Cracked baffle and the “Fisher doubler”
Unless you have a fairly new engine cooling baffle or have been very fortunate, your engine baffles are/will be cracked soon enough. This Technote deals with preventative or post crack repair.
The baffle and seal act as the walls of a dam to retain the air, which is forced into the chamber (Plenum), by the forward movement of the airplane. This air removes about 10% of the heat from your engine, and governs the CHT. The baffle attaches to each cylinder head by two screws, one on either side of the cylinder, and here in lies the major cause of the cracking. As the engine operates, the cylinders actually move about in the rough shape of a circle, when viewed from the end on view. The screws do not allow the baffle to move so there is a constant fatiguing motion in progress.
WHAT TO DO
First off, whether or not you have cracks, remove the baffle. If cracking is evident, stop drills these cracks. Do this by drilling a small hole about 1/8” ahead of the apparent progression line. Then take a slightly larger drill and re-drill this hole, thereby ensuring a nice round hole. The hole which is initially drilled will always be slightly triangular, which encourages the propagation of new cracks. When drilled the second time the hole should be about 1/16” dia.
Now, use the sides of each portion of the baffle and make a cardboard template as per the diagram below. It does not need to be too exact except for the cut outs around the valve covers.
Use this template to cut 2 new pieces of T3 032 aluminum, that are the same size as the shaded area, in diagram 2 but add 1.250” to the line “A” in diagram 2. Bend the aluminum outwards in a break along line “A” through 90°.
Rivet the doubler to the outside of existing baffle with counter sunk rivets and drill the attachment holes shown as solid black circles in diagram 3.
This procedure allows for optimum spacing between screws and greatly reduces the chance of any new cracks starting. Reinstall this strengthened baffle after careful cleaning with a good solvent. Remember when bending aluminum to drill a good-sized radius hole at each end of the bend line before bending, also put a small 45° on all sharp corners/edges. When reinstalled, use a tube of High Temp. RTV and seal all the leaks between the baffle and the crankcase especially in the front and around the accessory housing. In doing this, visualize the air flowing over, around and between the cylinders and prevent air escaping from the plenum without taking some heat with it. Sometimes it will be necessary to re-fabricate the cowl support catch, of which there are some mighty fine examples. If you have a baffle with a bead rolled into the side just above the valve cover line, you will have to make the doubler only as high as the bead, the end result is about the same and only minor changes need to be made to the original template. I first saw this doubler in Salinas on Col. Fisher’s baffle and used it successfully on my 165 engine and re-fabricated the entire piece for several new 220 installations.
A special thanks to my good wife “Shirley Who Must Be Obeyed” for doing the diagrams for this Technotes.