I finally finished building my 4l80e with a lot of help from people on here! As I am mating a 1996 4l80e to a 2006 L33 5.3, I knew I would need a flat flexplate, spacer, and longer bolts.

Here are links to the products I bought:
Flexplate: https://www.jegs.com/i/JEGS/555/601062/10002/-1

Spacer and bolts: https://www.jegs.com/i/JEGS/555/601010K/10002/-1

I followed the instructions and torqued the bolts to 15 lb/ft, then 37 lb/ft , then 74 lb/ft. As I was torquing the last bolt, it wrung off. I put a dab of assembly lube on the threads of each bolt as I installed them. I was able to get some vice grips on what was left of the bolt and got it out of the crank. What would have caused this? These parts should all work together. I also know this particular torque wrench is accurate. My only thought is that the whole in the flexplate isn't big enough as the spacer is very tight in there. I will need to use a mallet to separate the flexplate and spacer. Any thoughts or advice would be greatly appreciated!

On second glance, it looks like the space might be slightly crooked in the flexplate. Is it possible this could cause the problem? Does that mean I have a faulty flexplate?

Here are some pics:

https://imgur.com/a/0EYpCcZ

 

I am assuming that you are saying that the bolt/ fastener spun (threads stripped) in the crank-shaft flange...
Otherwise what I say next will be incorrect.

I have only ever used the GM bolts intended for use the the GM crankshaft to flex-plate spacer... or custom made ARP replacements.
You have the correct torque-spec...

One of the more important factors here (in terms of the fastener being properly secure) is that enough of the threads on the fastener engage enough of the threads in the crank-shaft flange.

Should the fastener have been too short, or even just the thread depth of the fastener have been too short... the fastener can easily strip/ spin at those frictional loads (torque-specs).

If you actually snapped the fastener/ broke the head off...
Then what I said above does not really apply.

Maybe you bottomed out the fastener or threaded it off-center...
A burr or poorly made thread could have increased the amount of rotational load/ friction... and broke the fastener.

 

A word of warning to you all...

Fasteners for engine components and many other critical components are far too often over-looked... and thought of as sufficient.
This is a bad practice to fall into.

Premium fasteners should be used as often as possible; and always double check the fastener thread measurements yourself before use.
Ensure that your measuring equipment: calipers, micrometers, torque-wrenches, ETC... are calibrated often.

Measuring tools and gauges that are "Fixed" (non-moving) and do not require calibration, can be a massive aid when building and/ or designing.
Pin-gauges for example have been amazing for hydraulic work, transmission work, fire-arms work, ETC...

Lastly, use lubricants when instructed to... and DO NOT use anything on a fastener intended to be used dry.

 

I did not strip the crank. The bolt head twisted off. I checked all the holes and the depth is the same.

 

sometimes you get a bad one. sucks too b/c they have a short head and you cant just get them at the hardware store

 

Jegs is sending me another set. I have a dumb question, it is slightly dished where the torque converter bolts to the flexplate. Does the concave side go towards the engine or the torque converter?

Also, how tight should the spacer be in the flexplate? Very snug, or need a mallet to get it off tight?