Kev

I would recommend you go at it with the holesaw, but flare the holes a la aircraft structures. A bent sheet of steel is a hell of a lot stiffer than a flat sheet, and flaring a hole just adds stiffness. The box section is doing the same thing for the relatively flat floor section. It is not overall tensile strength that will be critical there but resistance to buckling in compression and bending.

You can get flare tools but these tend to be a two sided jobby that you use before the sheets get welded together + exy - http://www.allphaseoffroad.com/products.html?id=65

I reckon you could come up with a good way to flare it in situ....

examples






Anyway, by making the holes yourself, you are just saving the time it would take to rust the holes

michael

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20mm holesaw shouldnt cause any problems Doc, as long as they were few and far between and not like swiss cheese.

I would then find some rubber bungs to suit (or alternatively, find the bungs and then geta holesaw to match).

You want bungs, not grommets, grommets have holes in them, bungs dont!

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