If you own a classic Porsche or are restoring one, you’ve likely come across part number 901-559-491-20. This specific component is essential for certain air-cooled models, and understanding its role, compatibility, and availability can save you time and money. In this guide, we’ll break down exactly what this part is, which vehicles use it, and how to ensure you get the correct version for your project.
What Is Porsche Part Number 901-559-491-20?
Porsche part number 901-559-491-20 refers to a rubber bellows or boot used in the heating system of early 911 models. It is a flexible connector that joins the heater box to the fresh air blower housing. Its primary function is to seal the ductwork and allow air to pass from the blower into the cabin while preventing leaks. Over time, these rubber parts can crack, harden, or collapse, leading to poor heater performance or unwanted air intrusion.
Physical Characteristics
- Material: Durable rubber (original was natural rubber compound, later replacements may use silicone or EPDM).
- Shape: Convoluted (accordion-like) to flex with movement of the engine and body.
- Dimensions: Approximately 4 inches in length, with a 2-inch inner diameter at each end (exact sizes vary slightly by production year).
- Color: Black.
Which Porsche Models Use Part 901-559-491-20?
This part was originally fitted to Porsche 911 models from the mid-1960s through the early 1970s. Specifically, it is used on:
- Porsche 911 (1965–1971) – all variants including 911, 911S, 911T, and 911E.
- Porsche 912 (1965–1969) – shares the same heating system as the early 911.
- Porsche 914 (1970–1976) – some early 914 models may use a similar boot, but verify compatibility.
The part number indicates it’s for the left-hand drive (LHD) versions. Right-hand drive (RHD) cars may require a different part number (e.g., 901-559-491-21).
Important Compatibility Notes
- Model years matter: In 1972, Porsche redesigned the heating system, so later 911s (1972–1973) use different bellows.
- Engine size: The part fits all engines in early 911s (2.0L, 2.2L, 2.4L) because the blower housing remained similar.
- AC vs. non-AC: Air-conditioned cars may have different duct routing; check your vehicle’s configuration.
How to Identify If You Need This Part
If you’re restoring an early 911 or 912 and notice one of the following, you likely need a 901-559-491-20:
- Heater blows weak or cold air – cracked boots allow outside air to bypass the heater core.
- Whistling noise from the dashboard – air leaks through deteriorated rubber.
- Engine bay heat entering the cabin – missing or torn boots let hot engine air mix with cabin air.
- Visual inspection shows cracks, tears, or missing boots – especially near the blower motor area under the dash or in the engine compartment.
To confirm, locate the heater blower assembly (usually behind the left side of the dashboard near the footwell). The rubber boot connects the blower outlet to the heater box. If it’s split, hardened, or absent, it’s the culprit.
Purchasing a Genuine vs. Aftermarket Part
When buying a replacement, you have two options: genuine Porsche (OE) or aftermarket (reproduction).
Genuine Porsche Part
- Part number: 901-559-491-20 (usually marked on the rubber).
- Quality: Exactly matches original specifications – correct rubber compound, fit, and durability.
- Price: Typically $40–$80 from Porsche dealers or specialty shops.
- Availability: Can be backordered; early 911 parts are increasingly scarce.
Aftermarket Options
- Brands: Pelican Parts, Stoddard, URO Tuning, and others manufacture reproductions.
- Quality: Varies – some are made from silicone (more heat resistant), others from rubber that may not last as long. Check reviews or ask fellow enthusiasts.
- Price: $15–$35.
- Fit: Most modern aftermarket boots are designed to fit the same dimensions, but some may be slightly stiffer or softer than original.
Recommendation: For a concours restoration, use genuine Porsche. For a driver car, a quality aftermarket boot works fine. Avoid no-name parts from random online listings as they may fail prematurely.
Installation Tips for Part 901-559-491-20
Replacing this boot is a straightforward DIY job, but access can be tight on early 911s. Here are some practical tips:
- Prepare the area: Remove the fresh air blower cover (typically held by screws or clips). You may need to disconnect the blower motor wires.
- Remove the old boot: It’s usually held by spring clamps or wire ties. Cut them off if rusted, and slide the boot off the plastic housings.
- Clean the flanges: Use a scraper or wire brush to remove old rubber residue. Wipe with a clean rag.
- Install the new boot: Slide one end onto the blower outlet, then the other onto the heater box inlet. Push firmly until seated.
- Secure with clamps: Use new spring clamps or high-quality stainless steel worm-drive clamps (avoid overtightening to prevent cracking the plastic).
- Test: Reconnect the blower and run the heater. Check for leaks by feeling around the boot edges.
Note: On some models, you may need to remove the glovebox or knee panel for better access. Consider replacing both boots (left and right) if they are in similar condition.
The Importance of Using the Correct Part
Using the right part number ensures proper sealing and longevity. An ill-fitting boot can cause air leaks, reduce heater efficiency, and lead to foggy windows or inconsistent cabin temperature. Moreover, incorrect boots may rub against moving parts (like the steering column) and cause noise or premature wear.
Where to Buy (Apart from Large Marketplaces)
You can source this part from:
- Your local Porsche dealer: Will have genuine part or can order it. Bring your VIN for exact match.
- Specialty Porsche restoration shops: Often stock NOS (New Old Stock) parts or offer high-quality aftermarket options.
- Porsche enthusiast forums and clubs: Classifieds sometimes list unused parts from restorations.
- Online parts retailers specializing in Porsche: Websites like Pelican Parts, ECS Tuning, or Stoddard (not named for endorsement, just examples).
Avoid generic auto parts stores – they rarely carry vintage Porsche heater components.
Final Recommendation
If you’re restoring an early 911 or 912 and need heater duct boots, part number 901-559-491-20 is the correct left-hand drive version. Order a genuine Porsche piece if originality matters; otherwise, a quality aftermarket boot will serve well for daily driving. Inspect the boots during any heater or blower service, and replace them in pairs for consistent performance. With a proper replacement, your classic Porsche’s heating system will function as designed, keeping you warm and comfortable on cool drives.
Remember: a small rubber boot can make a big difference in your car’s usability. Don’t overlook it during your restoration checklist.