Hi All,
This is my first post on this website, and I wanted to start with some advice on a little bit of a creative project. I recently bought a manual 5-speed 2001 BMW 325iT and got to work on maintenance and repair. (I know, where are the pics? I only have them on my phone they'll be posted soon). I am preparing to rebuild the cooling system and had an idea about making some custom coolant hoses with high-pressure/temp clear hose on McMaster, new o-rings, and new plastic fittings.

I know that DIY coolant systems aren't usually a good idea, but l am stubborn and think the final product would look awesome. What would be the best way? Also, I have seen websites online that make custom clear hoses, but why would I pay someone to do something that I have no idea how to do?

First thing, I need the hose specifications — coolant max temp and max system pressure. I would want these hoses to be rated for ~200 degs C and I need some advice on max pressure. I know that many of the coolant caps are rated for around 2 bar (29psi). I could be wrong, as I am a young engineer, and this is my first project, but I can't imagine that fluid operating in a system at 95 degs C, would only reach 29 psi. What pressure does my cooling system operate at? What would be a safe rating for custom hoses?

Now, I also am unsure about the dimensions of the hoses. I read on one of these forums that its 1.5" ID, 1.75" OD, but I refuse to accept freedom units when working with a European car. Does 38mm ID and 44.5mm OD sound correct?

My plan is this:

• I buy hose from McMaster and purchase new OEM hoses from FCP Euro (gotta love the lifetime guarantee)
• From here, I just cut the clear hose to length and swap it onto the connectors

McMaster Hose PN: 9176T7
Another thought: Will Polycarbonate work? Maybe I could use some diary hose from an agricultural supplier (I read this is an option somewhere)

For anyone interested in this DIY, depending on whether or not this forum deems this project manageable, I will continue to post pictures and updates.

Thanks for any help y'all can give me!
Bubba

Bubba,

You're a young engineer, so you should know what the water P-T diagram looks like. You know the water/ice/vapor diagram with the triple point, etc. At 95 degC, which is less than the water boiling point at STP, pressure is less than 1 bar. Look it up .

The diameter of the hose is not all that critical as long as it's reasonably close to the diameter of the connector ends. It's not all that critical because the cooling system is not volume flow limited. Just get reasonably close and you will be fine.

If you cut the hose from FCPEuro, your warranty is void. If you replace the hose with something clear, you need to take into consideration the following:

1. The factory hose is reinforced to handle repeated expansion and contraction due to temp changes. Will your clear hose handle the mechanical stress over time?
2. The factory hose is shaped, how would you handle that with your clear hose? Or do you not care? Watch out for interference.
3. Do you have a way to crimp the clear hose onto the factory ends? Or would you simply use hose clamps? How can you be sure it would be leak free?
4. Polycarbonate is too hard. It won't expand and contract like the factory-reinforced hose. In addition, how would you bend it (see #2)? A straight, stiff, polycarbonate pipe/hose won't fit.

The hoses have to flex as when the system cools down, there's slight vacuum and something has to give. You want the hoses to give, rather than seals or o-rings.

 

Hi Kaboom,

Thanks for the info. You're right about the PT diagram. I read a forum on here a while ago that explains the process for changing the cap pressure relief to occur at 1.2 bar, so maybe I will buy a new cap and do that to the old car for when I test my hose. This is just so the cap fails instead of a possible hose or fitting bursting.

As for the hose, you're also right. PC is too hard, which is what I was worried about. I began looking at silicone rubber or high-temp dairy hose, and I think these might work a little better.

Here are some possibilities I found:
McMaster PN: 5157K51
McMaster PN: 51735K34
McMaster PN: 3184K17

I have two thoughts after this. These options are all semi-clear, which I can't help but think defeats the purpose of clear hose. It also seems that there is a limitation when it comes to materials technology. Maybe science just isn't at the point where we can get clear silicone rubber hoses that can withstand high pressure and temps (Or maybe I'm just not good at finding it).

Some McMaster Alternatives:
1.25" (1-1/4") ID, FDA, USP Class VI Platinum Silicone w/Polyester Braid (Food and Pharma-Grade)

1.5 inch I.D. Tigerflex Milk Hose / Tubing (per foot)

hambydairysupply.com

1.5 inch ID M34R Milk Transfer Tubing 60 foot

hambydairysupply.com

Though, I will need to reach out to these companies to confirm their specifications.

As for mounting the hoses, I would just use hose clamps. If this doesn't seem to work well, then I would need to develop an alternative.

 

You are placing a lot of your requirements on high temp and pressure but not enough on physical material properties. In reality, with a properly functioning cooling system, the coolant doesn’t get that hot in absolute terms. The pressure is also not high (see PT diagram). Lots of materials can handle that. What is less trivial are other mechanical properties. Off the top of my head:

If the hose gets hot, would it deform and lose its shape? Would it droop, for example?

Can you bend it and have it hold its shape? Remember it’s not a straight path from the thermostat to the upper radiator. It’s also not a straight path from the radiator bottom to the thermostat. If you bend it, would it kink?

Would the outside of the hose handle the harsh environment under the hood?

In summary, you need a clear hose (doesn't have to be silicone, but it's a good start) that is flexible enough to bend, yet won't lose its shape if it gets hot. It has to handle moderate temperature (200 degC) and pressure (2+ bar) and vacuum and retain its shape. It has to be "clampable" to the OEM hose ends without leaks - practically that means you need the proper size to prevent leaks. It also has to handle the environment under the hood. It needs to last heat/cool cycles for more than 60k (?) miles without hardening or degrading. While this is not impossible, it's not trivial. Good luck and let us know what you end up doing.

 

Do you need the whole hose to be clear? What if you cut out a 4" section in the middle of the upper rad hose, slipped in some metal tube inserts into the existing hose, put your clear hose in the middle and clamp it up with hose clamps? You could then maybe get a bit of a straight section and it wouldn't have to be super-flexible since you would still have some of the original hose for flex on both ends.