I found the ST series auto regulators were still only good for upto 40 V - when running on a 24v system (with alternator running upto 30V) i figure that's too close. I did however find some devices termed as a TVS transient voltage suppressor. They are sortof (not quite) like a couple of grunty zener diodes that are back to back that have a distinct clamp voltage. They are called TRANSIL