The suspicion as to why the block threads failed during 02-04 (all we have, Toyota never disclosed why) is heat. Because its always the same 3 block threads located where the mold of the aluminum block is at its thinnest. Molded to compensate for the curvature of the plastic intake. Because the plastic intake passes so close to the block, foam insulation is used between the plastic intake and the block. Protecting the intake (but IMO trapping heat on the block) Its always the same 3 threads in the thin area of the block behind the foam that fail. My 02 2AZ-FE Camry has this potential. Can't claim the frequent (40k) drain and fills are why I have not experienced this, but I can say that researching the background on reported issue the majority neglected coolant, mixed coolant, or overheated prior to the event. So anything that can be done to prevent overheating or running hot IMO is helpful. But as stated there's no real proof it can prevent it reliably. IMO it was a design issue with the 02-04 block that leant itself to heat related problems.
The stripped block thread problem disappeared after 04 when Toyota changed something (possibly design of the block, maybe deeper threads, etc) for 05+. Not clear on all the changes but what is known is Toyota changed from Red LLC to pink SLLC in 05+. Coincidentally a rash of water pump failures occurred during 05-06. Why the pumps failed is anybody's guess. Some say it was the Pink coolant. Others (like me) suspect a new water pump design to help keep the engine cooler, assisting with the former stripped block thread (AKA stripped headbolt) issue.
What should be noted is the problems being discussed are from different engine designs, or different versions of the same design. They cannot be linked together as one big problem. They are different problems experienced over a decade of change.
