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Thread: M725 fuel filler neck

  1. #1
    Join Date
    Aug 2015
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    Portlandia, Oregon USA
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    202

    Default M725 rubber fuel filler neck, custom replacement

    My M725 filler rubber elbow had actual cracks/holes in it that could have eventually caused a fire if I ignored it. I gave up on trying to find a durable replacement rubber fuel filler elbow for my Ambulance and decided to make one myself for around $100.

    I researched options for petroleum friendly rubber tubing and 16 ga stainless steel tubing so this part will be something I’ll never have to mess with again.

    If you want to build one of these, you’ll need to be able to weld and cut stainless and mild steel. The finished product still looks stock from everywhere except under the vehicle where the new stainless steel filler elbow can be seen- but it looks pretty awesome from down there too, just not stock.
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    Last edited by brycer1968; May 21st, 2018 at 03:45 AM.
    1969 M725 ambulance

  2. #2
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    Aug 2015
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    Portlandia, Oregon USA
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    Default

    I thought I'd go through the build process for this upgrade with pics along the way.


    But first, some stuff I learned on here and read on-line about the M725 fuel filler neck:

     The Ambulance rubber filler was never reproduced and only one source in Greece exists that still sells what appears to be an NOS one. There’s lots of opinions on here about how long these factory 50 year old NOS rubber filler necks last when they are placed in contact with modern fuel.

     The metal M725 fuel filler neck frame is not the same as the M715 filler neck. Its design is different.

     The ambulance filler neck rubber tube has a 45 degree bend while the M715 has more of a 90 degree bend

     For all those reasons, the M715 NOS and reproduction necks don’t fit the ambulance at all. I wouldn’t even try to force one to fit.

     The factory gas tank inlet measures 3.00” OD and the factory metal fuel filler neck metal frame measures 4.25” OD.

     Very few composite rubber tube products are both fuel resistant and come in the sizes we need (but I dug into it and found some).


    Materials and costs:

     Ebay, $25 delivered: 16 ga 3” OD mandrel bent 45 degree 304 stainless steel exhaust pipe. Can be either brushed or polished finish and you’ll end up cutting the legs on each end of the bend to just a few inches long

     Ebay, $27 delivered: Climax metal products C-300 one piece shaft collar with a single set screw. These come in stainless and lightly galvanized mild steel versions. I bought the mild steel type because I wasn’t willing to pay for stainless.

     Pacific Rubber, Eugene, OR: $25/foot and for a one foot section of 4” Novaflex 841 and $20/foot for a one foot section of 3” Novaflex 841. This is a marine exhaust tube that is rated for fuel filler use. This is a multi-layer product with a bonded inner tube that is rated for contact with fuel and fuel vapor, polyester plies in the center and a bonded outer neoprene/fabric cover for durability and abrasion resistance. Please note that for this product to last for years, the inner tube surface needs to be made of nitrile (or Viton is even better) to resist gasoline and diesel and fuel vapor (Novaflex 841, Titan SS269 or Gates 4169A all have nitrile inner tubes, but can be challenging to find in stock locally) These are all soft wall tubes without the wire reinforcement. The lack of wire reinforcement allows flexibility/movement between the gas tank and the fuel filler similar to what the factory rubber elbow provided when it was new. I believe the wire will also keep the 4” tube from stretching slightly to fit on the 4.25” filler neck. I found out the hard way that 4.50” exhaust tubing is too big and won’t clamp down and seal around the 4.25” metal filler neck frame. The 4” soft wall rubber tubing is the way to go.

     Associated Hose Products in Portland: $20 for 4 ea T-bolt style heavy duty hose clamps to fit both sizes of marine rubber exhaust tube. Since the clamping ranges of this style of clamp is very limited, I found that I needed three different sizes to complete the job.

     Ebay, $8 delivered: A new fuel filler tube housing rubber seal from an M38 that I found on ebay
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    Last edited by brycer1968; June 4th, 2018 at 11:19 AM.
    1969 M725 ambulance

  3. #3
    Join Date
    Aug 2015
    Location
    Portlandia, Oregon USA
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    202

    Default

    Removal and fabrication:

     Remove the sheet metal M725 fuel filler shroud in the patient compartment under the bench seat. Then remove the metal filler neck vent tube and the old rubber 45 degree filler rubber elbow (I cut mine out in pieces). I found that removing the outside filler housing sheet metal (seven screws) was required to facilitate removal and replacement of the filler neck.

