I would like to revive this discussion of radial engines. The forum has covered a wide variety of topics from history and , identification of different engines, to common problems even fuel grades. In post 31 Dave Hebert mentioned that one issue was the failure of the 8 bolts holding on the crankshaft gear, meaning the rear gear case drive was disconnected from the front section. Thats what this new addition is about. I would appreciate input from forum members as I am looking for solutions to this problem. Anyway here is the story.
ABout a month ago, one of our fellow MV enthusiasts had his C4 equipped M18 out for a number of successful drives. At the end of the day when he went to restart it to garage it for the night, it would not start. The starter would run but the engine would not turn over. The engine could be turned using the cooling fan but there was no connection back to front. A magneto was pulled off and the fan rotated but the mag drive did not turn. So the engine had to come out and the rear case removed for a look see. It had, as Dave had pointed out, sheared off bolts otherwise fairly clean and had no apparent collateral damage. The bolts were removed without difficulty and the gear seemed to good enough to reuse. pictures enclosed.
Note the unusual shape of the sheared bolts, they are essentially a tapered shoulder cap screw with a square driving head on top with holes to put through lock wires. The manual identifies C1 and C4 crankshaft bolts as
"Screw, machine, sq-hd, dld-f/1kg-wire, alloy -S, 5/16-24NF-2 (19/32) x 1 7/32 overall lgh" it is part number G104-0102120 or CO-200439 , also B206663. Not quite sure what all of the description means, can anybody translate ?
Now before I ask you for your respective opinions on how to repair this, lets look at the later -46 engine gears. The Wright R975 started out with much less power than the finished C4 version. It appears that one of the weak links remained the 8 bolts holding on the rear crank gear- a design unchanged since the mid 1930s?? By contrast, the late 1940s designed 975-46 was built much heavier to handle its boosted horsepower. Some of this has been previously referred to as we noted the heavier case construction, heavier rear main bearing etc. If you check out the next photo you will see a rear crankgear for a -46 and the much heavier method of attaching it. While it uses 8 capscrews, they are flat not tapered. The gear/crankshaft have an aligning dowel on one side. Most importantly, there is a heavy metal projection attached to the gear which is drawn into the rear of the crankshaft witha heavy bolt. A LOT stronger than the C1 and C4 version!