Hi All, Hoping I can get some assistance with a few questions on the famed 39 box. I have already done countless searches and there are a lot of threads on this but I have some specific questions and was hoping those more knowledgeable might be able to help. I am in the process of rebuilding two of the 1939 gearboxes (one to install in my pickup and one as a spare). I am using the parts I have accumulated and being in Australia there isn't an abundance of this stuff. So I am trying to use what I have. Gearbox #1 - this gearbox has the thrust washer #7071 as per the below photo and i have eliminated end play. However the main shaft #7061 does not have the retaining pin #7073 that holds the thrust washer #7071 from spinning. The retaining pin seems to have sheered off flush and there is no way I have to remove what is left. The question I have is if this will cause any issues with the thrust washer #7071 being able to spin? It wont be locked in to place by the retaining pin. Thurst Washer 7071 by Dirty Fuel posted Jan 12, 2025 at 4:33 PM Broken Retaining Pin 7073 by Dirty Fuel posted Jan 12, 2025 at 4:33 PM 39 Gearbox Diagram Disassembled by Dirty Fuel posted Jan 12, 2025 at 4:33 PM Intermediate Gear End Play Schematic by Dirty Fuel posted Jan 12, 2025 at 4:33 PM Gearbox #2 - This gearbox uses a different main shaft (no idea of part number or year) and it does not allow for a thrust washer #7071 to fit. All other components fit and work as they should with minimal end play (0.008). My question here is, are there any issues with this set up as it does not have the thrust washer #7071? No Thrust Washer 7071 by Dirty Fuel posted Jan 12, 2025 at 4:33 PM Thanks in advance.
The 81A-7061 mainshaft has the pin and needs the 81A-7071 spacing washer (which butts up against the backside of the 81A-7102 second gear. If you don’t replace the sheared pin, you could enable the 7071 washer to spin against the leading edges of the 16 splines of the mainshaft. This would defeat the point of having the wear occurring on a $9.00 washer vs a $125.00 mainshaft. The next version of the mainshaft is the 51A-7061 and the primary difference is that its helical spline section is roughly 0.185” longer to the front…..essentially the thickness of the 81A-7071 spacer washer, which is not utilized. The downside of this mainshaft is that your second gear (which is in constant mesh) is always spinning against the leading edges of the 16 helical splines. When the wear here is sufficient to cause end play problems, there’s no way to close up the gap. The only solution is to replace the mainshaft with a new one (if you could find one). Forget about filling the gap with a shim washer…it would not last 200 miles. The little pin is sometimes a common steel and frequently a hardened steel pin. Try to center punch the pin and use a tungsten type drill bit with plenty of cutting fluid and lots of patience. Best done in a drill press with a press vice. The pin is 1/8” diameter and I believe it’s 0.400” long.
Thanks Mac, your knowledge is invaluable. I just saw a photo of the differences between the 81A-7071 versus the 51A-7061 on your website. I am assuming that the 51A-7061 would be the 2nd choice between the two mainshafts but designed to be used without the 81A-7071 spacer so would still function as intended (notwithstanding the wear due to no use of the spacer)? Is this right? In the gearbox with 81A mainshaft, I have a 51A/8M type synchro and when bench testing it all works well with no endplay. However I noticed on your site you mention that 'the 51A/8M synchro hub is slightly thicker than the 01A synchro hub, you can not use it in place of the 01A on the earlier (81A-7061) mainshaft', should I change this set up? Given the different mainshafts, I checked both second gears I had and one of them has the oil slot/cut-out which I believe may belong to the 81A and the gear without the oil slot/cut out belonging to the 51A. Is this correct? I would assume the slot/cut-out may bind with the helical spline of the 51A given no spacer is used so should only be used with the 81A. Can you confirm? I just want to ensure I keep the right gearset together as much as I can with the parts I have. Also, I have a NOS low and reverse gear #7100 which I intend to use. Are there any issues that may arise using a NOS part meshed with a used/slightly worn reverse idler gear #7141? I will try and drill out the sheared pin and replace it on the 81A mainshaft, thanks for the tips. Thank you so much for your help.
The 2nd gear with the oil slots on the back face would be normally used with the 81A type mainshaft. The 2nd gear with the smooth rear facing is the one you’d want to run against the 16 helical splines (ie the 51A-7061 mainshaft). Both of these 2 nd gears have 22 teeth and measure 3.03” diameter. The 01A-7124 and the 8M-7124 synchro units are interchangeable but you have to add the 81A-7069 copper washer to the 01A synchro type to equalize the front to rear length of the two inner hubs.
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