The Hose Knows

Working on a hybrid of 20 year old Japanese Subaru technology and 40 year old German Volkswagen technology in a metric hostile country can be maddening. Most of the time, you can get the parts that you need from a dealership...for the Subaru anyway, if you know exactly what part number you need. For doing an adaptation, you're going to need a specification list to know what sizes and types of hose you're going to need to buy.

Ordering by part number will improve your chances of it fitting a stock installation, but there won't be any information about the physical properties: Length, Inside Diameter, Outside Diameter, and material specifications (fuel, oil, EVAP vapors, coolant, etc.) If the part happens to be a hose and is No Longer Available (NLA), getting accurate measurements from one which has been in service for 20 years or more is often fruitless: Rubber shrinks as the volatiles in it leech out, so you can't be fully sure of the size of the hose you need. This is where you can really run the price of a conversion through the roof: having to buy one of everything, in every size, because you don't know what you need.

So even if you hit the jackpot and a molded hose is still available, that won't help you if your adaptation has rearranged the orientation of components the hose was meant to connect to. Then you must revert to using generic hose and determining its Inside Diameter by measuring the Outside Diameter of the fittings with a caliper. Once you've got a Metric measurement, convert to U.S. Customary (aka "Standard"), because for generic hose types, fractional inches are all your going to be able to find here in the Land of the Free and the Home of the Pig-Headed.

Units of Measurement in the USA don't have a proper acronym. No, not SAE (Society of Automotive Engineers) or even a simple 'Imperial.' The UK (from which 'USA Standard' is derived') standardized their weights & measures in 1824, and metrication begun in 1965 is still ongoing today. The United States is still doggedly and self-importantly stumping along on a system from the Middle Ages: A mongrelization of German tribal units and Roman units enforced on the English population by William the Conqueror after the Norman Conquest in 1066. 

My Country Tis' of... Nevermind.

This is what "traditional" hose for a vintage VW looks like after a year
of pumping 10% Ethanol. This is a wonderful way  to burn your Bus.

I'm going to toggle to fuel hose quality for a minute here, discussing both available sizes and fuel permeability (how much the hose 'sweats' fuel volatiles right through its skin which leads to degrading hose.)

If you're trying to use modern hose material available in the USA for your vintage VW, you'll probably run into the old school nuts who insist VW expects you to use the 'right' hose: In a Fuel Injected Bus you should be running the 7mm hose, since even the fuel hose is 'engineered' to stretch just the right amount to make a good seal around a slightly (.09mm!) larger fitting.

Let me just say this once: No vendor makes modern Fuel Injection safe, ethanol compatible fuel hose in 7mm. You can find them in the right size from overseas sources, but those hoses are not Fuel Injection pressure rated and allow gasoline in them to vent right through the hose walls about 33x more than modern hose.

How ironic that the more militant a vintage enthusiast is about the look and exacting size of a hose, the more likely they are to burn down their own Bus from using poor materials. (Passions run high on the subject of fuel hose.)

The sizing issue is partial hogwash. Yes, if you aren't careful, you can do something stupid and ruin your car. Put hose that is oversized for the fitting (because you are too careless to convert Metric to USA Standard) and the most monstrous hose clamp in the world won't keep it from leaking. Now add a third variable (Subaru fittings) and finding the right hoses for your adaptation can get complicated fast. I do pity the folks who need to use a 5.5mm hose: There is literally nothing on the market for them.

VW & Subaru prefer different standard metric sizes (VW: 3mm, 5mm, 5.5mm 7mm, 11mm // Subaru: Anything BUT the VW metric sizes.) What might be made to fit on the VW side still fits sloppy on the Subaru side. Or vice-versa. Regardless, you still have to go shopping in Fractional USA Standard measurements.

Depending on what you're putting through a hose, you can sometimes outwit this Machiavellian conundrum. I did one today: The brake booster. The Subaru side is 10.12mm at the fitting on the intake manifold. The VW side is 12.33mm on the fitting at the body that runs to the front of the vehicle and the vacuum booster. The fittings are on opposite sides of the vehicle.

