TOPIC:

Hi, I'm looking to verify that 1980-81 kz550 rear axle is 15mm and I'm trying to find the length of it. If someone has one to measure... Or better yet sell, that would be greatly appreciated! 

Howdy. Partzilla lists the castle nut as 16mm.  The axle part number is 41068-1086 & the cross-reference there at partzilla
says it was used on a bunch of 550/650/750 models, so you should be able to source one on EB.
Search eb by either part number or part name (KZ550 rear axle for example). 

ps: topic title kinda confusing. pivot shaft would be the swing arm or center stand I guess.

Yep same part # as many other bikes including the 1980 KZ750E1  as Martin said.

The shaft part of the bolt measures 29cm (or 11.5 inches). 30cm if you include the bolt head

There are a number of kz750 rear axles on ebay,com at the moment. When searching you only want the axle from 4 cylinder models

 

By JR:  ... When searching you only want the axle from 4 cylinder models.

This, with one exception being the 82 KZ750 CSR Twin. 
An earlier 750 Twin axle would be too big as these are 18mm.
The 83-84 750K Ltd Belt are 16mm but unknown length. 
Whatever you get, double-check the x-ref before purchase. no sense goofing on something so mundane.

Thank you for the responses.  I apologize for the title.  I'm actually planning on using the rear kz axle on the pivot shaft of the bike I'm building, must have been tired when I typed up the subject.   But to verify, it looks like the early 80s rear axle is 16mm? And not the 15mm that I require. 

Okay.  I checked a few smaller models (250/305/440) but they're either 14mm or 16mm. I tried looking at some Hondas of the era, but the ones I saw were either 14mm or18mm.  14/16/18 might be a Japanese standard. guessing.