Corsair “Special Nut”

[loggerhead]

At times, I think the subject in question pertains to the owner of the motor instead of the motor itself for attempting to revive this old beater but, here I am. I’m sure it would have been a parts motor to a large majority but sometimes, we get weak and can’t help ourselves, right? This motor came to me in a three motor deal and sat in corner of my shop for a few years until this past summer when ai was cleaning the shop out. Short story is that the lower unit was removed, the cap was missing, and forward gear and its thrust washer were long gone. Model #3335 research identified it as a ‘53 7.5 HP Corsair by Scott Atwater. Under the aluminum shroud, the motor had been sharing rent with a sizable rat nest, not a Mickey type mouse but judging by the size of the deceased guy’s skeleton, a pretty good sized rat who as his health declined, was unable to leave the “Cozy Corsair” motel and had urinated all over the carburetor and surrounding areas. Surprisingly, the motor was free but both the throttle shaft and choke shaft were frozen in their bores and corrosive white powder coated the cast aluminum. After substantial effort put into cleaning and sanitizing, I checked compression and it was impressive but there was no spark. I replaced the coils, condensers and plug wires but discovered that the points were new. Moving forward, I didn’t have a service or parts manual yet and took a chance on a ‘ 49 lower unit being the same as a ‘53. Well, it is…to a point. I found the ‘49 lower unit on ebay cheap enough but when it arrived, it was found to be extremely worn and very few usable parts. Now for my questions, #1 I have made what I believe are the necessary modifications to the front cap to be able to use it on the ‘53. I have since purchased CD of the Applicable Scott Atwater service and parts manual so I could see what parts I still need, after thorough inspection, forward an reverse gear appear to be identical and can be interchanged, can this be confirmed by someone? #2 the original “special nut” was also missing from the Corsair but was included with the ‘49 ebay unit. Most everyone is aware that the nut for the wire or cable operated shift models is a smaller diameter thread than the nut made for the ‘50 thru ‘53 models with the rod. I had planned to drill and tap the ‘49 nut to fit the ‘53 rod but I found out very quickly that the nut is tempered steel and cannot he drilled. Plan “B”, once upon a time I was a machinist so with patience, my Dremel tool, a square file, more patience and material donated from the lower ‘49 driveshaft, I fashioned a new “special nut” drilled and appropriately threaded for the ‘53 rod. Problem solved? Kinda sorta. When the nut is drilled and threaded, the sides that fit between the shift fork are so thin that the threads actually extend slightly into the fork slots. Here’s the “kicker”, no pun intended but how is it that the parts list has the same #493-3162 shift fork for the ‘49 thru ‘53 motors but there are 3 different part numbers listed for the “special nut”? From my experience, SA only changed prefix numbers whenever they changed the design of a part. My research found the following “special nut” part numbers for the ‘49 thru ‘53 7.5 HP motors, #493-3161, #509-3161 and for ‘53, #1325-3161. This is odd, so what changed? How was the same shift fork used while the nut had to be changed??? Doesn’t make sense so anyone with more knowledge and expertise, please explain? What am I missing?

[frankr]

Don’t know the whole answer, but the 509-3161 nut is for a 16hp. Very different gearcase. I’m suspecting the differences may be thread size, but have no idea if that is correct or not.

EDIT: Looking deeper, the model 509 (16hp) also uses a 509-3162 forked piece, whatever it is called.

[loggerhead]

Frank, are you sure? The only part number I saw listed for ‘49 thru ‘53 redardless of motor size was the 493-3162.

[frankr]

Model 509 is a 1950 16hp, uses the 509-3161 nut and 509-3162 fork

[Author]

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