Littlebeefy Posted February 1, 2021 Share Posted February 1, 2021 Hey folks, Been plowing through upgrades on my 84 Bronco. I was finishing reassembly of the rear axle yesterday and ran into a small problem and wanted to double-check against the vast wealth of knowledge that y'all happily share. Here's the background: I have a 31 spline 9" rear that was stock on my Bronco with 3.50 gears and an open carrier. I have a 9" chunk from a 78 (possibly 79) that I have regeared to 4.56 and installed a clutch-style Yukon LSD. I bought 2 new chromoly axle shafts for the rear from Alloy USA (http://www.alloyusa.com/rear-axle-shaft-kits/alloy-usa-15102.html). These shafts are sold as being L/R interchangeable. The passenger side fits snugly and installed fine. The drivers side appears to be approximately 9/16" too long. So the obvious answer is "cut it 9/16"". I just wanted to check with you first though because I see quite a bit of conflicting info. There seem to be three schools of thought on rear axle lengths in the 9". The first school of thought is the axle shafts are the same length and interchangeable L/R. This is supported by the 1981 factory shop manual on this site, as well by virtue of the fact that manufacturers who sell aftermarket shafts sell them as one sku for the 9" (vs. 2 skus for the unequal length 8.8" axles). A second school of thought is that the shafts are unequal length as a standard. This would (seemingly) be the belief held by JBG who sells them as specifically either left or right side units. However, it's worth noting that the description of both parts on their site lists the length as the same and uses an identical Ford replacement part number. Additionally, the Ten brand axles that they sell do not actually come in unequal lengths according to the manufacturer website, and they only sell one SKU. Lastly, there is a significant number of people on-line who say "it depends" and "it varies". It's worth noting that the axles that I removed were not identical in length. One was about 1/4" longer than the other. Additional, the cut end of the shaft did not look identical; they could have been different manufacturers or one could have had the end shortened but I can't tell which. So.... thoughts? Should I just trim this thing 9/16" and move on with my project? Or is this an indication of a more sinister problem? Thanks! Chad Link to comment Share on other sites More sharing options...
85lebaront2 Posted February 1, 2021 Share Posted February 1, 2021 I don't suppose you tried swapping them side for side? 9/16" is a big enough difference that if the short axle is where the long one should be you may not have real good engagement in the side gear, It's been years since I built a 9", but from what I remember the side gears are only about 3/4" thick where the axles go in. That could mean only 3/16" engagement on that side. Link to comment Share on other sites More sharing options...
Littlebeefy Posted February 1, 2021 Author Share Posted February 1, 2021 I don't suppose you tried swapping them side for side? 9/16" is a big enough difference that if the short axle is where the long one should be you may not have real good engagement in the side gear, It's been years since I built a 9", but from what I remember the side gears are only about 3/4" thick where the axles go in. That could mean only 3/16" engagement on that side. Good question. I'll tell you what we tried and the result. The new chromoly axles are identical length, same part number. When either was inserted into the drivers side of the housing after the chunk was in it stuck out ~9/16" too far. We also tried both of the old axle shafts and neither of them fit, but we didn't measure exactly how much each was too long by. We also did an experiment where we tested how much spline engagement there was by first measuring the length where engagement first occurred (approximately) and the ultimate point of insertion and found that it was making 3/4" of engagement without allowing for difficulty in determining "first engagement occurred" which seemed reasonable. I'm guessing that once I threw a Yukon LSD in there, all bets are off... Link to comment Share on other sites More sharing options...
85lebaront2 Posted February 1, 2021 Share Posted February 1, 2021 Good question. I'll tell you what we tried and the result. The new chromoly axles are identical length, same part number. When either was inserted into the drivers side of the housing after the chunk was in it stuck out ~9/16" too far. We also tried both of the old axle shafts and neither of them fit, but we didn't measure exactly how much each was too long by. We also did an experiment where we tested how much spline engagement there was by first measuring the length where engagement first occurred (approximately) and the ultimate point of insertion and found that it was making 3/4" of engagement without allowing for difficulty in determining "first engagement occurred" which seemed reasonable. I'm guessing that once I threw a Yukon LSD in there, all bets are off... Man, that jives with my memory on the 9". The 9" I had most experience with had a Detroit Locker in it, so was completely different internals. Link to comment Share on other sites More sharing options...
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