Detroit Diesel- 12V71TAC Replacement Zincs

[Big Chief]

Are the brass plug replacement zincs 3/4" diameter with 3 3/8" length? How often should they be replaced for preventive maintenance?

 

[Briankinley2004]

The number on my 12V71 TI was Detroit 8515850Z


https://www.boatzincs.com/8515850Z.html
The dimensions seem the same as you listed. I changed mine annually