Double flaring brake lines

To finish fitting up my rebuilt Toyota 4×4 calipers to my 240z, I had to bend up and double flare a new brake line, so I thought I’d snap a few pictures while I was at it.

Most car’s hard lines seal with a 45 degree double flare. I hear about single flares and bubble flares but haven’t ever seen one. Since they seal by deformation, there’s some contention as to exactly how resealable¬†this type of connection is- theoretically, they’re not meant to be resealable at all, but in practice you get about 10-20 goes before they no longer seal. I’ve never actually had this happen. Aircraft fittings, incidentally, are 37 degrees and don’t plastically deform so don’t have this problem, but I won’t get into that. Here’s what a 45 degree double flare fitting looks like straight out of the factory:

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To make one, you’ll first need a length of brake tube. People call it bundy tube around here. You can get it in stainless or mild- I’ve found that mild steel is a bit easier to work with. You’ll also need a couple of the threaded fittings (I reuse the old ones unless the hex head is buggered) and a flaring tool, pictured below. The tool consists of a split tapered collet which tightens up on the brake line to hold it, a button die which forms the end of the tube into a sort of bubble and a conical die which forms the flare. There’s also a little tube cutter and a de-burring tool.

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So here’s the tube set up. There’s about 4mm of tube protruding from the end of the collet. If you leave too much hanging out, you end up with a nice flare with a not-so-nice bulge immediately behind it. If your bit of tube isn’t cut square and bent straight, the flare will end up uneven (not quite doubled over on one side). Once you get it right, if you’re lucky, at this point, you’ll realise that you forgot to put on the fitting. If you’re unlucky, you’ll realise after you formed the flare.

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Below on the left shows the position of the button die after the first stage of the forming process. It takes a few tries to get the hang of how much to squish it- this is a photo of one that I got just right (attempt number 5, I think). On the right shows the conical die tightened in. Again, this is a photo of the successful attempt. If you squash it too much here, the flare will start to split around the periphery.

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And below is the finished product. Time to fill the system up and bleed it.

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2 Responses to Double flaring brake lines

  1. Joe Gaskill says:

    Just curious, I know that a ’79-81 280zx master cylinder will work for the Toyota upgrade, but what about an ’82-’83?

    • Hyllest says:

      Well, I’m not sure to be honest, but I have fitted master cylinders from r30 skylines, 280zxs, patrols, etc, etc and I have yet to find a master cylinder that won’t work. The main point to be careful of is that you get one with large enough a bore. The stock 240z master cylinders are a 7/8″ as are (I think) 280zx 2-seaters. The 280 2+2’s have a 15/16″ master which is definitely the preferred of the two.

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