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Engine Mounting Structure (cracks and such) From: Classic Aero Support- D. Combs Doug Combs Managed the Luscombe Aircraft Type certificate and engineering design updates to the Luscombe aircraft from 1993 to 2003 when he resigned that management position. He consults on repairs and FAA approvals, and supports light aircraft with parts and technical advice through Classic Aero support at www.luscombesilvaire.info Original question RE: The bolts that attach the engine mount to the firewall seem a bit too small. Is it ok to drill out the firewall hole and place in larger bolts? Is there any A.D.’s that would require a larger bolt size? Answer Hint: The engine mount installation is certified and quite adequate for up to the O-200 engine installation (105HP). The top engine mount should be installed with a long 5/16 bolt that goes through the V strut, mount, and mount rubbers with 1/8" thick washers. The two bottom mounts are supposed to be 1/4 inch bolts with similar hardware. There is no need for larger attachment bolts as the weak points are not the bolts, but other areas discussed below. 1. The lower engine mount bolts were originally installed with standard AN960-416 washers that concentrate vibration and stress on the internal reinforcements INSIDE the firewall. These areas under the firewall cover must be inspected carefully and thoroughly each annual. These reinforcements are often cracked and require repair/replacement during inspection. After repairing and replacing any cracked reinforcement on the firewall, the service "Fix" to prevent future cracks is to install AN970-416 washers in that location to de-centralize the loading of the firewall and reinforcements. This Mod is approved by FAA approved E.O. 2855, and is an excellent method to prevent such cracks. 2. Vibration causes the rivets adjacent to the lower engine mount reinforcements to become loose. This can often be repaired with larger rivets, and sometimes requires the fabrication of a larger internal bracket to re-distribute the load to more rivets (6 vs. 4). This repair is authorized by Section 4 of the Luscombe repair manual and 43.13 1B. 3. Weak link. The engine side of the engine mount uses a large Aluminum washer that is 1/8" thick to retain the rubber vibration mount and the engine mount (with engine. The washers can fail in overload and out of balance conditions, releasing the engine to be restrained by only the cowl- if at all. Steel washers to replace these aluminum washers were approved by E.O. 2874 R-1, and are recommended as a retrofit. 4. Most airplanes use 3/8" bolts to attach the engine mount through rubber cones to the engine. Luscombe specified a 7/16" bolt and nut for this application through the engine mount. Because the cones are drilled with a 3/8" hole, some interference in the installation is a little problematic. A large Continental washer (proprietary PN) was then used on the front end of the rubber cones to restrain the engine with a rubber cushion effect. The size of the bolt determines the size of the wide area washer, and the size of the wide area washer must be large enough to cover the rubber cone for full effectiveness. Therefore, the size of these bolts (7/16" or AN7) has less to do with structural integrity than it has to do with the desired outside diameter of the retention washer. Upsizing bolts at the firewall is not only unnecessary but would also be considered a major alteration of the design, (and several parts themselves) and require at a minimum, FAA field approval, and likely per AC43-210, a full engineering workup and FAA 8110 form, and or STC. Little would be gained and much expense would come to that project. Materials, many parts, and further technical assistance are available from Donna Losey at 480-650-0883 or dlosey@luscombesilvaire.info Please post your further questions or clarification needs. Doug Combs |
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