     I took this opportunity to clean 50 years of dirt off of the metal filler neck frame inside and out and also the removable filler extension tube. Both of these parts on mine had deposits of stuff on it that looked like burning man dust??

     Reinstall the now naked metal filler neck frame with the factory hose clamp that attaches it to the metal spot-welded bracket sticking up at the bottom of the fuel filler opening in the side of the ambulance. This will hold the filler neck frame in its installed place so you can mock-up and trim your 45 degree stainless tube. I did this with the metal collar installed on the stainless tube using its large set-screw to hold it in place and that let me hold the tube in place by hand and mark it/trimming off the extra length. (see second picture below). Go slow cutting off what you don't need so both ends of the 45 degree tube meet the bottom of the filler neck frame and tank neck squarely. Then assemble and test fit the tube one last time to make sure it fits.

     Mark the shaft collar location on the stainless tube, then remove the collar and cut the tube off flush with the top of the collar.

     Prep the heavy collar for welding by wire-wheeling off any coatings (zinc/galvanized) where you will be TIG welding it. I used 304 stainless filler rod and welded it in the two places shown. Last, you will need to weld up the set-screw hole and dynafile (or roloc in a die grinder) that weld bead so its smooth and flush with the metal around it to allow a good seal with the 4” hose.

     I painted the outer welded section of the custom filler neck tube with OG green spray paint to keep the welded area from rusting

    I'll note that there is some adjustment allowed in the installation of the M715/725 fuel tank, both side to side and front to back and since my tank was out for EFI return line modification, I located the tank more towards the drivers frame rail to keep well away from the now relocated rear drive shaft and centered front to back. You may want to verify/fine-tune the tank location before you begin fabrication of your filler neck. Though I don't have pics of this, I also took the opportunity to insulate the four welded-on steel "tank landing pads" under the ambulance box with four bonded thin pieces of black viton rubber that keep there from being fuel tank to body contact once the tank straps are tightened down.
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    Last edited by brycer1968; June 4th, 2018 at 12:31 PM.
    1969 M725 ambulance

  4. #4
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    Aug 2015
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    Portlandia, Oregon USA
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    Default

    more pics of the build
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    Last edited by brycer1968; May 21st, 2018 at 03:04 AM.
    1969 M725 ambulance

  5. #5
    Join Date
    Aug 2015
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    Portlandia, Oregon USA
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    Default

    You are ready to assemble the new filler neck, starting on your work bench:

     Cut an 8” long piece of the 4” tube. I know 8” is too long, but don’t cut the tube to length right now because it will lose up to 1/4” of length as it stretches onto the filler neck frame- really. Since the filler neck frame measures 4.25” OD, you MUST warm the 4” marine exhaust tube in boiling water for several minutes first, also soap up the metal filler neck frame and then start wrestling the rubber tube onto the metal filler neck frame until the vent tube stops it from going any further. This wrestling match was the worst part of the whole filler neck project. I started with a rubber mallet hitting the rubber tube end then moved onto a 2.5lb hammer and a board across the rubber end of the tube. A hydraulic press might also work. I used a blue plastic body panel tool (see second picture below) to help ease the cut edges onto the filler neck frame to get it started. You have to hit the block of wood surprisingly HARD to drive the rubber tubing all the way onto the filler neck frame. It will go. Also, make sure to protect your gas cap sealing surface with scrap wood or rags to keep it from getting damaged by the workbench while being pounded. Keep soaping and dipping it in hot water too as you go. One other note: Once the tube is on the filler neck frame about a couple inches or so, it’s not coming off and the only way to remove it is by cutting it off with a razor knife.

     Install the two larger T-bolt clamps- Since t-bolt hose clamps have relatively small ranges of adjustment, the filler neck installation required three different sizes for the upper and lower clamps.

     Cut a 3-4” long piece of the 3” rubber tube that connects to the fuel tank inlet and install the T-bolts loosely on the tube.

