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I searched TM and ORD to no avail. What is the size of the master cylinder filler cap without the vent (installed on my M38)? It seems to be 15/16", but that just seems like an odd dimension, and I don't want to round it off because of a slightly odd fit due to corrosion.

That sounds like the wrench size - I know it wasn't 1". I just checked my fluid the other day. I'm not sure about the actual thread size.