as the old aluminum part did, but allows access to the cylinder base nuts without removing the baffles.  Thus, retorquing the cylinder base nuts goes from an eight-hour job with the original baffles to a one-hour job with the modified baffles.  

If you consider that you will need to do this retorque project twelve times over the life of your engine, the cost savings at today’s labor rate will be about $5500.  The best time to make the modification is the next time the hundred-hour retorque is due.  The approval that the FAA granted us allows us to modify original baffles that are sent in for repair, and the process usually requires that we have the baffles in our shop for two days.