Define "warmed up". I almost never pull cold, but it has been known to happen. I know in the winter this thing takes forever to warm up, I can start it and leave it idling for a half hour and the needle hasn't moved. The sooner it's rolling, the sooner it starts warming up. I always give it a couple minutes in the morning before heading out, and drive it pretty easy until the needle starts moving, but sometimes I have to bring a loaded trailer home to start the next day with. Should I be finding out more about this coolant bypass? Never heard of it.

its a line the takes coolant from the rear freeze plug and goes around the t stat. It is there to help coolant flow for the back cylinders for pullers and drag racers. For cold weather turn your heat on low while the motor is cold will help take move some coolant before the tsat opens

Hm. So I guess the question is- whether or not it's actually necessary- if it isn't necessary, what can it hurt?

Yeah I was romping on it after the new TC was installed ... high EGT's and the T-stat not open or even warmed up yet .... caused a high pressure spike due to the extreme egt's (1500+*)... and blew the HG out under the T-stat...

if I had warmed up the truck to opperating temps before nailiing it to the floor ... migh have been able to just do one stud in each hole one bye one ... now i have to pull head and such .... O-rings are gonna be going in , then we can play

Personally for a street truck I don't think you need it but its not going to hurt to run one

So when you blew the gasket it was on just stock head bolts?

Yes ... And they are toast ... Now bought a new head, studs , and saving for a gasket set from Cummins.. One step at a time ...

and in all this, I am nearly ready to pull the IDI from my F250 so I can pull the Cummins and get ER done ..