How soon do I need to display DOT number?

Correct. Any time spent serving or conditioning a CMV or performing work (regardless of compensation) on behalf of a motor carrier is on-duty time and must be logged appropriately. In you above example, the 6,400 GVWR, you would log that time as on-duty not driving per FMCSA guidance, same as you would driving a car or riding with someone to go give the urine sample.

 

No, in your example your F350 is clearly not a commercial vehicle unless you camp for prize money LoL (like the DOT has been busting the dirt track racers and competitive horse riders). And, yes, if the scale is open you are still required to stop when using your tractor off-duty (PC driving) and they can ask you to justify the trip.

What makes this so complicated is the lack of a single rule that says "thou shall or shall not.....", instead it is a mix of different regulations applied to different situations or, as in the case of personal conveyance, the lack of prohibition in the regulations that allowed FMCSA to issue the guidance document supporting the use of PC time. I make my living with these rules but believe me it would not hurt my feelings if all the regulations were reworked to be common sense rules, and much fewer in number. I would be happy to see the need for my services, along with most lawyers, done away with.

 

Not to be nit picky, and only because you will appreciate the exact details, but in Pennsylvania it is 17,001 GVWR/actual weight or above that is the threshold for an intrastate regulated commercial motor vehicle. We have three ways it can be registered (outside of special plates such as repair towing, salvage, MMVB, historic, antique, etc.) which are just a truck plate (regular plate same as a little pickup would get), recreational vehicle (if it meets the criteria) or apportioned.

Now, just because it has a truck plate and is over 17,000 GVWR doesn't automatically make it a CMV in PA. They still apply the intended or actual use test, following the same guidelines as the FMCSA since PA adopts the FMCSRs with only a few minor tweaks such as the GVWR change to the definition of a CMV. Where enforcement gets picky in PA is once you get over 26,000 pounds because they have, and enforce, class A, B and C non-CDL licenses. So, if you have your bobtail historic tractor with say a 52,000 GVWR you need at least a non-cdl B to legally operate it if you are a Pennsylvania resident with a PA driver license. That said, they do honor (as required) other states that do not offer non-cdl class A & B licenses.

 

Not saying your wrong because I could be misremembering, but...
A couple years ago a member was on here trying to get his show truck registered as anything that's not a CMV so he didn't have to get a dot number and pay commercial insurance. He ended up having to get a dot number, put the proper lettering on a door magnet so he could pull it off when he got to the show.
All because in PA he couldn't register it as a personal vehicle. It was 100% funded by him, and he didn't make any money at any of the shows so not commercial. He also had a class a, so no license issue.
If you remember it was the guy doing the Optimus Prime show truck.

 

Yes, I know Joe with that truck, provided some help (for free given his cause). He got it all straightened out, has a PA truck plate on it but it is not a CMV. No US DOT or PA PUC numbers required and he was able to insure it just like his collectible KITT replica (the car from Knight Rider). It took some fighting but he got it done. He travels all over the US still with that truck as a private vehicle, no numbers and a non-apportioned PA truck plate.

The biggest issue he had was PA doesn't differentiate between a "truck" and a "commercial truck" regarding their in-state registration plates. They only have different weight classes, the same plate that was on my F-150 as a class 1 was also on my Mack tractor as a class 25. This causes some confusion with insurance companies, and in PA since we do all our title and tag work thru agents they get confused with anything slightly out of the norm.