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  1. #1
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    Default How to measure O-rings and save $$$.

    Most gasket and seal kits aren't cheap. Fortunately makers use standard O-rings for nearly every application. You can do the same for metric rings but I'm not working anything metric at home right now so post refers to non-metric sizes.

    Next time you remove one (if it's not mangled) or install a new ring, measure the outside diameter (OD), inside diameter (ID) and thickness. Write that down and you can find bags of rings easily for dirt cheap.

    You'll get a feel for standard sizes. For example the rings for the small primary chain inspection covers on Evo Sporties seal 1/4" screws.

    Their inside diameter is 1/4", their outside diameter is 3/8", and their diameter is 1/16" thick.

    The OD minus the ID equals TWICE the diameter because they are circular.


    I started taping removed rings to white cardboard along with dimensions so I have sample sheets for my O-ring stash for quick visual reference. Standard aircraft practice to to have wall boards of such small parts for easy ordering and the principle is fucking wonderful.

    You can find bags of Viton or Buna rings online for dirt cheap, and ring vendors sites discuss what materials are good for a given application if you want to learn more. For motorcycles Viton or Buna is fine. Viton is a bit better and the same material carb needles are tipped with.

    Some S&S Super E sizes. "Exploded view" references S&S Super E instruction sheet. Text is as I labeled them hence the caps.

    1/16" THICK X 3/8" OD X 1/4" ID = ACCEL PUMP CAP #42 IN EXPLODED VIEW. (2 EACH)

    1/16" THICK X 1/4" OD X 1/8" ID = ACCEL PUMP SQUIRTER "FREQUENTLY LOST" WHEN PULLING BOWL. (1 EACH)

    3mm ID x 5mm OD x 1mm THICK = IDLE FUEL MIXTURE SCREW. (1 EACH) - Too thin for my liking. Was torn on my new carb. Use the accel. pump ring instead.

    1/16" THICK X 9/16" OD X 7/16" ID = SEAT AND BOWL PLUG. (2 EACH)

    I just ordered the above so here are some example prices. I'm not affiliated with the sellers.

    Accel pump cap:
    https://www.ebay.com/itm/O-RING-Buna...72.m2749.l2649

    Accel pump squirter:
    https://www.ebay.com/itm/ID-1-8-x-OD...gs-50-PACK-006
    /292387646765?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

    I use a cheap Harbor Freight digital caliper for that stuff to save my nice Mitutoyo and Tesa. They work fine though the feel is gritty, and their key advantage is instant fractional conversion which is veryfuckinguseful for SAE hardware. For about 20 bucks it's like a sore dick, you can't beat it.

    BTW Super B or E bowl gaskets are usually good for several uses. If you trace or scan your gasket you can use that as a pattern when cutting your own. You can also email scans to others which might come in handy.
    Last edited by farmall; 04-08-2019 at 1:01 PM.

  2. #2

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    Thanks farmall!

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    Ya, good stuff.
    I just read an old 2006 thread, in the 'Fits 'n Bits section', a couple days ago with similar info:

    O rings and bearings
    "Most 'o' rings and bearings you find on your bike are standard sizes.
    Check out local bearing place before ordering 'genuine' part. This includes 'o' rings in brake cylinders. Dav just found out the hard way by paying $17 for his first 'genuine' set of brake 'o' rings and then 70c for his second set from CBC!"
    http://www.choppersaustralia.com/for...php?f=24&t=528

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    Example fluid resistance guide shows why Viton is outstanding:

    https://plastomatic.com/viton-fluid-...ance-guide.pdf

    You do NOT have to go this deep to find if a ring suits you, but know these guides exist and also apply to seal materials.

    Every time I think in industrial or aviation terms when choosing and sourcing parts I get the result I'm after.

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    I couldn't agree with this more. great info, thank you farmall
    Last edited by xMPRx; 10-01-2018 at 1:40 AM.

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    just to add to the good advice.. mcmaster-carr is a great site for probably every o-ring on the planet.

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