3 shortblocks, rebuilt heads, still knocks. Help!

[fasterdamitt]

Ok guys, I have a good one. I bought this 347 with a bad crank so I thought I'll just reuse the block and build from there. Well three teardowns later, it has the same knock in all three builds. Help!
The knock is only below 190 deg. It's noticable at idle, but loud under load. Once it warms up to 190 it sounds like every other motor with a Comp Xtreem cam. I removed the belts, still there. It sounds like it's timed with the firing order, not every rotation of the crank. I thought a valve could be tapping the top of the piston but I can't see evidence in the piston top. Using a stethoscope, The knock comes from the middle of the engine most knoticable under the intake area as I have a Victor Junior intake. What can go away after warming up? I have check piston to wall clearance on each block. The last build is only at .00175 Is it possible the heads could cause this somehow since that's the only thing I did not replace? This motor pulls hard and I rev to 6k. I had the short bolck builder listen to it and the guy who did the heads, they both are puzzled.
Here is a list of things done.

One rebuild 347 shortblock myself utilizing DSS Racing Block and Probe kit.
Two rebuild 331 shorblocks by M&R engines
Rebuilt the AFR 185 heads, needed intake valve guides bad.
New Comp Xtreem XE Roller cam 216-224 .555 lift degreed
New FMS roller lifters
Used exsisting DSS racing pushrods and roller rockers after inspection
New Cloyes timing roller set
New water pump
New oil pump
New flex plate
New vibration damper
Had headers surfaced for possible leaks
Pushrod hight checked
Rocker/valve contact checked
Cam endplay checked
Crank endplay checked
Crank checked for contact with anything
Checked oil pan clearance
Timing at 16 initial and 38 full
Inspected distributor

 

[347HO]

The first thing that comes to my mind... the fuel in your tank is the same each time?
Other than that, I'd have to be there... since I'm not... I don't know.

 

[fasterdamitt]

I have driven each one through a tank or more after trying to diagnose. The last one I have in now has been driven for probably 600 miles and still sounds the same. Once warm, no one notices, but let it cool down for a half hour and you can hear it. Almost like something is hitting somwhere inside.

 

[302RollinHard]

For starters, I don't like the combo of stock type lifters and a fairly aggressive bumpstick. Also, are your springs matched to your cam (rate and installed height)?
What weight oil are you running and have you tried different weights. That might be a contributor to the temp based knock.
just somethings to think about as you have done most of the mechanical checks you should do.

 

[strtrcr50]

Piston slap maybe?

 

[fasterdamitt]

For starters, I don't like the combo of stock type lifters and a fairly aggressive bumpstick. Also, are your springs matched to your cam (rate and installed height)?
What weight oil are you running and have you tried different weights. That might be a contributor to the temp based knock.
just somethings to think about as you have done most of the mechanical checks you should do.

The springs where tested when the heads rebuilt. I have run Crane lifters in one set up but no improvement. I was thinking of the oil weight before, I ran 10-30, 10-40 and 20-50 after break in. Did'nt really notice any difference. I thought the oil may be thining when the knock is getting less noisy but not the case.

 

[fasterdamitt]

Piston slap maybe?

I know, it's a tipical diagnosis. But in every build I got the pistons tighter. There's no way it can be piston knock at .00175 fit. Thanks

 

[fasterdamitt]

Does anyone know someone in SoCal that could diagnose a HP engine? The machine shops that I work with don't do in car repairs.

Thanks

 

[fasterdamitt]

I bought a new stephoscope. Sounds like the knock is coming from the back of the block or right at the trans mounting/bell housing area. I have a new Lentech converter and flexplate. Hmmm...