I bought a ZB last week that the engine was loose on. Turned over fine by hand. I hooked a battery up and the starter turned it over fine. It didn't have spark, so I resolved that issue, and put some gas in it. It backfired twice and blew a mouse nest out of the muffler. Figured it must have a stuck exhaust valve, and sure enough #3 was stuck. I got that coming around but when I went to hit the starter again it wouldn't turn. I figured the starter jammed with the ring gear so I pulled the starter out. The engine will not turn either direction with a pry bar on the ring gear. Next I took the plug out of the side cover to see the hydraulic pump gear. The gear rocks back and forth, so that eliminates the theory of the hydraulic pump locking up. Pushing the clutch in or out does not make a difference either. I suppose next will be to pull the side cover. I should add, with the valve covers off to fix the stuck valve, everything appeared it was getting oil. I really find it hard to believe a rod or main went tight turning it over for maybe two minutes intermittently with the starter. Has anyone ever seen the engine oil pump lock up? My other thought is if the front main (roller bearing) is shot and decided to lock up. I guess I'll figure it out as I dig in deeper. This just seems very odd. Any ideas?