New fuel line cracking

Help Support

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-known member
Sep 19, 2020
Reaction score
Smithville, Texas
I replaced the fuel line less than a year ago with the A1-15 hose. On my jon boat with portable tank. Even covered the hose with gorilla sleeve for extra protection. The other day I noticed the fuel line is cracking/splitting at some of the fittings. Not past the clamps though. It’s 5/16” hose. The fittings are 8mm I think which is supposed to be compatible. I know 5/16” is a little bit under 8mm. I think 7.8. Anyone else have this issue? Should I just cut the ends off and redo the fittings?
I don't know if 5/16 hose is compatible with 6mm fittings or not. I bought flexible rubber 6mm hose at my local racing shop. I also sleeved it, not so much for protection from damage but for protection from the sun.
5/16 is 8mm for all intents and purposes. The only difference between an 8mm and a 5/16 socket is the engraving, it's one of the few conversions where the small difference is within tolerance of most things.

Go to the local farm supply and just get some plain old 5/16 fuel hose. The sun will rot it in a couple years, but at least it's cheap, and it doesn't have that inner plastic lining that will disintegrate and clog up the whole fuel system like the marine style lines.

The cracking that you're seeing is likely a combination of flexing and UV damage. Not much you can do about it.
I could be wrong, but I don't think A1-15 is a legal requirement for outboard where hose is out in the open.
Todd hit the nail on the head.
There are already a few threads in here covering the low cost hoses that are out there. The silver colored hose was pure junk for example.
Go back and do a search and take a look at photos and some of the posts.
Primer bulbs are in the same category. An OMC system matched bulb or a Yamaha will do the job for years. The object when you buy things that can leave you stranded out there is to buy quality one time and not look back.
Pappy I thought I was going all out. This is the black hose A-1 15 that’s for below and above deck. $3.50 per foot . Yamaha fuel water separator, and Yamaha primer bulb. The bulb started leaking the other day and when I used my back up is when I noticed the line cracked at the bulb on both sides. Then I found a couple more fittings the same way. Just on the very end of the hose and not past or into the clamped area.
They’re sending me a replacement bulb since it was less than a year old. It started leaking at the seal where the valve is I guess. I’m wondering if it was the way the hose hung. Should I use a longer hose between the separator and the motor so I lay the bulb down on the deck?
Well you may have stumbled on to something. Let's see if a trend develops with this.
As far as special routing goes just use your common sense in routing with no tight bends.
One question. What type of clamp were you using? The metal screw type clamp can be tightened to the point of compromising the hose. Again common sense prevails when using these.
I will not use them anymore at
Pappy said:
Well you may have stumbled on to something. Let's see if a trend develops with this.
As far as special routing goes just use your common sense in routing with no tight bends.
One question. What type of clamp were you using? The metal screw type clamp can be tightened to the point of compromising the hose. Again common sense prevails when using these.
I will not use them anymore at

They are the metal gear clamps and I noticed yesterday it looks like they could be over tightened. The fitting to the tank is fine. Doesn’t look over tightened and no cracked end.
What do you use for clamps now?
There’s no sharp curves but the hose seems heavy to me to the point the weight of it is pulling down on the motors fitting some. I’m afraid I’ll have a leak there next. I leave it connected all the time but I’m fixing to make a practice of disconnecting it and running the motor out of fuel at the ramp. Seems like when the sun hits the water separator filter it gets hot and the pressure pushes the fuel in it to the carburetors while the boat is sitting in the driveway. Which is fixing to change to being kept in my shop after a tree fell on it Sunday and cracked the transom. Let me know about those clamps sir.
Thanks, Pappy


  • D71A2C23-7DFA-4493-A2F3-CFD8CF05A60C.jpeg
    1.9 MB · Views: 494
Fuel expands EVERYWHERE when in higher temps. I'd disconnect the motor-side and would run it for a few minutes, but wouldn't necessarily run it out until empty ...
I have not used a worm gear clamp in years. I use these instead.
Having said that I will tell you that you must do the research into the proper size for your application. Not all 5/16" or 7mm, 8mm hoses have the same outer diameter for example and the "O" clamps have many different sizes to help get it right. You can overtighten these as well so again....common sense. There are assortments available however and you would do well to purchase a proper tool as well.
There are areas where I use a proper tie wrap designed for fuel lines that have the radius, such as these.
Dale H also hit on something that we touched on a long time ago and probably needs to be repeated as well.
Today's fuel tanks will not vent to the atmosphere. Even the old two line pressure tanks had a pressure relief valve in them to help protect the tank and fuel system. Since they do not have any way to vent it is wise to say the least to disconnect your fuel line from the engine specially if the boat/engine combination is left in a home garage! Why? A hot garage can and will pressurize a gas tank. If the fuel line is still hooked up and there is any kind of issue with the needle and seat in a carbureted engine you will get gasoline free-flowing into the hull and into the closed atmosphere of a garage. Think about the possibilities!
I forgot...........
After looking at your photo I see two things you can improve on.
First is adding a 90 to the line coming out of the filter. Point it down and your bend will simply go away.
Second. Move your primer bulb to where the outlet is pointed "UP". (Closer to the engine) Why? Because there are check valves in the primer that open and close and by simply pointing the bulb up these will pretty much automatically drop and close as you pump the bulb.
Thanks again Pappy. There’s several options as far as clamps on the site you gave me but I get the idea. I have the zip ties for fuel line under the cowling and they work great. I wondered if larger ones would work on the main line. Yamaha sent me a replacement bulb. I got the 90 degree fitting coming and will relocate the bulb closer to the motor ( you read my mind on that one). I have always held the arrow up when priming but it’ll be handy having it stay that way. Boats at the shop right now getting the transom welded from the tree that fell on it. I just need to get some more fuel line. It’s hard to find just a few feet of the marine line. I don’t have a boat shop near by to go to.
I’m familiar with the new tanks and the pressure. I have a new one sitting in my shop. I have two of the old style tanks that vent from the cap still in great shape. Solved the problems I was having when I switched back to them. I didn’t have a fuel demand valve and the new tank was shoving the fuel up stream. After reading about the demand valves failing a lot I went to the old tank I had. On the line, the A1-15 I have is overkill. Any suggestions for the small section I’m replacing? I’m getting a lot of suggestions for automotive but I’m not thinking that’s a good idea out in the uv and less fire protection.
Lol I just realized after looking at my picture I can probably take the longer side off the motor and put it on the separator and the shorter side on the motor and have plenty of hose. Duh
I’m back up and running. Got the new welds all primed and painted. Redid the fuel line from the fuel water separator to the motor. Put the 90 degree fitting in and the new Yamaha primer bulb is in a upright position closer to the motor.
Took the wife to the lake today and had a good day. I only had to prime it on the first start up. We’d fish an hour or so and I’d check the bulb before starting and it stayed primed all day. Not one squeeze needed.
Thanks for the advice and sharing the knowledge.

Latest posts