Loud knock after rebuild
 

After a very long rebuild I started the engine - and this is the disappointing sound. I thought everything went well but there is an obvious problem. Does anyone have an idea what this could be?
https://www.youtube.com/embed/Otrvvb5kOTg
Thank you,
Bill

Specs on the rebuild? at first I was thinking piston slap?

Maybe an excessive game(set,play) of valve. I hope that your valves do not touch your heads of cylinder.

Sounds like valves. Better check clearances again. Maybe some were set at wrong crank position.
if so, prob not fatal - just real sloppy.
Alan

is it too loud for an exhaust leak?

I had one bolt missing on one of my exhaust flanges and it sounded like that.

Does it get louder when you rev it ?

An intake leak can also make that noise.

Sounds mechanical, not a leak.

I vote for bad cam timing. I had the exact same noise and went back to check the cam timing, which was significantly off on one side. The good news is, you can retime the cams with the engine still in the car.

Have heard similar from clutch area at idle; slight rise in idle and things would quiet down. Use a stethoscope to isolate the offending area before tearing into anything.

I am with Alan L here more like valve area, hope its not to bad for you
Bill after all your work, cross fingers,

What the heck, I'll throw my guess behind exhaust leak. Hard to tell from just the video.

Valve train. Pull the cover and check the clerances.

Try removing the fan belt, you never know.

Check tensioners. They have a tendency to lose prime after sitting awhile.

probably valve train noise... do a VERY PRECISE & ACCURATE compression & leak down tests... repeat the tests a second time to validate your findings... your answer lies within these tests...
if you do not know how to do these tests accurately and most importantly how to interpret the results and how to listen for where air blow by is happening, then ask for help... good luck...))

I'm going with worn rocker and rocker slap
Bruce

The simple answer will be to first pull some valve covers and check the slop in the rockers.
Alan

I had exactly the same after my rebuild. I had accidentally set one of the valve clearances at .040" instead of .004"! My excuse is that they are difficult to get to and the proper tool is pretty useless! After a reset all was quiet.

I wishing you an easy fix like a rocker, but man that sounds like a main bearing.

Fingers crossed for you, I'd be *****ting bricks myself.

Of course my valve covers would have been off in minutes of hearing that.

Have you tried a stethoscope? I had an engine with piston slap once I could tell exactly which one it was, it sounded almost like a bell with the stethoscope. Also, try rotating it over very, very gently, you should be able to feel the rocker action and maybe sense something. Good luck.