12A vs 13B weight

[Jeff20B]

Yeah, 10 to 15 pounds. The 13B rotors are 11.5 compared to 9.5ish for the '83-'85 12A rotors. The 13B rotor housings are about 1 pound more each, and the shaft is only slightly longer so not much weight gained there. The tension bolts by the same amount, but there are 17 or 18 of them so it adds up to a small amount. The rest is in the water pumps (same weights if both are cast iron) and the flywheels. 26 pounds stock for "31" (late 12A) and "304" (GSL-SE) style. You can drop the 13B weight below a 12A just in the flywheel if you go aftermarket. Maybe do an FC aluminum waterpump too.

15-16 pounds for full cast iron. An aftermarket GMB aluminum housing with the stamped steel impeller drops the weight a bit. The other option is an FC waterpump which is about 6 pounds if I remember correctly.

One option which you might consider but shouldn't do unless you're running an electric fan is an FC housing and a GMB 1st gen impeller. Ultra light weight but not very strong so you shouldn't use a stock clutch fan with it. I did this for my rotary baja project. I added a stud to the alt bracket hole on the GMB and filled the hole in the FC housing with sealant. The engine plates are 1st gen Y castings so no provision for the FC alt braket bolt. If you already have an idea as to what I'm talking about, you should be able to replicate this setup for the lightest waterpump just short of the no longer available racing aluminum waterpump from the '80s. Otherwise don't do this because I haven't run it long enough to know how the parts hold up over time.