How could it seize?

[Macspsi]

Here's the story, recently replaced headgasket, water pump, timing belt, and put in arp head studs. I've been easy on it for awhile since I put the 16g on since I did not have a tune, but I was only running 12lbs to be even more sure I wouldn't hurt it. Last week I was coming home, I slowly eased into it and watched it build to 10lbs and it was pulling pretty good and then it blew a coupler off. Thought that I had tightened it up good enough. It cut off of course and I just coasted it into my driveway and let it sit because I had to go back to college. Yesterday I went to start it, and it didn't sound right almost like a clicking when the motor was turning then it locked up. I have no idea what could cause this, good oil pressure, fresh water pump. I know it couldn't have sucked up a rock when the coupler came off because it was on the j-pipe so that would be next to impossible to happen. I also popped the valve cover off to check everything there and everything looks perfect. Tried to turn it by hand and it would not turn. Please help me diagnose this.

 

[ricktb]

i though i blew a coupler too when my timing belt broke, its just a sudden loss of power feels about the same. my engine is sittin in about the same condition at the moment. Priced out a head rebuild for $1500, assuming the bottom end is ok.. closer to 3,500 to do the whole thing. I came the the conclusion that it would be cheaper to buy a used running engine, than to rebuid even to oem spec, pricey.. I found a 6 bolt for $600 cept its 2000 miles away, could pay another couple hundred to ship, and then my own labour to install, and do the swap.