New build, 350, .040 over, comp roller 8-502-8. Kept the TPI, with a new chip with a "baseline" tune. Will datalog and burn a better tune later.
Initial startup went great. Set timing to 6* BTDC with computer disconnected. All is good with the motor (phew)!

Issue is, when I put it in reverse (only in reverse), I get a knocking/clunking type sound that changes with engine speed. Forward gears, idle, etc. are all fine.
Some research has suggested that it is possibly a flexplate issue.

My question: When I had the motor built, I had the entire rotating assembly balanced (it is an externally balanced 350 one piece roller block).
The builder was very specific about using the flexplate with which the engine was balanced. (I will reach out to him tomorrow, but wanted to see if the gurus on here could advise).
What is the harm with using a new flexplate if the existing one is indeed damaged/cracked?

 

It is unlikely that the flexplate is the cause, but it never hurts to check the engagement of the drive lugs and converter bolts just to be certain. The engine, flexplate, torque converter shell, and even the converter turbine turn the same direction regardless of what the transmission range might be selected, The direction reversal is accomplished via the input revers clutch and planet/sprag arrangement. It may be theoretically possible that the front half of the input drum is not secured to the turbine shaft, and that could allow forward pressure (and motion) on the pump and possibly converter when the trans is in reverse range. That could be observable if the flywheel dust cover is removed.

 

Probably not the flex plate. Almost certainly not in fact, because nothing that the flex plate is doing, changes when put into reverse, as opposed to what it's doing in forward.

I'd be looking at the exhaust. Specifically, hitting something under the car. The direction that the engine torques itself in the body DOES change in reverse as compared to forward, thus leading to things happening like ... exhaust clearance being inadequate on the opposite side. Potentially motor mounts, for the same reason; although not as likely.

 

Found the issue to not only be in reverse, but there also in 1st and second, just not as audible.
It was the gear on the proform starter, just barely making contact with the ring gear on the flexplate once torque was applied to the motor.

Took the starter off, installed the shim to move the gear a bit back off the flexplate. Problem solved!

On to the next issue--leaky injector on cylinder 2, 4 (or both)...

Getting this thing back on the road is like playing whack a mole, LOL!

 

It might be "solved" or possibly just "addressed" temporarily. There really should be no relative motion between the block (starter mount boss) and flexplate (carrying the converter and trans input shaft) for any reason. It might be a good idea to verify that the dowels (solid and/or tubular) are installed in the transmission flange where it is bolted to the rear of the block.

If the trans housing is deflecting under torque and allowing the input shaft/converter/flex plate to move relative to the crank and block, there may still be a problem which eventually reveals itself again.

If the crank and flex plate are deflecting under load relative to the block, that will almost certainly return as a problem sometime.