Air Cooled VW Beetle Flywheel Gland Nut Size

Does anyone know what size a tapped hole would need to be in order to accommodate the large flywheel gland nut on the old VW Aircooled beetles?

I want to try and make a part that has a tapped hole to fit this large fastener in place but it doesn't seem like it's a standard size.

After measuring and calculating, I get the nominal diameter of the fastener to be 27mm but I calculated the thread pitch to be ~ 1.3 mm which doesn't match up to any standard size.

Does anyone have detailed info on the size of this bolt?

Thanks!

PS - here's a picture of the bolt in question: http://cgi.ebay.com/ebaymotors/VW-Beetle-Fastback-Flywheel-Gland-Nut-61-74-75-76-77-78-/290496042213