Changing the Evaporator Fittings from Flare to O-Rings

[thehackmechanic]

In this post:

http://www.bmw2002faq.com/component/option,com_forum/Itemid,0/page,viewtopic/t,376398/start,0/postdays,0/postorder,asc/highlight,/

I detailed what the original fittings are on a Behr a/c system, and how they change if you’re replacing the original compressor, drier, and condenser – which all have flare fittings – with modern versions that have o-ring fittings.

The piece that remains original is the evaporator. The original copper flare fittings are integral to the copper tubing that feeds the unit.

However, you can easily adapt them to o-ring fittings.

First, understand that with modern o-ring fittings, it is the o-ring that creates the seal, so you don’t need to crank down on the fitting nut with all your upper body strength. In fact, if you do, you’ll crush the o-ring and it won’t seal. This is bad. Don’t do it.

In contrast, with the older-style flare fittings, you’re sealing a metal face against another metal face, and you do need to crank it down really tight to get it to seal. On BMWs, like on other cars from this era, flared copper crush washers were used to help ensure the seal. It’s not that these flared copper crush rings are impossible to find – they’re available on Amazon – but the size is confusing, as they appear to be named for the inner, not the outer diameter. The big #10 flared fitting is 0.765” which is very close to ¾”, but takes a 5/8” flare gasket. The small #6 is 0.53”, very close to ½”, but takes a 3/8” flare gasket. As you can see, they’re exactly the right size.

When you’re cranking down on these fittings to deform and seal the crush washer, you need to put wrenches on both the pipe side and on the fitting side, and hold the pipe side steady while you turn the fitting side. If you don’t, you risk shearing the pipe side off. It is much easier to do this and crank down hard on the fitting to seal the crush washer when the evaporator assembly is out of the car and you have unimpeded access on all sides, but obviously you can’t do that if one of the fittings you’re cranking down on is on a hose coming through the firewall.

If you look at the fittings on the original Behr hoses, they actually use an angled adapter on a straight hose. The angled adapter has an odd long tapered thread on it. It’s possible that this was done so that the end-most fitting could be cranked down before the evap assembly was put into the car, allowing the connection to be made and broken at the tapered fitting, which is much easier to reach when the evap assembly is installed, but that’s just a guess.

This is simplified when you change over to o-rings. With the evaporator assembly out of the car and access on all sides to use two wrenches, you thread the flare-to-o-ring adapters on over the crush washers and lock ‘em down nice and tight.

Then, when you’ve put the evap assembly back in the car and crimped your nice new #6 and #10 o-ring fittings on your two nice new hoses coming through your firewall, you can easily tighten the o-ring fittings, which do not require a dying strain. Of course, if you’re building your own hoses, there’s nothing to prevent you from simply crimping on an angled flare fitting, but it would be damned near impossible to get a wrench on the pipe side to hold it still while you tighten the flare fitting. Note, in the picture below, how easily accessible the flare-to-o-ring adapters and the o-ring fittings are, and how inaccessible the original flared pipe ends are; you can barely see the top one peaking out behind the squirrel cage housing. This shows the added utility of using the adapters.

[jgerock]

Good stuff Rob. I still have all the old components in my car.

[austinmf]

Are you prelubing the o rings or crush washers with PAG oil before assembly? Any idea on the torque for o ring fittings?

[thehackmechanic]

Jim, looks to me like those hoses have been replaced, using a single angled flare fitting instead of the compound fitting I've seen on the original hoses. And it looks like your pipes peek out a little further than mine, making it a bit easier to put wrenches on 'em to hold 'em still if you need to pull the fittings off.

I pre-lube the o-rings and crush washers with Nylog (or, "snot," as it is affectionately called).

I don't know the torque settings for either the flared crush-ring fittings or the o-ring fittings. I use my informal wrist-o-meter, set to "really tight" for the flared crush-ring fittings, and "snug but not too snug" for the o-rings.

[TobyB]

Out of context,

these look very much like part of field- assembly fittings-

the outer shell retains the hose, then this tapered

fitting drives into the inside, wedging the hose

into a compression fit.

t

[thehackmechanic]

Interesting, Toby, I didn't know that. Two Behr-conditioned cars recently passed through my hands in addition to the tii, and all of them used these on the evap hoses. I assumed they were part of the original Behr installation.