This service bulletin applies to all Type 4 engines that utilize either a 66mm or 71mm stock VW crankshaft that were modified by Great Plains Aircraft, Mosler Motors or Total Engine Concepts.

There have been two confirmed cases where the bolt that holds the prop hub onto the crankshaft, having unscrewed while in service.

In the first case, the bolt unscrewed almost all the way, but the prop hub and prop stayed on the engine. After a field repair, the aircraft was placed back in service with the same retention bolt. The tapered spacer used on the 71mm crankshaft was observed to be locked in the prop hub, and not on the end of the crankshaft. A short time later on take off, the bolt lost it's pre-load and unscrewed again. The prop and hub stayed with the engine, but did not continue to produce static thrust. The engine was a 2400cc Type 4 engine. Total engine time is unknown.

The second incident occurred at 80 hours total engine time. The prop hub had been removed about 10 hours prior to the incident to replace a seal. The tapered spacer that is used on the 71mm crankshaft was observed to be attached to the crankshaft and not in the prop hub when the seal was replaced. When the failure occurred, the tapered spacer was in the hub and not on the crankshaft.

Upon investigation of the second incident, it was first thought that the bolt had broken. Closer examination revealed that the bolt did not fail. The problem has been determined to be with the tapered spacer that is used on the Type 4 crankshaft. About 51 percent of the torque is transmitted through the tapered spacer and to the tapered crankshaft. If the tapered spacer breaks looses on it's taper, 100% of the load is then transmitted to the rest of the crankshaft. The tapered spacer is about 1.250" in length. The remaining length of the crankshaft/prop hub taper is about 1.200" in length. When this happens, the instantinous torque load to the rest of the crankshaft causes the prop hub to break free from the remaining taper and the bolt unscrews from the crankshaft.

We suspect when the hub was removed, to replace the seal, the tapered spacer broke free from the taper on the crank, but was held in place by the gear (pop hub) puller. This was NOT a failure of the prop hub! It appears that the failure is the tapered spacer, not remaining attached to the crankshaft.


Several solutions have been proposed. The first involves pressing the tapered spacer on the crankshaft about .035" after contact. This would take about 12,000 pounds of force. The spacer would then be tapered to match the taper of the prop hub. This is similar to how the spacer is currently attached. One would still rely on the transmission of torque through the spacer to the crankshaft.

The second solution involves not using a tapered spacer at all. A new prop hub would be made that is 1" longer than the current prop hub. The crankshaft would be tapered back to the cam timing gear, eliminating the brass drive gear and the spacer. This would increase the bending load on the crankshaft as there would not be any actual crankshaft in the bearing for load transmission. It would also eliminate the brass drive gear, which would cause a relocation and re-development of the secondary ignition.

The third solution involves welding the tapered spacer on the crankshaft after it is pressed in place. The spacer would be welded at the front of the spacer and the rear of the spacer to the crankshaft. This solution still relies on torque transmission through the tapered spacer and the weld to hold it in place along with the press fit.

The fourth solution involves welding up the nose of the crankshaft and then grinding the taper on the full length of the crankshaft to match the prop hub. This eliminates the tapered spacer all together. It retains the option for secondary ignition using the brass drive gear on the crankshaft. It eliminates the need to fabricate a special prop hub for the Type 4 engine.

We are currently in the process of trying the 4th solution, welding the nose of the crankshaft. Two crankshafts are being modified. One to actually use and one to send to an engineering firm that specializes in metallurgy to have the crankshaft analyzed at the area of welding. We do not desire to fix one problem but to cause another.

Here are pictures of one of the intial cranks, with the welded nose.

This crankshaft is being used to study the feasibility of this solution and is will be used for costing purposes.

Another crankshaft is currently being welded up and will be split lengthwise to the number 3 main bearing for review by metallurgist company. At that time, if all results are positive, a third crankshaft will be done - that will be placed in use.

If you are using a Type 4 engine that is using a 66mm or 71mm crankshaft, and has a 1/2" fine thread bolt for prop hub retention, we ask that you proceed with caution if you are flying the engine at this time. It appears that the problem does occur when the prop hub is removed and the tapered spacer comes off the crankshaft and is locked in the prop hub. When the hub is reinstalled to the crankshaft, the tapered spacer is not firmly reseated on the crankshaft. The 1/2" prop hub bolt should be considered to be a single use item. It is torqued to 64 ft. lbs for a grade 5 bolt and up to 90 ft. lbs for a grade 8 bolt. All bolts should use Blue Locktite the full length of the threads. The crankshafts internal threads should be cleaned with lacquer thinner/acetone to eliminate any contaminates prior to assembly.

If you have any further questions or comments, please contact Great Plains Aircraft (telephone numbers & email listed below).

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Great Plains Aviation Supply LLC
7011 N 160 Avenue
Bennington, NE 68007
Orders: 1-800-922-6507 (available in the US Only)
Information: 402-493-6507
or Fax 402-493-3846