I know this topic has been discussed in the threads sticky and a few other posts here, and that the large inner thread for the fuel tank is 3/4" BSP however fittings for that thread don't fit my tank, so just wanted to post my thinkings on the topic.
Are people that say its 3/4" BSP forcing the fittings in, or drilling/retapping a new thread? Either way it doesn't fit my tank, instead I think it could be 1" - 12 UNF (the 12 representing TPI).
I took the tank to my local hydraulics shop where they tried all sorts of steel fittings including 3/4" BSP, but couldn't find anything that would fit. So I 3D printed a range of plugs with different threads to try and find something that would fit based off the dimensions measured.
My original fitting measures approx 25.30 mm externally (major diameter) and roughly a 2mm thread pitch (measured with calipers as I don't own a thread gauge).
1" - 12 UNF and M25 x 2mm were the only threads that tightened well with no binding that I could feel, all the others were either too large in diameter, wrong pitch, or slightly too loose.
I'm looking at this as I'm working on adding an EFI feed to my Mk2. My Mk1 already has EFI and the feed is simply the original fitting drilled out with a large copper pipe brazed in, for the Mk2 I'd like to use AN fittings from the tank ideally.
Both 1" UNF and M25 appear to be really uncommon threads, I'm struggling to find bolts with those threads online let alone AN adapters, so I'm currently set on welding a AN bung onto the tank but thought I'd post my workings anyway.
I also took some pics inside the tank to see how thick the metal was surrounding the original thread, not sure if there's enough meat there to tap out really, using photoshop I measured the lip to be around 1.9mm thick.
Also doesn't help that it looks like its tapped slightly off centre.
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