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Can't find that one on webike. Just an aluminium one for something north of €1300   The one in your pic is for the Z1 - did they have a 900A4/1000A1 version?

www.japan-webike.ie/products/9627926.html

There are 2 options, the standard cap and airplane cap and it for the z1000 A1 too, i just ran out of page on the screnn grab. its a bit of a cost out lay but might save a load of heart ache down the line and it comes with the cap...

I think I'll stick with the home made job I can choose the size cap I want and get a keyless cap. And I'll come out about 200€ better off even after buying the cap. Plus I get to have a go at messing up a tank

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Plus I get to have a go at messing up a tank

Messing things up is all part of learning new skills.  Go for it.

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I did consider that as well. I quite like the ring with the screws though

How will you get the ring to sit flat on the tank? It looks like the tank surface curves away from the fill hole. Is there a way to build up under the ring/flange to avoid the gap? Would you somehow flatten the top section of the tank?

The most complicated solution would be to find someone with a 3D laser scanner setup and scan the top surface of the tank and have a ring machined to match. www.faro.com/en/Products/Hardware/Quantum-FaroArms


 
 

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I'll cut out a slightly larger section than needed and make up a flange with a step down so the cap is flush with the tank (may need a bit of filler on the transition from the tank to the flange). I have a few ideas of how to do it but I need to wait until the tank is here to see where I need to cut it and how curved it is there. 
I'll pass on the laser scanner I'll get the flange laser cut, though. Will probably get two halves made that I can weld together when they are inside the tank - rough draft in the picture below. Will need another set with a slightly larger diameter to create a step down from the tank surface (large ones welded to the tank, smaller ones with the holes for the cap welded to the larger ones). Figured I can drill some holes around the cutout in the tank so I can "spot" weld the flange to the tank and won't have everything hanging off the thin edge of the cutout. If the curvature of the tank is too much it may be a bit tricky though...

 

This picture might make a bit more sense... I'll try and get the cutout in the tank flush with the step - hoping it's pretty flat at that diameter.

 

I see where you're going with that.  Do you know about press fit threaded inserts for street metal?  Looks like there are weld on parts as well.

www.pemnet.com/fastening-products/pem-se...ching-fasteners-new/

I'll either make up some short threaded bungs and weld them in place before it goes in or I'll just get the parts cut out of 2.5mm steel and cut the threads directly into the plate. 8 x M5 threads each 2.5mm deep should be plenty. Not really worried about weight.

If you fit the 400 series 8 bolt filler cap the original cap recess is all but removed when cutting the hole for it.
There is a small section of recess that remains towards the top yoke where the original cap hinge was.
This is easily made flush with the surrounding metal.

Continuing to think of overly complicated solutions, I was trying to think of a way to form the area around where you want to weld to make it into a flat ring.  I was thinking you could make two piece insert and then clamp the rim to form it to be flat.  No idea if that would actually work.  The metal might winkle or just refuse to change shape. 

The picture below shows what I'm thinking.  The two piece split ring goes on the inside, the full ring goes on the outside, and you clamp it together with bolts.


 

Guys this why I suggested the Golf panel, the recess is already formed so you have a flat mounting surface (you could weld nuts to the back) there is enough spare panel to either cut to fit your existing hole or overlay it and lead load the transition once it’s welded in (just a suggestion)

Thanks for the input guys. Tank should be here in a day or two and I can see what I'm dealing with.