Radiator/reservoir tank temperature differential question

[Rederic]

Radiator coolant reservoir temperature differential question
1991 LT 31 Florida
1G motor

Good day!
Not sure if this is the correct place to post -
Recently had complete top and bottom end rebuild including everything new head etc.
Had the radiator tested to make sure everything was good.
Recently blew a head gasket after 40,000 km.
Just got everything taken apart machined reassembled.
The mechanic is very meticulous and took some measurements of the radiator compared to the reservoir tank.
He said that there’s already while idling a The 20°C differential between the reservoir tank and the radiator.
He said that there’s not supposed to be more than 10° differential and this indicates that my radiator is clogged up and inefficient.
He suspects this is the cause of the recent head gasket failure.
Can anyone confirm this differential specifications between radiator temperature and reservoir tank temperature?

[ngoma]

I have some questions:

What type of head gasket?

How were the surfaces prepared?

Studs or (new?) bolts?

Torque procedure?

How was the radiator tested?

How old is the radiator? What is its condition? Was it ever subjected to oil/water (clogging) emulsion? Coolant condition? (what type/maintenance schedule). (Dexcool type known to create sludge if mixed with other types.)

Thermostat tested?

Waterpump type (stamped steel impeller or cast)?

More:

Was the head gasket failure a direct result of an overheat incident? If so, what was the engine doing at the time? What was the temperature? For how long?

I have never observed much coolant circulation through the reservoir during idle or while revving the engnie. Not so sure the temperature differential is a good indicator of radiator health. Is the LT coolant overflow tank plumbed much different than the Volvo system?

Is the LT radiator a crossflow type? The coolant flow through the radiator is from the inlet corner crosswise towards the outlet corner. There should be a measurable temperature difference between the inlet and outlet areas but not a distinct cold spot or region. Also, once the thermostat opens and the cooling system pressurizes, the radiator hoses will stiffen and become hard to squeeze. Sometimes a radiator blockage will cause the upper radiator hose to become rock hard in comparison to the lower hose.