Rather than bumping any one of a number of twenty year old threads, I want to provide some picture evidence of an often misunderstood issue about using AT831 bodies with AT853 caps. I own both of these types of mics in varying setups. I looked into old threads while preparing possible future sales and want to clarify varying degrees of information as there seems to be some lack of transparency and photo evidence.
AT831 bodies are compatible with AT U853, U853R, and 853a caps. The only cap that I am aware of that is NOT compatible with 831 bodies are the original 853 caps. This is because these are the only 853 cap (that I am aware of) with threads directly on the inside of the housing. All compatible caps have a recessed base that features threads on the outside of the recess. These screw on perfectly and provide a secure connection directly similar to the original 831 caps. I do not own any caps in the 900 series and therefore can not test these for compatibility. I am fairly certain that 853a caps are directly similar to U853Rx caps in threading.
I tested my U583 cardioid and 853a cardioid caps on 831 bodies and got exact/nearly exact measurement in quick tests as the original 831 cardioid caps.
Attached are pictures to visual clarify this post.