Since this is about Crank Seal installation for CR125, the cranks are not slip fit from OEM into the bearings and the bearings are not slip fit into the cases. Therefore something needs to be heated up to assemble the cases shut and you wouldn’t want heating being done while new crank seals are in place, so the crank seals go in after the cases are closed.
What this also allows is the check the crank spins freely with cases torqued up which otherwise could not be felt if the seals are in there creating some resistance.
I like the idea that Chris R has for crank seal installation, while it may be a little overboard for someone who does this job once every 2 years, it’s certainly a effective tool and nice way to do it if your doing this on a regular basis. The last thing you would want is a seal popping out on the transmission side to suck in oil into the crankcase at a race meeting. Maybe even recommend some loctite sealing compound between the seal and cases even though I’ve never used a sealing compound on CR125 or personally had a crank seal pop out of any of my CR125 rebuilds, however free insurance if you have some laying around.
Ohhh and some advise that I’ve always taken from a reputable engine builder, always lube the inside of the crank seals that makes contact with the crank shaft with some Moly grease. Slips though nice to avoid damaging the lip and creates great sealing while bedding in. ;-)