What causes a lifter tie bar to break
 

Just like it says
Trying to figure out what happened.

Found it
Intake body broke allowing bearings, wheel and axle out. Body grabbed cam spun and broke tie bar.
Tie bar hit oil splash shield snapped pin holding bar on exhaust lifter and that spun too....grrrrr

Weak valve springs. When the springs get weak, the lifter flys off the lobe, stressing the tie bar rivet until it eventually fails and lifter spins, roller skids on cam until roller gets loose....etc. Been there and learned the importance of checking valve spring pressures as parts of the maintenance program.

Why in the world would you routinely check the most stressed part of the engine? Isn't it more important to wax and polish the valve covers rather than check what's going on underneath them?

Wouldn't expect a mild 80 hr+/-engine that's never seen 5k rpm to need valve springs. 😏

If it failed that soon, there's a good chance the springs were never up to the task from the start. There's a wealth of info here. Contact RM Builder (Bob Marada, Marine Kinetics) when it's time to buy the components. Bob will set you up with all matched valve train components so you don't have to go through this again. And his prices can't be beat.

I did some back tracking, you have stock 502MPI's ? I found a thread you where asking about upgrading 502MPI's to 540's ? Best of my knowledge, stock 502MPI's came with flat tappets at first, and then OEM GM dogbone roller lifters that don't use the tie bars.

So, what do you have ?
If aftermarket stuff, what spring pressures ? What cam ? What other valvetrain pieces ?

beat me too it,,what lifters do you have?will help us figure it out.

First off they are 540" with Dart Iron Eagle 305 heads and 502 mpi intakes with reflashed ecms.
I did not build them so I do not have all the info on the builds.
The lifters are PBM, cam specs unknown until I pull timing cover, I need to get back with a spring tester to check pressure, but they are double springs with damper (Orange stripe on outer spring)
Crane 1.7 roller rockers with guide plates

My plan is to call Bob M to discuss valve train. IMO 80 hrs is unacceptable for valve train failure on such mild builds, not like they are 1200hp solid lifter beasts.

lifters sound chinese and cam could have come from SB and my friend.:D

LOL. Could be....maybe one of his roller buttons (he and his ex wife made in the garage) broke, the cam moved far, and a lobe caught a lifter.