Our old Franklin engines are prone to leaks in very predictable areas, with that in mind, and how difficult it can be to track down a hidden leak I thought I would share my experience with our members.
The very first thing you want to do is get yourself a set of silicon seals for the push-rod tubes, and valve covers from Real Gasket Co in Corvallis, Oregon, phone 800.635.7325, also if you can afford an extra $50 or so, there is an especially nice push rod tube puller, available from Frank Bailey in L.A, call him at 310.835.2836.
Clean the entire engine with a blaster and solvent, the secret to locating a hidden leak is to start with an absolutely clean and dry engine. Remove all the top spark plugs and ground the magnetos. Set #1 cylinder to top dead center, remove the valve cover on # 1 cylinder, and the rocker assembly. Pull the push rods and then remove the push-rod tubes. After cleaning every thing up, re-install the tubes with the new seals, note that there are 2 sizes of seal, the larger fits on the bottom of the tube.
Now inset the push rods and reinstall the rocker assembly, using new lock washers, available from A1 services, phone 903.626.5115. At this point you want to confirm that the piston is at top dead center, and then set the valve clearances to .040” on both valves. Replace the valve cover with the new gasket. The gasket goes on dry and you must be very careful not to over-tighten the cover screws. The torque on these screw is 20 inch pounds, so little torque in fact that you will be convinced that the screws are too loose and will fall out!
Proceed to cylinders #s 4,3,2,1 in the firing order sequence. You will find that you only need to rotate the prop about a ¼ turn to each advancing top dead center. When this is completed you will have reset your valves and cleaned up 90% of your oil leaks.
Next, beg, borrow, steal or purchase an Ultra violet lamp, either fluorescent or incandescent will do. Then add bottles of dye, available from most auto parts stores, (I get mine at the local GM dealer) to the crankcase. Re-cowl the engine and take the airplane around the patch a few times, then back into the hangar, de-cowl, close the door and inspect the engine with the ultra violet light.
Any leak will show up as a yellow line or smudge. This part of the process can get very frustrating, since too little operation causes no leak, and too much operation causes so much leakage that the source can’t be located. You should be prepared to remove and refit the cowl several times before the exercise is complete.
On the Franklin most of the leaks that remain after the push-rod/valve cover seals are replaced seem to occur at the rear of the engine where the accessory case bolts to the crankcase in the area of the oil pan flange. The fix that I use is to get the area absolutely clean and grease free, (Use M.E.K.) and then seal the joint with a bead of R.T.V., smoothing over with a wet finger, will get the leak most every time.