gcthree

This is the front male nipple on the passenger side fuel rail of my '89 S4. Despite the threads looking absolutely perfect, the nut goes on with some resistance, and I need to chase these threads. I tried a light clean-up (there was no discernible issues with the thread) with a pointed file, but it is just marginally better.

Is this the same as an NPT pipe thread?

Thanks,
Bob

jcorenman

Not a pipe thread. The fuel rail fittings are all metric ball-flares, including the test-port at the end of the right-side rail (car's right side). What happens is that the attached fittings get over-tightened, which expands the end of the "female ball" into a "bell" shape, expanding the last thread or two and making it tight fit. You can cut back the threads with a file or metric die, but then you are making the wall thinner, and more likely to get expanded.

I am not sure what the best fix is (other than replacement), it needs a magic "ball-flare fitting compression tool", sort of like a crimping tool.

gcthree

Got it, Jim.

It is the only fitting on the car where I've had this issue, but your explanation makes sense. Perhaps just a light pass over the threads with a very mild file will do it. I can bring the fitting all the way home, but compared to other fittings on the car (which come home easily by hand) this one required a more 'help'.

Bob

dr bob

Mine is similarly 'stiff'. I discovered this when I pulled the factory cap to install a pressure test gauge there, and also use it for a fuel injector cleaning cannister connection. On reassembly, I quickly figured out that a smear of TFE thread sealing paste helps a lot, both on the threads as well as on that ball bearing and the two seating faces for it. It doesn't need to be tightened nearly as tight now. You can use that Loktite Anearobic Sealer 474 (IIRC), Hondabond, Dreibond, etc, as you would for other flared or ball-type fuel system connections. As tempting as it is to add just another little bit more oooomph to the wrench when trying to solve a leak at any fuel system connection, go easywith the wrench and let a little sealant do its magic for you.

James Bailey

I often ask this about many of the threads on this forum.... "What Type of Thread is This? "

dr bob

This is a fine, normally-straight thread, Jim.

James Bailey

Probably simpler than the tapered threads which ultimately go no where....

gcthree

Let me see if I've got this right: (I have some Hondabond). You should use Hondabond on the threads and the ball? Isn't there a chance that some of the sealant gets into the fuel system and clogs an injector?

17prospective buyer

They're precise mechanical fits. In a perfect world no sealant is required. They get scored up, or tightened down with stuff in there that embeds the globe seal.

Hilton

I'd be very careful messing with the thread - you really don't want to end up with it not sealing.

Out of interest, is the ball bearing still present which seals the end? The cap and its thread are only part of the solution.

dr bob

For my fuel pressure test gauge/fuel injector cleaner injector setup, a used cap is drilled and tapped 1/4-28 (fine thread) for a GM-style connection. Inside the cap, the ball bearing is replaced with a 1/4" brass tube compression ferrule.

Were I having a sealing problem not solvable with the recommended products, my next step might be to use a 1/4" brass ball, maybe one just a little bigger if available. The softer brass would more easily conform to damaged/distorted areas of the bell'd end of the fuel rail.

To the question of contamination of the fuel system, the sealant is applied very sparingly to the flared/bell end of the rail where the ball sits. Your pre-start leak-check procedure runs the fuel pump with a jumper in the fuel pump relay socket, pressurizing the system so you can check for leaks with everything cold. During that run period, fuel is circulated through the rails and back to the tank via the fuel pressure regulator. If you were a little too generous with the sealant, any squeeze-out will be flushed back to the tank where it will be diluted. If you choose the Loktite 474, the sealing protocol is to coat the socket with a very thin layer, then install and tighten the cap, leaving the system dry and unpressurized for 24 hrs to the sealant can set. Then the leak-check.

I use some handy Teflon pipe thread sealing paste for both the ball and the threads. The ball always seals easily (so far), and the lubrication in the threads makes removal next time a lot easier. Most folks it seems, when presented with the large hex of the nut, tighten it to wheel-lug torque. Instead, treat it the same way you'd treat a 6-8mm bolt into aluminum, with 7-10 lbs/ft when using the TFE paste as thread lubricant.