I am pretty sure I did something I never do, and threw away a perfectly good OEM gas cap for my 038. I went looking for it as the gasket on the "new" one is cracked, leaking, and currently sitting in the trash. I bought a set off eBay when I lost the bar oil cap. Stupid me went "new" with the gas cap, hey they need to match ya know. The cap threads very poorly so while I could make a gasket, I'd be better off getting a better cap too. Anyone have a reputable supplier/ vendor for these things? Very common caps, they are used on many Stihl saws. The replacements looked like this
I'd try your local Stihl dealer. They will look a bit different as they substitute to the 036 style caps (taller for better grip) but they aren't so expensive as to take another chance on an aftermarket replacement.
I have been able to in the past just go to hardware store and match up rubber flat gaskets or o-ring type depending on saw.
Does your 038 have the old school oiler cap too? Ones like the AFM cap pic? JJ was saying those have been NLA oem for like 20 years.
I am only showing OEM http://www.ebay.com/itm/stihl-038-m...467?pt=LH_DefaultDomain_0&hash=item35e1df8b43