I have a 40'x42' four bent post and beam bank barn cica 1850. The tie beams were cut at bent B and bent C for a hay trolly. Mortise and tennons are pulling apart and the roof sags down aprox 8" in center. I plan to use allthread in four places(bents B and C at eve and lower tie heights) to stablize barn, and possibly close mortises and raise sag. My question is: will 1/2" allthread,run 42'through all posts be under sized for the job?

Allthread is a bit too generic a term to place any values on. The standard hardware store mild steel grade is IMO not up to it, when “it” comes to structural requirements. I kinda think of it as wimpy metal, reduced in section and made wimpier still by having an endless shear plane cut into it.

I’ve not seen the scenario you describe used, and I’d think it would be far more expensive than sets of forged eyes, cables and turnbuckles.

I have been involved in the occasional use of coupled 1” grade 8 rod to suspend debris mats and staging below covered bridges…

Available from specialty suppliers – a quick search found me this one, and some values to help you in yours > http://www.williamsform.com/Threaded_Bars/150_KSI_All-Thread_Bar/150_ksi_all-thread_bar.html

I'm with Will. "Allthread" is not rated for high stresses. Plus it screams "ugly."

If you don't like the look of cable, it might be practical to have hot-rolled steel rod welded or machined to use turnbuckles. I use temporary cables to take up the slack before fine tuning within the limited range the turnbuckle provides. Half inch diameter or larger ought to do it, in my experience.

In restoration work we have spliced in new tie-beam sections with keyed scarfs, using the cables only temporarily. Perhaps you can consider that option, if enough length remains on the original tie beam stubs.

You may indeed be able to reduce sag, but it will be a hard pull unless you provide some vertical jacking force to lift the rafters and ridge while drawing in at the tie-beams.

Your hay-carrier problem is a very common one, by the way, ruined a lot of barns.

Good luck.