This article was first published in 2005.
Whether you’re playing with a turbo boost control system, an intercooler
water spray system, a fuel system or a turbo, there’s one aspect you’ll always
need to consider: plumbing. We’re not talking high-end, expensive stuff here –
braided line and special anodised fittings and silicone hose. Instead, in this
series we’ll cover the sort of cheap bits and pieces you can buy from industrial
and truck suppliers and put together to make all kinds of systems – from those
that control boost to those that supply fuel.
This week we’ll cover small diameter hose and fittings and next week we’ll
take a look at the different valves you can use with this type of plumbing. In
the third and final part, we’ll examine large diameter plumbing.
In most automotive applications, the rubber hose designed for EFI systems is
a good general purpose hose. That is, it can be used in fuel systems, boost
control systems and water sprays. It’s not as cheap in the latter systems as
thin-wall “vacuum” tube (or vinyl tube), but it’s also much more durable in the
hot environment of an engine bay. Fuel hose also has great resistance to
puncturing and abrasion, so it’s good for applications where you don’t want
leaks, even if the hose inadvertently rubs against a metal component.
Fuel hose is available in a wide variety of imperial sizes, including ¼ inch,
5/16th inch, 3/8 inch, 5/8 inch and so on. (To get these dimensions
in mm, divide the top of the fraction by the bottom and then multiply by 25.4 –
ie 3/8 inch is 9.5mm.)
In some cases, the fittings on which you want to place the hose may be a
fraction too large for any of the available fuel hose sizes. If that’s the case,
use a heat-gun to warm the end of the hose until it is slightly softened and
then push it onto the fitting.
The most widely used clamps are worm-drive – those that everyone knows as
normal hose clamps. When buying worm-drive clamps, make sure you’re getting
stainless steel clamps – some are just plated mild steel and they’ll corrode.
Worm-drive clamps also vary a lot in build quality – a brand name clamp will
almost always be better than a cheapy no-brander. Buying clamps individually can
get quite expensive – better to see if a bulk lot of clamps is available rather
than going down to the local petrol station and buying them one at a time as the
Many OE manufacturers use spring clamps instead of worm drive clamps. In the
applications in which manufacturers are using them (constant hose diameter,
constant fitting diameter) they work very well, even on high pressure fuel
systems. However, if you change hose brand, or use a fitting with a different
diameter barb, this type of clamp can become unreliable. You may find that to
get it on the hose, the clamp needs to be stretched beyond its normal elastic
limit, or conversely, that when placed on the hose its clamping force is
insufficient. Always keep these clamps as you come across them, but be wary of
using them unless their fit is perfect.
You’ll find that on very small diameter plumbing there are no worm-drive
clamps available that are small enough to fit. In these situations there are
three alternatives – use the abovementioned spring clamps, use cable ties, or
use super glue. What?! One example where super glue (ie cyanoacrylate) works well is where a car has been
converted to forced aspiration. The
very small diameter hoses (say with an internal diameter of 4mm) that previously
carried vacuum (so pushing the hose more firmly on their fittings), now carry
boost. You don’t want them to blow off so you need a way of positively holding
them in place. However, because the hose diameter is so small, the magnitude of
the forces that are developed is also small – superglue or small cable ties work
fine in these applications. Gluing the hose on will obviously make it rather
hard to remove, so don’t do this if you expect to be frequently taking the hose
Some manufacturers are now using hoses in their fuel systems that don’t have
clamps. Instead, the fuel hose consists of an inner hard plastic tube and an
outer protective rubber covering. The inner plastic tube appears to be factory
heated before being pushed over the fitting where it hardens to form a permanent
assembly. To replace the hose, use a sharp knife to carefully cut along the hard
plastic tube, allowing it to be pulled off. Normal fuel hose and a worm drive
clamp can then be used as a replacement.
It’s important to realise that this type of OE hose can be kinked badly
without this being apparent – the inner hard plastic hose kinks but the outer
rubber covering still looks fine. You’ve been warned!
Using Worm Drive Clamps
Nothing could be simpler than using a worm-drive clamp, huh? Well, not really
– there are plenty of ways of stuffing it up. Firstly, don’t use a screwdriver.
Instead, use a socket and an extension bar. (Don’t have any small diameter
sockets? Go buy them – even just a cheap set will last a long time as they’ll be
used only rarely.) Using a socket drive allows you to determine the torque that
you’re applying far better than when using a screwdriver, and the socket is much
less likely to slip off and damage the hose – something that’s dead-easy to do
with a screwdriver.
Secondly, make sure that the clamp is not positioned right at the end of the
hose – instead, it should be located back from the hose end by about one-quarter
of the width of the clamp.
Thirdly, use a penetrating spray lubricant (RP7, WD40, etc) on the worm and
thread before fitting the clamp. This reduces the chance of stripping the
thread, and – as with the use of the socket – allows a much better feel for how
much torque you’re applying.
