Fuel line and fittings assembly - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 07-20-2010
I33 I33 is offline
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Fuel line and fittings assembly

My new fuel system has many fittings between the tank and the lift pump--filters, valves, electric pump, etc. Most of these pipe fittings are 1/4-inch brass with standard pipe threads. In all my previous plumbing experience I always used Teflon tape on the threads to prevent leaks. Some are saying that Teflon tape is not appropriate in a fuel system--pipe dope is the preferred sealant. What is the collective wisdom of this forum? Thanks.
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Old 07-21-2010
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NEITHER PTFE tape nor PTFE 'dope' is appropriate for fuel system connections !!!!!

The common connection fittings on fuel system are either very inferior compression fittings or flared/double flared connectors whose mechanism of 'sealing' is by METAL TO METAL (ring-seal) CONTACT - the 'threads' of the connector only providing the necessary force to 'drive' the metal mating surfaces together and make a 'seal'. Such sealing methods are always DRY and one should NEVER use dope or tape to make them seal.

So, if a typical fuel system connector is leaking it means that the metal-to-metal seal in not 'integral' and using dope or tape will only cause the leaking fluid to be 'trapped' on the outside of the sealing surfaces.

Of the two, (and when used correctly in 'threaded' - NPT or BSB joins) PTFE tape is SUPERIOR because it will stay in place and wont generate 'particulate'. Dope isnt very strong because the 'carrier' which holds the PTFE particles together can easily 'wash away' leaving only the PTFE partices, which after the carrier migrates, so too in a short time the residual particles will also migrate.

BTW - the so-called compression fittings are the LEAST desireable for a fuel system; double flared is the choice for MANY reasons:
1. The compression fitting is considered to be a ONE-TIME-ONLY usage/tightening. If opened the tubing under the 'ferrule' should be 'trimmed back' and the ferrule should be reinstalled over 'fresh' or NON-COMPRESSED tube.
2. Compression fittings are subject to 'stress-relaxation' of the copper tube, thus leaving the connection very subject to leakage over time .... very important on 'VACUUM' motive systems.

Whenever possible one should choose 'double flared' connectors for 'surety' and 'safety'
Boat builders use compression fittings because they dont require 'special tools' - cheap
Copper tubing is REACTIVE to most diesel fuels - a 'prime' system should be with stainless steel tubing.

Last edited by RichH; 07-21-2010 at 10:03 AM.
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Old 07-23-2010
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Gil, I just did a complete overhaul of my mech system, from prop to the fan belt new. I was at the point where I was laying out my supply and return fittings and valves, in copper. I was advised by my engine supplier, also a certified mechanic, to use coast guard approved rubber hose only, to abandon the old copper system. I installed a new ballvalve at the tank and ran I believe 7/16" approved hose for supply and return lines. I clamped everything down and it was a good deal easier than the copper also. He said it was something to do with vibration and imbedded copper in the glass. Red
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