What are the dimensions of Volvo's "blind rivets", part number 982 565 used for fender liner and similar trim pieces? Are they simply black 1/8" diameter, 3/8" depth aluminum pop rivets?

@erikv11 thanks for checking. Yep, an M7 tap did the job.

Here are the factory service manual specifications. I'd measure the dish with a feeler gauge and a good straightedge and compare it it spec. https://charm.li/Volvo/2003/V70%202.4%20L5-2.4L%20VIN%2061%20B5244S/Repair%20and%20Diagnosis/Engine%2C%20Cooling%20and%20Exhaust/Engine/Cylinder%20Head%20Assem...

Appreciate the suggestion. Can I flush the core while it is still in place? Thinking of holding a garden house up to where the hoses connect at the firewall. Yes Perfect, thanks for the suggestion. I flushed the heater core at the firewall with the garden hose and that seems to have cleared the clo...