#238 Nitrile O-Rings (10 Pack) - Buna-N NBR 70A - 3-1/2" ID x 3-3/4" OD x 1/8" CS
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#238 Nitrile O-Rings (10 Pack) - Buna-N NBR 70A - 3-1/2" ID x 3-3/4" OD x 1/8" CS is backordered and will ship as soon as it is back in stock.
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Description
Description
#238 Nitrile O-Ring (Buna-N NBR, 70A) - 3-1/2" ID x 3-3/4" OD x 1/8" CS
Standard AS568 dash size #238 nitrile O-ring - the exact size used across faucets, pumps, filters, and fittings worldwide. Sold as a pack of 10 so you have spares on hand for every rebuild.
Specifications
- AS568 Dash Number: -238
- Inside Diameter: 3-1/2" nominal (3.484" / 88.49 mm actual)
- Outside Diameter: 3-3/4"
- Cross Section (Width): 1/8" (0.139" / 3.53 mm)
- Material: Nitrile rubber (Buna-N / NBR)
- Hardness: 70A durometer
- Temperature Range: -40°F to 250°F
- Color: Black | Pack of 10
Material - Why Buna-N Nitrile
Buna-N is the workhorse O-ring compound: excellent resistance to petroleum oils, fuels, hydraulic fluids, water, and silicone greases, with good abrasion and tear resistance. Not recommended for ozone/direct sunlight exposure, brake fluid, ketones, or strong acids - for those, use EPDM or Viton.
Common Applications
Pool filter tanks, pump strainer lids, large canister housings, tank fittings.
How to Measure an O-Ring
Measure the inside diameter (hole to hole) and the cross-section thickness of your old ring, or measure the groove: groove inner diameter = O-ring ID, groove width slightly wider than the cross section. Match both numbers to the chart below - the dash number takes care of the rest.
AS568 O-Ring Size Chart - Sizes We Stock
| Dash # | Inside Dia. | Outside Dia. | Cross Section | Metric ID |
|---|---|---|---|---|
| #010 | 1/4" (0.239") | 3/8" | 1/16" (0.070") | 6.07 mm ID |
| #011 | 5/16" (0.301") | 7/16" | 1/16" (0.070") | 7.65 mm ID |
| #017 | 11/16" (0.676") | 13/16" | 1/16" (0.070") | 17.17 mm ID |
| #121 | 1-1/16" (1.046") | 1-1/4" | 3/32" (0.103") | 26.57 mm ID |
| #228 | 2-1/4" (2.234") | 2-1/2" | 1/8" (0.139") | 56.74 mm ID |
| #230 | 2-1/2" (2.484") | 2-3/4" | 1/8" (0.139") | 63.09 mm ID |
| #238 (this size) | 3-1/2" (3.484") | 3-3/4" | 1/8" (0.139") | 88.49 mm ID |
| #357 | 5-1/2" (5.475") | 5-7/8" | 3/16" (0.210") | 139.07 mm ID |
Installation Tip
Wipe the groove clean and apply a thin film of silicone grease before seating the ring - it prevents pinching and doubles seal life on anything that gets opened regularly, like filter housings.
Why Buy From Rubberfeetwarehouse
Genuine 70A Buna-N, ships fast from our South Carolina warehouse. Need a different size or larger quantity? Contact us.
Buna-N Material Guide
Buna-N Material Guide
Buna-N (Nitrile/NBR) 70A: excellent with petroleum oils, fuels, water, and hydraulic fluids. -40F to 250F. Not for ozone, brake fluid, or ketones - ask us about EPDM or Viton for those.
Delivery and Shipping
Delivery and Shipping
Usually ships within 24 hours from our South Carolina warehouse. All orders include tracking.
O-Ring Specifications
| AS568 Dash Number | -238 |
|---|---|
| Inside Diameter | 3-1/2" (88.9 mm) |
| Outside Diameter | 3-3/4" (95.25 mm) |
| Cross Section / Width | 1/8" (3.18 mm) |
| Shape | Round (torus) |
| Material | Nitrile (Buna-N / NBR) |
| Hardness | 70 Shore A |
| Finish / Color | Black |
| Temperature Range | -40°F to 250°F |
| Pack Quantity | 10 |
| Fits | AS568 -238 glands: 3-1/2" ID grooves, 1/8" cross section |
| Applications | Pool filter tanks, pump strainer lids, large canister housings, tank fittings |
AS568 nominal dimensions. Measure your groove ID and cross section to confirm fit.
O-Ring Size Calculator
Enter your O-ring's inside diameter and cross-section — get the outside diameter and recommended static groove dimensions (25% squeeze).
Measuring a worn ring? Use calipers on ID and thickness — not the stretched old ring. Compound questions: rubber compatibility chart.
Frequently Asked Questions
How do I measure an O-ring?
Measure the inside diameter and the cross-section thickness of your old ring, or measure the groove: groove ID equals O-ring ID. Match both to the AS568 chart on this page.
Is Buna-N right for my application?
Buna-N (nitrile) handles petroleum oils, fuels, water, and hydraulic fluid from -40F to 250F. It is not for ozone/direct sun, brake fluid, or ketones - ask us about EPDM or Viton for those.
Should I lubricate O-rings at install?
Yes - a thin film of silicone grease prevents pinching and roughly doubles seal life on housings that get opened regularly, like water filter sumps.
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