Nota Bene: Hose, no matter what is transferred through it, should be chosen by specification for the job. Find out what you need and buy it. Don't use a 1/2" ID chunk of garden hose on your car because 'its what I had lying around the garage.' This line of thinking leads to disaster.

Yup. Maddening. Here's how I outwitted it:

The closest that I could get on the VW size to that 12.33mm is to convert to USA Standard, and then choose the next smaller size. In the case of the vacuum hose, forums like thesamba.com have been singularly unhelpful. No one seems to have an actual source for the power brake vacuum hose, or truly know what the material requirements are. I have had it recommended to me to re-use the 40 year old hose "because we know that works." Which is great...if you're doing a 40 point Concours Restoration. A brittle, 40 year old plastic hose of unknown material is not a smart bet for holding vacuum for another 10 years, let alone another 40. The old school mentality which used to decry form over function is now doing so in the name of 'we know it works.' This is a sign of dotage: All it really means is 'twenty years ago, we knew they still worked.'

So here's what I've learned: The properties of a hose designed for both vacuum and pressure are very similar: They tend to have thicker bodies, multiple layers, and spiral braiding. They also don't take well to tight radius turns due to their thick bodies. A thinner hose kinks; the thicker hose will just refuse to play.

So having measured your fittings, let's walk the math:

VW side fitting: 12.33mm x .03937 (converts to inches) = 0.4854321

Ugh. Decimal inches, the worst of both worlds.

Thank God for conversion tables. They include regularly spaced fractional equivalents, the only way most of the knuckle-draggers know how to sell things here in the U.S. of Pig-Headed. If I work my way down a chart, the first stop down below 0.4854321 which has a usable fractional equivalent is .46875, or according to my table: 15/32".

Mercy me! Gates just happens to make a 15/32" power brake vacuum hose. P/N GAT 27233

That does the VW side. Now I have to do the Subaru side of 10.12mm at the fitting.

Subaru fitting: 10.12mm x .03937 (converts to inches) = 0.3984244

Hmm. First stop down on the table yields the unlikely size of 25/64 (.39063"). Nobody sells anything in that size. But one very tiny stop down the table from that at .375" is dear old 3/8". In USA Standard, they sell everything from coffee to condoms in that size. But not power brake hose from Gates.

Still, finding heavily reinforced hose is not hard in a 3/8" size. You can get away with it in this case because the only thing the hose needs to contain is A-I-R. No concerns about material suitability for fuel, coolant, hydraulic fluid, etc. Yippy!

Nevertheless, you'll have to dig for it since your local franchise parts shops are unlikely to have what you need and because they want a make/model/year of your car, they have no way to look it up by specification, instead of by application. Requesting "Seven feet of 5/16" SAE30R14r2 fuel hose" is only going to earn you a dumb look. NAPA is often the only chain where you can order a part by its specifications, and it is still safer to walk in with a manufacturer name and part number than the raw specs. Local or online Speed Shops may be other places to buy by specification, even if you aren't interesting in building a racer: They're the only ones who still speak the language.

I'm just moving air: a brass adapter for the power brake
vacuum hoses isn't any problem at all.

In my case, I'm using PVC jacketed 3/8" ID reinforced rubber air hose, aka the stuff that's designed to take about 300PSI of pressure from your industrial air compressor. So I've got both the Subaru end licked, and the VW end. There's just one problem: The different size hoses still have to join up in the middle.

My solution was to use a brass fitting adapter: Male/Female threads in between with 3/8" male barb on one end and 1/2" male barb on the other. Screw those pair together with teflon tape and a dab of blue lock-tite, and you have an adapter for about 8 bucks. All told, I fabricated this unobtainium variable size hose built to take heavy vacuum and not fold up for about $24. I had about $80 of my TIME researching source material for it, which is frustrating. But my time is already spent and I've shared my results, so at least you shouldn't have to waste your time, too.

