DSE master cylinder uses a 37 degree flare???

[jlwdvm]

The directions I have for my DSE master/booster combo say to use a 37 degree inverted flare with the 3/16" line to metric fittings provided for the master cylinder lines. I'm confuse...is this a typo and should I be using a 45 degree double flare like the rest of my lines?

[572Camaro]

Jlwdvm,
Sorry no one has replied yet.
I don't know the answer, but recommend you call DSE tech support and question them. They are good and when it comes to brakes, I would call them for sure.

[jlwdvm]

Did that....twice. They said the directions were right, one guy even said I could use a 37 degree single flare?!?!? I wasn't too confident in the info I received from the DSE tech line. I ended up buying a 37 double flare die set for my Eastwood flare kit and did 37 flares. No leaks.

[65 Drop Top]

I think by 37* single flare, he probably meant AN fitting. A lot of guys will use AN fittings through out the brake system.

[jlwdvm]

The whole debate that nobody seems to be able to answer is:what is the flare in the master cylinder? The kit comes with metric to 3'16" line adapters....but what is the actual flare inside the master?

[65 Drop Top]

The instructions definitely say to use a 37* inverted flare

[Oleyellar]

Metric Vs SAE Flares
Metric double flare and SAE double flare fittings are constructed from the same principles but with different angles. Referring back to the schematic, SAE double flare uses a 45 degree angle:



JIS metric double flare, as used in Miatas and other Japanese-manufactured vehicles, employ a 37 degree angle.

The far right image on the above illustration shows an SAE double flare with a metric (M10x1.0) tube nut. This is not to be confused with a metric double flare with the same metric tube nut - the angle of the flare itself is critical for the junction sealing.

A proper flare junction uses the same flare type on both male and female ends. Cars like Miatas are configured like this from the factory: they employ M10x1.0 tube nuts and female JIS metric double flares on hard lines and male JIS metric double flares on soft lines.

Issue arise when one mixes SAE and metric fittings and fasteners in a single brake system, because in the US the SAE fittings and fasteners are more commonly available. Due to the differences in flare angles (45 vs 37 degrees), the junction of an SAE inverted flare and a metric inverted flare will not seal properly if tightened to normal specifications. However, if the tube nut is overtightened, eventually one of the ends will form the other end to the same angle, thus creating a seal. In my experience the female end of the hard line alters the male end of the soft line.

Because higher than normal force is required to squish the mismatching flare ends together, assembling a junction in this fashion increases the risk of rounding off the tube nut. Subsequent disassembly also carries increased risk of rounding off the tube nut as it was tightened to a higher than normal torque. Furthermore, as the shape of the tube itself is changed during assembly it is possible to reduce the size of the opening below the diameter of the tube, though I believe this to be rather unlikely given that the angle change is relatively small (8 degrees, from 45 to 37).

On the topic of tube nuts, if one uses SAE flared hard lines with SAE tube nuts the size of the tube nuts - 3/8" or 9.5 mm - is close to the 10 mm size of the metric tube nuts but not quite the same. Attempting to loosen SAE tube nuts with the metric 10 mm flare nut wrench, especially if the SAE nuts were overtightened to compensate for the flare angle difference, is likely to result in rounded off tube nuts during disassembly.

[SuperB70]

As I live in Europe and build US cars I have to play w/ SAE and Metric fittings. So here is my .2 :

37 degree is nor.al AN-fitting. Its also JIG 37 in Metric system. Those two are mixable together. No matter male or female fittings.