Finally, don’t over-tighten the clamp! If the clamp is well fitted, and the
hose size is well matched to the fitting, the clamp shouldn’t need to be
ultra-tight. (Remember the OE factory spring clips used on EFI fuel hoses
mentioned above – they’re often not very tight at all yet they seldom leak.)
Even if it’s not very tight, the clamp won’t come loose as the springiness of
the rubber hose will hold it in tension. If you start seeing the hose bulge out
from each side of the clamp, chances are that you’re over-tightening it.
If the plumbing run that you’re working on needs to split in two, you’ll need
a T-piece. T-pieces can be found in boost control systems; fuel supply systems
running multiple pumps, pressure regs or fuel rails; and intercooler sprays with
The best T-pieces are formed from a single piece of brass. These are
relatively rare and can be differentiated from brass T-pieces made from multiple
pieces by the absence of silver solder joins. A one-piece brass T-piece is
nearly indestructible! At the other end of the spectrum are plastic T-pieces.
Even those made for automotive fuel use (eg of nylon) can easily fail,
especially if you need to remove and replace the hose a few times.
T-pieces are made in as wide a variety of sizes as hoses, although it’s
harder to get brass T-pieces for very small and very large diameter hoses.
The flow behaviour through a T-piece is dependent on its orientation. If the
flow is up the vertical arm of the T, equal flow will be available at each of
the upper outlets. But if the flow is enters through one of the upper arms, much
more flow will go through the other upper arm than out through the vertical part
of the T. (The actual flow split also depends a lot on the rest of the system,
so this rule of thumb doesn’t always hold true. But it usually does.)
This asymmetric flow behaviour can be very useful. For example, the on-road
behaviour of a boost control system can be quite dramatically changed by the
orientation of the T-pieces in the system. Together with variable nozzle sizes,
it can also be used to direct more water through one intercooler spray nozzle
In short, keep in mind the orientation of the T-pieces when installing them,
and if it could be advantageous, try them in different directions.
The T-pieces described above uses three barbed fittings onto which the hoses
are pushed. ‘Barbed’ simply describes the bumpy end of the fitting that’s
designed to make it harder for the hose to slide off. However, pretty well all
ball valves, needle valves, one-way valves, pressure regulators, pressure relief
valves (and so on!) don’t have barbed fittings. Instead, they use female
threaded fittings. So how do you attach a hose to these valves? The answer is to
use barbed fittings called hose tails.
Barbed hose tails have the hose barb at one end and a male thread at the
other. These threads are normally either BSP or NPT in type. In addition to the
type of thread, the internal diameter of the fitting varies. So for example, if
you ask for a “¼ inch BSP barbed hose tail” you’ll get one to suit ¼ inch hose.
But what about the diameter of the threaded bit? Well, that can vary!
Long experience and many return trips to industrial plumbing suppliers has
taught me that you should always either buy the valve and barbed fitting
together (that way, you know the thread type and diameter are right) or you
should physically take along the valve when buying the hose barbs.
Like T-pieces, barbed hose tails are available in brass or plastic. For much
the same reasons as already mentioned, brass is preferable and since the
construction of a hose tail is much simpler than a T-piece, you don’t need to
look so hard at quality – any brass one that fits will do.
Some valves don’t require male fittings – instead they need female. This is
less common but it certainly can occur so until you physically sight the valve,
never presuppose that just a male barbed hose fitting will be needed.
Most threaded fittings use a tapered thread design. That is, as you screw it
in, it gets tighter and tighter. This is important to know as beginners have
been known to get out a big spanner and just keep on tightening until the thread
bottoms out – or the fitting fractures!
Sealing of the thread can be done with Teflon thread tape, cheaply available
from any hardware store. Wind the tape on the male threaded fitting in the
opposite direction to that which you’ll be screwing it – this prevents the
thread tape being nicely unwound as you screw the fitting into place. Apply only
two or three layers of the tape - you don’t need to wind on a heap. In fact,
putting on too much thread tape is a big negative as the chances of the tape
getting into the flow path is greater. For the same reason, make sure the
wound-on tape doesn’t cover the internal opening of the fitting. (Because of the
chances of it getting into the system, some authorities recommend that you don’t
use thread tape at all. However, if you’re careful in the application, this
With thread tape applied, a tapered thread fitting should be tightened to
only a snug level – not super tight. Brass is much weaker than steel so it’s not
hard to shear off a threaded fitting, especially if you hold the valve in a vice
and use a large spanner.
The difference between a plumbing system that leaks, gets holes abraded in
hoses, fails in service because a hose pops off, or has cracked fittings, is all
in the parts selection and the workmanship. It costs no more and takes no more
time to get it right!
Next week – all types of valves