     I was a little bummed to find that the assembled filler neck with new 4” tube installed was now too wide to easily install into the oval sheet metal hole in the ambulance floor, so I removed the factory rubber seal and marked the sheet metal around the oval hole to remove a small amount of material from the inside of this hole. Most of the material should be removed from the driver and passenger sides of the hole. I used an electric dynafile and took my time. See measurement pics before and after to know about how much to remove. When you are done reinstall the factory oval hole gasket. Note that there is now about an inch of gap at the factory rubber seal ends that I ended up sealing that area with black RTV after the install was completed.

     Test fit the filler neck again to see if it will lay against the spot welded filler neck bracket without distorting or pulling the ambulance floor up more than a few mm. Adjust the hole with the dynafile further if it is still too tight.

     Now that the neck is in its installed position, test fit the stainless elbow/collar and see how much of the 4” rubber tube you need to trim off the lower end. I needed to leave a little over 1-1/8” below the metal filler neck frame. Make that cut on the work bench with a sharp knife and then reinstall everything. I located my t-bolt clamps with the t-bolts at the 12 o-clock locations to reduce the chance of fuel leaks.

     Reinstall the original vent tube breather tube.

     Install a new M38 fuel filler neck to housing seal and reinstall the sheet metal fuel filler housing onto the M725 body. I know someone on here can tell us what type of body seam sealer is needed between the exterior fuel housing sheet metal and the side of the ambulance.

     Tighten all your t-bolt connections, gas er’up and check all your connections for leaks.

     I also installed a locking gas cap made for an M35A1 that “those Military Guys” are selling on line. Its un-vented like our factory cap is and even has a hole to attach the cap retention chain as well. I works great and gives peace of mind here in the inner city where idiots sometimes put things in gas tanks for fun.
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    Last edited by brycer1968; June 4th, 2018 at 12:13 PM.
    1969 M725 ambulance

  6. #6
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    Aug 2015
    Location
    Portlandia, Oregon USA
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    202

    Default oval filler neck hole dimensions before

    oval filler neck hole dimensions before adjustment with the dynafile:
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    1969 M725 ambulance

  7. #7
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    Aug 2015
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    Portlandia, Oregon USA
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    Default And after adjustment with the dynafile:

    And that same opening after adjustment with the dynafile.

    Like many of you, I don't like making changes to the ambulance like this because it its something I can't un-do and I like to leave things factory-stock when I can. That said, I committed to making the adjustment and I was surprised how much material I had to remove after I reviewed the before and after measurement pics.

    I must have removed and reinstalled the rubber gasket and test fit the filler neck in this hole about 15 times as I marked and filed the opening bigger until the neck filled the hole without pushing the floor up and the metal filler neck frame sat down on the spot welded bracket.
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    Last edited by brycer1968; June 4th, 2018 at 12:32 PM.
    1969 M725 ambulance

  8. #8
    Join Date
    Aug 2015
    Location
    Portlandia, Oregon USA
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    202

    Default More pics of install

    More pics of the install.
    You might notice some tank modifications- I've prepped my stock 715/725 tank for a return line and routed the tank vent tube into a 5/16" steel line that runs to a charcoal vapor canister- Both changes are part of my prep for EFI conversion down the road somewhat.
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    Last edited by brycer1968; June 4th, 2018 at 12:08 PM.
    1969 M725 ambulance

  9. #9
    Join Date
    Aug 2015
    Location
    Portlandia, Oregon USA
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    202

    Default I do have some left over materials

    Thats it for my journey into making my own M725 filler neck. Happy to answer questions or post other pics.

    I’ll Note that I did have some left over materials . . .enough to let me make up two other identical M725 filler necks - if anyone else wants to replace their M725 filler neck without having to do any stainless welding/fabrication. See the last pics if you are interested.

    I’m selling just the two pieces of rubber tubing and a completed custom welded 45-degree stainless tube/collar that I welded up at the same time I built mine. You would need to get your own t-bolt clamps or regular hose clamps to complete the job.

    I think $120 shipped in the USA is a fair price for one of these kits and covers the material costs and a little of the time I have into them. Let me know if you want one.

    Cheers, Bryce
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    Last edited by brycer1968; May 21st, 2018 at 03:26 AM.
    1969 M725 ambulance

  10. #10

    Default

    Nice!

    $120 is perfectly reasonable, IMHO, given VPW wants the same for the new fuel filler hoses they offer.

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