which nut? rear upper control arm

[BigShooter]

not sure which nut is correct. I had installed the normal looking ones, then found the nuts with the flat-washer looking end which matched the nuts on the other control arms, however, they are requiring an EXTREME amount of torque to screw on.
Which ones are the correct ones to use??

[edwardb]

My two IRS builds did not come with a flange nut for that location. So that would suggest the nut on the right in your picture is the correct one. I added a washer just because that's my preference. A distorted lock nut like that can be a little stiff to tighten. But it should go on relatively easily until it's nearly through and into the distorted area. None of the suspension bolts, this location or otherwise, require EXTREME torque. If that happens, very likely something is mismatched and continuing could cause damage. Stop and sort it out. Also recommend a little anti-seize as your assembling those bolts. Helps now and in the future.

[BigShooter]

Thank you! That is helpful to know -especially about your other IRS builds

[JohnMac]

I think you’ll find the nut that’s difficult to thread is a lock-nut (slightly distorted) thread area.

[CraigS]

I throw both of them away and use a nylock and washer.
https://en.wikipedia.org/wiki/Nyloc_nut