Ok I got an 81 blazer K5, with a 350. it's got a fresh rebuild probably around 1.5k or less.
The problem with it is that this afternoon I started up the truck, it was still cold, then probably 5 seconds later there is a very loud knock going on in the engine. I shut it off right away. I turned it on again just to see the oil pressure and it was at 50psi, what can it be causing that knocking noise even if it has the oil pressure at 50 psi? Please help!

 

Does the knock come from the top or bottom end. With good oil psi I'm thinking the knock is top. When you have a rod bearing on the cap slide under the top bearing on the rod it shims the piston up to were it makes contact with the head. The bearing clearance isn't that much to drop oil psi. Where the rod replaced or re-sized? If not you may have a stretched rod. One thing is sure it has to come apart. I would start with the oil pan and check the bottom end. If it is coming from the top pull that head to see if there is contact with the piston.

 

Mine knocked under load at first then began knocking all the time, it ended up being vacuum leak. Could also be timeing issues.

 

Yea it's coming from the top of the engine, I will drain the oil and hopefully I won't find anything abnormal in there.