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Making a fuel rail - best Loctite and best materials?

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
I'm making some fuel rails which will hold about 58PSI of gasoline. In most cases, the fuel injector sits directly in the fuel rail, which is an aluminum extrusion. But, I need to add extensions to the fuel rail...basically the bridge the gap between the injector and the rail. the extension will be about 1.5" long and 3/4" diameter.

The extension will have a female port to mate with the male O-Ring on the injector body.

For various reasons, I don't want to use O-rings for the other end (where the extension meets the fuel rail. I want to use a straight, 5/8-18 threaded fit. I am cutting the female thread with a high quality tap then single point cutting the male threads in the lathe...the final fit is a very precise fit, tighter than a normal nut and bolt.

I want to use some form of Loctite to seal the threads and permanently lock the extensions place. In other words, I never want the ability to unthread the fit.

If I read the Loctite site, it seems like practically every product they sell will work - they all are said to both seal and lock threads, and some say permanently. And most have temp ratings which will cover anything the fuel rail will see.

So...my questions are....should I use Loctite or should I use JB Weld even? As above, I want this fit to be final and un-leaky.

Then....the fuel rail is aluminum, probably 6061. I can make the extensions out of the same material, or use stainless steel. I guess I'm wondering if aluminum will bond best with aluminum, or SS? Loctite's site gives no guidance on this.

The photo is just a test piece made of scrap but it shows the concept.
 

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  • fuel rail test.jpg
    fuel rail test.jpg
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If you just want to make sure they don't leak and never come off, auto parts stores carry fuel tank epoxy. It's two-part and looks just like JB weld when you're mixing and applying it. This is on the fuel rail that you're trying to get a seal, or the manifold / head?
 
It's the fuel rail to the injector extension that I'm wanting to permanently seal. Basically, the threaded fit shown above. I hadn't thought about tank repair, thanks. I wonder if it's rated for 250F or so?
 
The loctite will probably work but id probably just tig weld around it, will never unscrew or leak then. Could still thread it for initial alignment. Fuel leak fires are no fun at all.
 
First, I question your size you are choosing. It is way larger than needed in gasoline that I can imagine. The flow rate at 6 PSI will support in excess of 1,000 HP and you are using 58 PSI. Do the arithmetic. I'm sure you will not need more than .5 lb/HP hour. lastly, no special threads are needed. Use normal NPT with Loctite SI 5331, that is what it is designed for and is fuel proof. Give it 24 hours cure time before use. I have successfully used the stuff with NPT threads to 4500 PSI with no issues. I don't like to preach, but I see no reason to deviate from standard industry practice in engineering solutions. These standard practices have been in place through thousands of successful uses over many years. In your application you can an use NPS (gas fitting) thread with seal or the international standard BSPP with a seal or a BSPT which seals on the thread like NPT with a sealant.
 
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Tough to tig aluminum to stainless, tigging al to al will put a LOT of heat in there. Loctite product if anything. JB weld is a consumer-grade bodge IMO.
 
"but id probably just tig weld around it"

As noted, welding is out of the question. I can no longer TIG weld aluminum thanks to my eyesight, and I don't know anyone who can do a 'high end' job of it, and - most of all - I do not want the real estate used up by weld bead as I need the room for a mounting bracket between the two injectors. Every bit of it.

"First, I question your size you are choosing. It is way larger than needed in gasoline that I can imagine. The flow rate at 6 PSI will support in excess of 1,000 HP and you are using 58 PSI. Do the arithmetic. I'm sure you will not need more than .5 lb/HP hour."

The size is not needed for pure 'on paper' horsepower production. Rather, real world experience dictates there is good reason to use a large fuel rail and large diameters wherever possible. The actual rail I will use has a .969 ID, and after I get done making the extensions they will have an ID of right around 10MM. Why? The volume helps keep the fuel cool and most of all minimizes the effect of pulsations. Those who have looked into the matter (not me) report that the constant stop/start of the injectors creates a good deal of pulsation in the vessel (the fuel rail) which produces an uneven dispensing of fuel. A small diameter rail will flow enough fuel - on paper - but won't give as good a result in terms of actual running.

I know JB Weld is consumer grade but ... it does seem to work.
 
Use normal NPT
I thought of NPT at first but decided that it would not provide the exact depth of installation that I wanted from injector to injector. Plus, the size I would need to use is 3/8NPT and it created other space issues.
 








 
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