The adapter in place: 15/32" to the right of the adapter, 3/8" to the left back
to the intake manifold. NOTE: The 15/32" section is NOT bent: That is a
molded curve in that piece of hose. B/W picture because the PVC jacket on the 
air hose is exactly the color GREEN of a garden hose, and I don't want 
someone making a foolish assumption.

So what do I do about those little size mis-matches? The ones that the well meaning old school says will make my VW BURN if I try this with fuel hose? They object to a hose which isn't exactly undersized from the VW fitting than VW says it should be. Their assertion that stretching the wrong size rubber fuel hose will just split the rubber: leak, ignite, burn. This remains true now...if what you're using is just Continental Brand cloth weave covered rubber hose, just like grand-dad used on his VW.

Today we're using different types of hoses which have a fluoro-elastomer liner on the inside diameter. The rubber can stretch these kinds of hose over a slightly larger fitting without fear of them splitting and bleeding out all over the place: The layer that takes the brunt of the stretch is the extruded liner of the hose. Because of the separate layers, you're unlikely to split the way a single layer extruded rubber hose might. The PVC layer on the outside aren't in contact with the contents: they're just there for mechanical reinforcing. The interior liner takes all of the abuse from the E10 gas or whatever unsavory thing you pump down it. Even hoses which aren't transporting fuel are moving to this multi-layer design with an separate inside liner for the hose.

My stunt for making slightly undersized hoses go on the slightly too large fittings is simple: a deep travel cup of boiling water, and foaming anti-bacteria hand soap. No kidding.

This Performance Electric P37 fuel pump from Sweetwater, Texas is
a better stand-in for the stock Bosch pump: It permits the removal of
the pre-filter and allows me to run a stock Subaru filter in the engine
compartment. Both fitting accept 5/16" hose which fits the
Subaru engine fittings (8mm) nicely. Use VW #: 111-941-539 boots 
to cover the spade ends for the power.

Pop the end of the hose that you intend to attach to a fitting into the boiling water and let it set for about 10-15 minutes. The warm up time is to give the normally non-stretchy materials permissions to 'get loose.' You'll want to wear gloves for this, because that hose will be HOT when you pull it out.

When you're ready to pull out the hose, first use the hand soap to lube up the fitting. Then pull the hose out of the hot water and give it shake to remove excess water from the inside. Then add your hose clamp. Now push the hose over the fitting. Between the heat giving the materials a new-found willingness to stretch, and the lubricated fitting, your hose will likely go on with a firm, steady pressure. Now tighten down the hose clamp while the hose is still warm. Finally, if you need to make bends in the warm end of the hose to clear occlusions or to lay the hose into guides, do so now.

As the hose cools, if everything is kept under compression (like a bend or something) the hose will shrink down to its former size and stiffness, except now it will have conformed itself to the fitting and the turns you have forced into it. Remove it from its guides, it will want to go straight again. Left alone however, you'll probably never have to touch this fitting again. Even the water in the soap dries out, leaving a sticky residue that tends to seal the connection even better. It will make it quite difficult to get the hose off the fitting in the future, but unless you're replacing engines or components frequently, this is an acceptable trade-off for the daily driver.

This is not the guaranteed outcome of using non-metric hose on your Bus.

What are the products I've used for my adaptation? I've linked the specification list as a separate page which will grow as I finish the project. While it might be more expensive to use a product that is designed for more than the minimum specification to do the job, sometimes you can only find the size of hose in Fractional USA Standard that you need in a higher rated hose. You might use a fuel hose that is a better size match for an air hose which is not available in that size.

So look over the spec list! There are only two times a hose can be 'wrong' for a conversion: When it won't fit right (seal) and when it isn't made of the right stuff to do its job for 20 years. Make the effort for it to do both. Imagine the horror when you complete your conversion project....only to burn it down or wreck the engine because you used 'the wrong hose.' Look at the specs.