Installed valve-head depth (or protrusion) in the head certainly makes a difference in cold-starting in some engines, but not a huge difference.
When valve-heads are too deep (usually after a bad valve reseating job), and all else is perfect, it can make a tractor skip and smoke at a certain temp - when otherwise it wouldn't. Often by a 10-20 degree F margin. Or make a tractor start poorly at 50 degrees F, whereas when the valves are correct it might not start poorly until it's down to 30-40 degrees F, etc. And on some tractors, it makes no difference at all. Much depends on other factors.
When an engine is first built, there are plus-minus tolerances on many parts of the engine. Crankshaft throws, piston standout, cam profile, top ring to piston-top distance, fuel delivery, etc. For that reason, same-model engines do not all behave the same when new or used.
I haven't heard specific details, but . . . if his engine won't even try to start at 40 degrees F, but starts fine at 70 F, valve-head position is not going to be the problem. If his tractor smokes and tries to start at 40 F, and starts fine at 70 F, then I'd suspect the valves as part of the problem.
In your sitution . . . if somebody reworked your head to get valve-heads in the correct place, then more was done then JUST making them protrude more. To do so, they proably used thicker-headed valves, installed hardened seat-inserts, or both. So, at the least - your valves got newly seated and would of been tighter and sealed better, then before - thus giving more compression at cranking and starting. I also assume they had the injectors out and checked them while the head was off, and made sure the valve-lash was dead-on when it went back together. My point being, that more then just the valve-in-head-depth was probably altered and improved.
I had to pull apart many brand new direct-injected diesel tractors in the 70s-80s that had cold-starting problems. We had a long check list of things to check, including valve-head depth or standout. That alone never made a huge differece, but it was one of many factors when put together, did.
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