Unidentified nut fell off during oil change

Today I started on an oil change for my 2008 FSC600D. I was in the middle of loosening the old filter when a nut fell down from the undercarriage. I couldn't tell where it came from and I don't see it in the part schematics I've consulted, though it's possible I'm overlooking it. One oddity is that the nut is immaculately clean even though the undercarriage (and, alas, the scooter itself) is pretty dirty. I'm wondering if it might just be a piece that somehow got picked up on the road and was knocked loose in the course of my work, but that feels too good to be true and in any case I want to be sure. I've attached some photos of the nut. Thanks in advance for any help.

Those are typically used to hold plastic panels in place and are usually accompanied by a sleeve or bushing that is inserted thru the oversize hole in the plastic. The sleeve allows the nut to be tightened securely without compressing the plastic and preventing it from moving slightly during temperature swings. It also keeps from compressing and breaking the plastic. I own a Silverwing, Reflex, and Helix (sold a Honda Spree) and have seen those nut a lot of times. I seem to recall them being used on the exhaust studs of one of the scooters as well. I'm guessing the Silverwing would use a larger nut on the exhaust and that nut shows no sign of being heated. I would look into the air cleaner and muffler "chamber" for the air cleaner system. The 6mm variety (uses 10mm wrench) are all over on the body panels of the Spree, but I can't place them on the Silverwing at the moment. All my Hondas are in storage for the winter away from my house. I only have my antique Salsbury and total-electric R Martin scooters here right now - and I rode them both today.

By the looks of the nut, I have not seen any nut like that on my Swing. I have seen similar caps on lug nuts for cars and trucks. You probably picked it up from the road.

Looks like exhaust header nut to me bud

The Bern wrote:

Looks like exhaust header nut to me bud

As said nice and clean no sign of getting hot, if you are the 2nd owner or more it might be a where did that nut go as they heard the " clink " of it disappearing to places unknown, now you know.

The Bern wrote:

Looks like exhaust header nut to me bud

The Bern is right.  Found some on Ebay exact nut and they are a little rusty but no signs of heat.

Quality of older Honda parts is impressive. Newer stuff.  

Found a rock or two in lowers but anything can disappear if dropped.

Here is a nice photo of a rock wedged between sub-frame and transmission of a 2002 Accord.. It was tight and took a large prybar and carefully placed piece of wood not to split the trans. case.

The only part of my SW which I haven't 'touched' is the engine. I haven't come across anything like that during all my dismantling/reassembling work. Check the parts manual. It does seem rather shiny for an exhaust nut though!

Loosemarbles wrote:

..//.. It does seem rather shiny for an exhaust nut though!

 

Oh ye of little faith, some of us 'old hands' like to fit long dome nuts so that there is no exposed thread on the stud to get corroded .......  

https://www.bertrumsbikebits.co.uk/product/honda-yamaha-kawasaki-suzuki-m6-m-6-chrome-exhaust-dome-nuts-set-of-2/

I have to agree with Easyrider, that looks like a wheel lug nut. If it's 3/4" outside diameter then it is a lug nut.

Thanks for all the replies! It was indeed part of the exhaust system, one of four identical nuts holding the manifold. I was able to find where it belongs and (despite a very tight fit) get it back in place. It looks like the previous owner might've run into the same problem, since one of the four nuts had been replaced by a more conventional capless variety.

Wonder how long it was where you found found after it came off, or was dropped.

Odd, I've never heard a cap nut called a dome nut. Might be a transatlantic thing.

exhaust front pipe nut thats what it is