Recently I read the SRA article outlining the Sensenich Service Bulletin No. WSB-1 concerning the importance of maintaining a correct bolt-torque on wood propellers. This article explained how the driving torque of the engine has to be delivered to the propeller by the surface friction of the retaining plates vs. the more obvious thought that the attaching bolts were to carry the load. To be honest I am relatively new at aircraft ownership and propeller upkeep. Until this time my preflight in this area had consisted of merely ensuring that the front nuts were in place and the cotter-pins secure. I didn’t even own a torque-wrench. So, on my next trip to the airport, armed with my newly purchased micro-torque wrench, I did some investigating. I found that two of the nuts were loose enough that, with a little effort, I could rotate them by hand. Another clue was evidence of a small amount of yellow-brown dust in a halo that was centered around the bolts on the front face plate. The rear hub face of the prop was covered with the stuff. I have owned the plane for just over two years and for all I know that was the original torque applied since the existing prop was mounted in 1993. There wasn’t any evidence in my paperwork that the torque had been checked during the annual inspections. Worse still, on the backside of the hub one of the bolt heads was sheared completely off. O.K. – no flying that day.
The design of the Lycoming/Sensenich combination is such that because of the lack of clearance you can’t remove the prop bolts without first removing the entire prop-assembly from the engine. So, armed with a little knowledge from the Sensenich website, and the advise of an experienced mechanic who had was familiar with wood props, we went to work getting the propeller off. This involved cutting the safety wire securing the clevis pin, removing that pin with a pair of needle-nose pliers, removing the snap ring, and then inserting a breaker-bar through the 7/8 diameter holes in order to remove the hub-retaining-nut. The hub-retaining-nut (secured with a minimum of 450 ft/lbs torque) doesn’t come off easily. In fact it didn’t look like it was going to come off at all for quite a while. A few helping hands later and a borrowed ‘cheater’ pipe slipped over the breaker-bar and we were in business. That is I thought we were until I bent the breaker-bar like it was made of taffy.
The next day I returned to the airport with my newly purchased 7/8 inch diameter, 3-foot long, solid, cold-rolled steel, single-purpose, hub-retaining-nut removal tool. This hefty piece of metal made short work of the last remaining vestiges of friction and the hub nut rotated smoothly off. In the interim I had called Sensenich to order a new bolt. I was a little surprised to learn that Sensenich does not sell the hardware required to mount their products. I was referred to Air Repair Inc instead where I ordered the new bolt, and a spare, – for $24.10 a piece (ouch!). While waiting for delivery we used the down-time to thoroughly clean and inspect the hub hardware and to apply a new coat of silver paint to the metal. Another surprise awaited. The seven remaining bolts all showed evidence of chattering and vibration. Portions of the shafts were polished mirror-smooth where they had been rubbing against the inside of the bolt-holes and there were indentations collared just under the bolt heads where the bolts were in contact with the rear hub face. The good news was that there was only superficial rubbing to the finish of the actual propeller, and there was no elongation to the bolt holes or counter bores, or any other type of damage to the wood. Evidentially the old bolts had absorbed all of the abuse. So, another call was made to Air Repair to order a complete set of new bolts.
With all of the necessary parts in hand we went back to work. The rear cone, propeller hub, and front cone were assembled on the engine shaft. Moving diagonally across the face of the propeller I slowly torqued each nut to 250 in/lbs then secured the nuts in place with cotter pins. The hub-retaining-nut was then rotated on finger tight. To achieve the required 450 ft/lbs of torque I had first weighed myself before going to the airport then divided 450 by that weight and multiplied that number by 12 in/ft. I then marked off a span of 28.5 inches on my cold-rolled bar. The bar was then inserted through the hub-nut until the first mark was centered on the engine shaft. With a few helping hands holding the propeller in place I applied torque to the hub-retaining-nut until my weight was completely supported while hanging from the 28.5-inch mark. The clevis pin was re-inserted to secure the hub-nut in place, the pin was safety wired, and the snap ring was re-installed. Done.
In accordance with the WSB-1 I flew the aircraft for an hour then removed, and re-applied, the 250 in/lbs of torque to the 8 bolts. My personal maintenance tracking log contains the hour, or date, for scheduled maintenance events. Currently I change the oil every 25 hours so I intend to re-check the torque each time I change the oil.