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Its actually not the splines that are the culprit although they can cut the seal. Case designed this system to use a special Case installation tool that covers the spline and creates a flush surface for the seal ID to slide over when you push it up onto the sealing surface of the side gear. There is no entry bevel or any kind of entry allowance made for the seal at this point. (Its the square shoulder that catches your seal lip and cuts it or turns it inside out.) It was assumed that the installer would be using the Case installation tool. What I did on my 630 was mic the spline and the seal surface. I found the spline OD as 2.250" and the seal surface was 2.500" I went to the auto parts store and bought a $3 exhaust pipe reducer with a small ID for 2 1/4" exhaust pipe and an OD of 2 1/2". I smoothed off the sharp corners on both ends with a file and sand paper, slipped it on the spline shaft, wrapped a layer of thin clear packing tape on the junction between the the reducer and seal OD, greased all the surfaces real good and slid the gear housing on. No cut seals and no seal leaks. Make sure everything is lubricated well to keep the seal from turning back on itself. Make sure you put a new o-ring on the housing old o-rings are hard and clip easily (o-rings are pretty cheap) grease the o-ring and its mating seal surface before putting everthing together.
(The o-ring I didn't replace that looked good is weeping.) Also, on the first round of ruined seals I got a mirror and flashlight to look at the side gear spline end. When I had pushed the side gear up into the differential housingonce a got a squirt of oil from the side gear ID. What I saw in the mirror was that a previous owner had installed the plug seal with a center punch so imagine if you will what kind of leak you get from a .063-.093" hole. I couldn't get the plug out without disassembling the whole rear end so I took a sheet metal screw that would fit in the hole, made a flat gasket out of several layers of teflon plumbing tape and screwed the screw through the tape and snugged up the screw in the hole. No more leak there. Now I have to pull that side apart and replace the o-ring I thought was still good!! BTW, I drain my gear lube back in the pail and reuse it if its only been in a few days. Sorry for long post all.
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