I just did the same thing this morning. Bought one on EBAY with out hardware. I ended up buying a 10 pack from McMaster use a 25/64 drill bit. Also you need to make a installer tool Follow this link to built one easy. I did and worked great.
http://www.fjr1300.info/howto/rivnut-tool.html
Get 2 for the top 1 3/4 6mm x 100 pitch Hex Heads and fender washers. You'll need 4 more about 1in. I bought long ones and cut them down when I figured out what I needed. They might have been 1 1/4. Keep the front left one (not the top ones up front up near the bumper just long enough to catch the treads and tighten up or you will hit the engine. I measured with a drill bit by hand to see my clearance. (sitting on the sled or drivers side if it was a car)
Important thing to remember is to use the anvil to hold the rivit flush while you tighten the nut. I used a drill to oblongate the holes because they didn't line up good. Thus the fender washers
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I did what this guy said to do.
in fact there are two layers of metal to go through for a total of what I roughly measure as 5mm.. The specs for the 14.73 long AVK A-L insert(rivnut) that they send with the kit, state that the material thickness range is up to 4.2mm. What that tells me is that they are simply not long enough. So, if you wish to stick to using rivnuts, or replace your falling out rivnuts, the answer is to get the next size longer, the 17.27 long rivnut which is good for up to 6.6mm material thickness. Fits in your original hole. I ordered a package of 25, part number 94020A387, from McMaster-Carr for roughly 14 dollars plus 4 dollars shipping. Order online.
and this guy
I like to lift my sled up with two straps and go under it while I'm doing these. You need to mark out the four rivets on the corners that will have to be drilled out, you can also knock them off with a sharp cold chisel & hammer. They recommend a metric drill bit #M9.9 I never seem to have any of those darn things when I need them so I used a 25/64th and it works just fine, BTW if you have a drill bit stop use it so you wouldn't go to far into there and hit something with the bit. [if you wrap masking tape around the bit enough times and don't push to hard it works to] Now take that little riv nut installer they give you and screw on the riv nut till it's at the bottom, now push it through the hole and hold it there with a wrench while you use a little ratchet to tighten and pull the riv nut down tight, keep tightening till you feel it bottom out. Back it off and do the other three, then remove the two screws from up front on the nose cone as they give you longer ones. Install those two bolts and push the Skid Plate up and install the four bolts...........U R Done again.
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And this guy
I just installed a skid plate on my 2007 Venture GT last night, using a 25/64 drill bit, which worked perfectly. The factory instructions were emphatic about not drilling in more than 20mm deep. IIRC, the four mounting screws were M6 x 20mm, and used 6mm flat washers. My skidplate also has two mounting holes at the very front of the plate, which used M6 x 25mm screws and the 6mm flat washers.