Dow Corning Silicone for O-Rings: 7 Questions Answered by a Rush-Order Specialist

When Every Hour Counts: My Dow Corning Cheat Sheet for O-Ring Emergencies

I've been the guy who gets the 3 PM call for a Friday delivery. "We need O-rings by Monday morning. Our supplier just told us they're backordered six weeks."

When I first started in this role, I assumed any silicone would work for any seal. A $12,000 pump failure later, I learned the hard way: material selection isn't just about what's available—it's about what's right for the application.

Here are the questions I answer most often when someone's scrambling for Dow Corning silicone products.

1. What's the difference between Dow Corning silicone oil and silicone grease for O-rings?

I get this one a lot. And the short answer: don't use oil where you need grease.

Dow Corning silicone oil (like the 200 fluid series) is a low-viscosity lubricant. It's great for assembly—helps O-rings slide into place without tearing. But it won't stay put. In a dynamic seal application, it'll migrate out within hours.

Silicone grease (like Dow 732 or other high-consistency compounds) stays where you put it. For O-rings that need long-term lubrication, especially in static seals, grease is the better choice.

A quick rule I use: if the O-ring moves, use grease. If it's just installation, oil is fine.

2. Can I use Dow Corning 732 sealant as an O-ring?

This came up during a rush order in March 2024. A client needed a custom gasket for a prototype—48 hours before a trade show. Normal turnaround on a molded O-ring? Two weeks minimum.

Short answer: 732 is a sealant, not a molded O-ring. It works for formed-in-place gaskets (FIPG), but it has limitations:

• It cures at room temperature (takes 24 hours for full strength)
• Lower tear strength than molded silicone
• Not ideal for dynamic seals

Did we make it work for that prototype? Yes. Was it a permanent solution? No way. The client replaced it with a proper Dow Corning silicone O-ring within a month.

3. What's the deal with 'dow corning silicone dow 732 99179'—what is that number?

I've seen this search string so many times. The "99179" is a specific part number for Dow Corning 732 in certain packaging—usually a 10.3 oz cartridge for standard caulk guns.

Here's the thing: Dow Corning 732 is a multi-purpose silicone sealant. It's available in different colors (white, black, clear) and different packaging (cartridges, tubes, pails). The 99179 is just one SKU.

If you're ordering for a critical application, don't just search the number. Specify:

• Color
• Cure time (standard vs. fast-cure variants)
• Quantity needed

I've seen orders delayed because someone ordered 99179 (which is white) when they needed black. Save yourself the call.

4. When should I choose Buna-N (Nitrile) over silicone for an O-ring?

This is where conventional wisdom often gets it wrong. Everything I'd read said silicone is the go-to for high-temperature applications. In practice, I've found Buna-N outperforms silicone in some critical areas.

Property	Silicone	Buna-N (Nitrile)
Temperature range	-60°C to 200°C	-40°C to 120°C
Oil resistance	Poor	Excellent
Abrasion resistance	Fair	Good
Tensile strength	700-1400 psi	1500-3000 psi
Cost	Higher	Lower

Bottom line: If the O-ring contacts petroleum-based oils or fuels, Buna-N is the safer bet—silicone will swell and fail. For high-heat, dry environments, silicone wins.

One more thing: Buna-N has a shelf life of about 5-8 years; silicone can last 20+. For long-term storage, silicone is better. Just make sure the application doesn't involve oil.

5. What's wrong with using a black PVC sheet as a gasket instead of an O-ring?

I've seen this too many times. Someone needs a seal fast, and there's a sheet of black PVC in the workshop. "It's the same color, right?" Not even close.

Black PVC is not a gasket material for dynamic seals. Here's why:

• PVC has poor compression set—it'll take a set and leak within weeks
• It's stiffer than most gasket materials (Shore A 90+ vs. silicone at 50-70)
• Temperature limit is lower (about 60°C continuous)
• Chemical resistance is unpredictable with many fluids

I learned this the hard way in 2022 when I used black PVC as a quick fix for a water pump seal. It worked for three days. Then I spent four hours cleaning up the mess.

If you need a quick gasket and have nothing else, use a properly selected Dow Corning silicone sheet (like their custom-fabricated gaskets). It'll cost more upfront but save you the rework.

6. How do I measure an O-ring for replacement when I don't have the original spec?

This is the most common emergency call I get. Someone has a failed O-ring, no documentation, and a machine down.

Here's the method I use:

1. Measure the inside diameter (ID) — use calipers for precision
2. Measure the cross-section (CS) — the thickness of the ring
3. Determine the durometer — Shore A is standard for most applications

If the O-ring is damaged, measure the gland (the groove it sits in) instead. The gland width should be about 1.5x the O-ring cross-section.

For Dow Corning silicone O-rings, standard sizes follow AS568 dash numbers. A #214 is 0.984" ID × 0.139" CS. A #336 is 2.984" × 0.275". You can cross-reference your measurements with a standard O-ring chart.

One tip: if the original was Buna-N and you're switching to silicone, check the durometer. Silicone is often softer (50-70 Shore A) compared to Buna-N (70-90). You may need a thicker cross-section or a backup ring.

7. What's the fastest way to get Dow Corning silicone O-rings when I need them tomorrow?

This is why I have a job. When normal lead times are 2-6 weeks, here's what I do:

Option 1: Stock distributors with local inventory. Companies like Grainger, McMaster-Carr, or specialized seal distributors often stock common Dow Corning silicone O-ring sizes. Call them directly—don't just check the website.

Option 2: Custom fabrication shops. Some shops can splice silicone O-rings from cord stock in hours. Not as strong as molded, but functional for low-pressure applications. In Q4 2024, I used this method for a client's 48-hour turnaround and it saved a $15,000 production line shutdown.

Option 3: Rush order with the manufacturer. Dow's own distribution can sometimes expedite. Expect premium pricing—I've paid anywhere from 25% to 100% markup for rush service.

But here's what I've learned after 200+ rush orders: prevention beats cure every time. The 12-point checklist I created after my third O-ring emergency has saved us an estimated $8,000 in potential rework. Now I always verify:

• Material compatibility (silicone vs. Buna-N vs. something else)
• Size accuracy (measure twice, order once)
• Durometer requirements
• Lead time buffer (add 50% to quoted lead times)
• Backup supplier contact info

Five minutes of verification beats five days of correction. Every time.

Pricing for Dow Corning products varies by distributor and volume. As of early 2025, a standard 10.3 oz cartridge of 732 is around $8-15.