Or:
Instead of implying that the asker is a cheapskate, and not even attempting to answer his question, like maybe sharing any experience you might personally have about the o-rings going bad, you might have gone with something like this:
I've never been a big fan of the flippy tops (too clever by half as I like to say), I suppose that once one learns to think like a flippy top, there's a remote chance that they can actually be properly closed with reasonable success on something approaching a consistent basis.
Quirks aside, it stands to reason that the o-rings might just start to lose their effectiveness after a while, and might just be good candidates for replacement. It's also possible that a replacement o-ring might actually be of higher quality than the stock ones. Many manufacturers use, and a lot of hardware stores stock, "Buna-N", which isn't particularly good at anything except being inexpensive. Viton is exceptionally good at standing up to petroleum-based products like the oil and gas we run in our saws. Are the stock o-rings viton? Well who knows? And who really cares, since viton o-rings are so readily available from a wide variety of sources, and probably for a whole lot cheaper than you could get stock ones from Stihl for, if they sell the o-ring separately at all.
So the OP got me thinking, and I took the cap off of my 460 and checked out the caps and o-rings. Right off the bat, the o-rings are totally normal, off-the shelf o-rings, and the caps seem to be interchangeable, with the o-rings exactly the same size. Hell, of a start, I say to myself.
Measuring up both the caps and the o-rings, I make them out to be Dash 214, which has a fractional size of 1" ID x 1.25" OD, and an actual size of 0.984" x 1.262". This sorts well with the measurements I took of the cap (about 1.10" diameter where the o-ring sits), and of the o-rings themselves, which were about 1.01" ID and 1.28" OD (after being in service for about 3 years).
They actually seem to be in pretty good shape; still plenty pliable, not torn, and still fairly round; but could they stand to be replaced with something that might be a little closer to stock specs, and maybe a superior material too as an added bonus?
On closer inspection, the average wall thickness of my used o-rings was about .1315". They were a bit flattened - .1350" high, and 1280 thousandths wide (I would have expected them to be flattened the other way), but replacing them with new o-rings would give me a good .070" tighter seal (((1.262" - .984")/2 - .1315) = .070)), so what the hell, I'm going to replace them.
Plenty of places to buy them
As so often happens when looking into something, I learned something new. There's a material called PTFE (polytetrafluoroethylene - oh yeah, Teflon, of course) that might be even better than viton for our purposes.
Teflon vs. viton:
hydrocarbon resistance: Viton good, teflon excellent
water resistance: Viton poor, teflon excellent
tearing: Viton fair, teflon excellent
abrasion: Viton good, teflon excellent
compression set: Viton good, teflon fair
This last one, compression set (how well does it spring back to its original shape after being compressed) might actaully be an advantage.
"Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore, it has no "memory" and is subject to creep. This is advantageous when used as a seal, because the material creeps a small amount to conform to the mating surface. However, to keep the seal from creeping too much, fillers are used, which can also improve wear resistance and reduce friction".
Polytetrafluoroethylene - Wikipedia, the free encyclopedia
Hmm, creeps a small amount to conform to the mating surface. Just might be what the doctor ordered.
Hey, did you know:
"The coefficient of friction of plastics is usually measured against polished steel. PTFE's coefficient of friction is 0.05 to 0.10, which is the third-lowest of any known solid material (BAM being the first, with a coefficient of friction of 0.02; diamond-like carbon being second-lowest at 0.05). PTFE's resistance to van der Waals forces means that it is the only known surface to which a gecko cannot stick".
Polytetrafluoroethylene - Wikipedia, the free encyclopedia
I bet you didn't. Take that, Mr. Gecko.
Oh, and what is "BAM"? I had no idea.
"BAM is a ceramic alloy created from an alloy of boron, aluminium and magnesium (AlMgB14) and titanium boride (TiB2). It is one of the hardest known materials, after diamond and cubic boron nitride. It is also the slipperiest solid known, with a coefficient of friction of 0.02; for comparison, Teflon products have coefficients of friction (dynamic) of approximately 0.04. The material was discovered in the course of other investigations by Bruce Cook and Iowa State University professor Alan Russell in 1999 at the U.S. Department of Energy's Ames Laboratory. "Its hardness was discovered by accident. We had a terrible time cutting it, grinding it, or polishing it," said Russell".
http://en.wikipedia.org/wiki/BAM_(material)
Hope this answers your question.
Best regards, John D.