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Stubborn flare nuts on master cylinder
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Ive got some stubborn flare nuts on the master cylinder. Has anyone got any tips for freeing them up?. Ive used what I thought was a good sidchrome 10mm flare nut spanner on it and it feels like it’s gonna round the nut if I keep applying pressure
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They can be b!tches sometimes. Over the years I have had to use vice grips on the nut to undo them which often wrecks the nuts, and then had to get replacement pipes. But that was back when there were HQ-WB in every wrecking yard.
I’ve even had the nuts eventually undo but be rusted on the pipe and thus corkscrew the pipe.
Don’t panic if you wreck a nut or pipe though, you can buy any of them as new repro. Spray some Penetrene or WD40 or similar on them and leave them overnight. |
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Ok, am I right in say a 10mm flare nut spanners correct?. It
Wouldn’t be imperial by chance?
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If the 10mm spanner is the correct size it will be a very snug fit on the flare nut. They are made as a six sided hex so they won't/cannot tear the points off the nut. More than likely you need a 3/8" flare nut spanner. Just checked a NOS brake pipe that I believe is for an early Commodore. The flare nuts are definitely imperial. A 10mm flare nut spanner is not a snug fit on the nut whereas the 3/8" spanner fits perfectly. Edited by user Sunday, 24 October 2021 9:11:03 PM(UTC)
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Ive just tried a 3/8 open end spanner on it just for size so I’d know if I had to get a spanner tommorow and it was too small but going back to 10mm flare nut spanner does seem to have a bit of play in it
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It could be 3/8. If you have been leaning heavily on the nut it may have deformed slightly and be out of square. Maybe give the flats a rub with a smooth file and try again. Any slop, as the 10mm seems to give, will end badly.
Another tip is to give the nut a hard knock. This may not be easy to do but can give good results.
Also support the master cylinder as you undo the nut. Any flex or movement greatly reduces the effectiveness of the 'undoing'.
Hitting the spanner handle can work but, as above, you need to support the master cylinder.
Finally grab the spanner head with a pair of vicegrips or multigrips on the opposite side to the spanner handle. Then turn both at the same time. This puts the twisting force at the centre, not offset as when using just the spanner. Sort of like using a t-handle.
Good luck, they can be a pain.
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Doesn’t look like 3/8 as I’ve tried a 3/8 flare nut spanner. The nut is in good Nick so far but 10mm feels too loose to lean on it
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Try a normal 10mm spanner on it to make sure you haven’t got a dodgy split spanner. If 10mm is loose though 3/8” should fit as the next AF imperial size slightly less than 10mm is 25/64 and I doubt it’s that as I have never seen that size spanner, it is 9.922mm which is probably within the manufacturing tolerance of M10 anyway and could normally be interchanged like 7/16-11mm and 9/16-14mm. |
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Yea tried both, no way a 3/8 will go on it and 10mm seems too lose. I might try a different spanner brand
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It is a Holden you're working on? Not a pommy car with Whitworth nuts?
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Originally Posted by: Sandaro  It is a Holden you're working on? Not a pommy car with Whitworth nuts? yeah.. a Holden I guess with a PBR master and bundy from the maker in Qld = you can never be sure the exact size is the correct one :) |
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Hi can you undo the other end of the pipe and slide a 3/8 ring spanner from the end or may be able to slide a single hex impact socket along the pipe.
Another option is to cut the pipe next to the nut then use a single hex socket and sometimes a rattle gun will shock it , then grab a new nut and re double flare your pipe if there is enough length.
Cheers
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3/8" flare nut and 3/8" flare nut spanner  3/8" flare nut and 10mm flare nut spanner. 
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You could try measuring the flare nut with vernier calipers and take the guesswork out of the equation. Just a thought.
If it's one of the oddball sizes, and you've been wailing on it with an oversize spanner the flats are probably done for. If they aren't, I've had some success with copper soft jaws on vicegrips in the past.
Not for the faint-hearted, but a 20 second burst from propane torch can help things along, particularly on the aluminium master cylinders.
And right about now I can hear Dr Terry gnawing his own arm off.
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You got that one right. This 3/8" AF vs 10mm AF thing with tube nuts has been a botch since day one. My guess here is that you have a 10mm tube nut & a worm or bent spanner.
With nuts & bolts they are either metric or imperial, right ? In other words a metric threaded nut or bolt will have a metric size AF hex (AF = across the flats). A UNC or UNF nut or bolt will have an inch AF size hex. We won't talk about Whitworth here if you wish to remain sane.
Tube nuts are a different animal. It seems to depend on who makes them & in what era. Way back when (say 1950s & 1960s) in Australian manufacture just about all threads were imperial (inches) & so was the stock material used to make the tube nuts. A standard tube nut had a 3/8" UNF thread & was made from material which began life as a 3/8" hex rod. A simple machining process to make the tube nuts from plain hex rod.
When metric came along in the 1970s, the most common pipe nut thread was 10mm x 1 & was usually made from 10mm hex rod, but in some cases it was made from the regular old 3/8" rod. Perfect a metric nut with an inch size AF hex, just to trap the uninitiated.
In recent years we have the opposite, the old 3/8" rod is now hard to procure, but 10mm hex rod is easy. For older cars 3/8" tube nuts are still required, so today if you buy new 3/8" tube nuts they come with a 10mm AF hex, so you'll need a 10mm pipe spanner to work on your old Holden.
Been caught several times. Go figure.
Dr Terry
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Then some times it’s just weird. The new bonnet cable I bought has a Whitworth nut on it. It definitely did not fit either metric or imperial (quality minimax tools I have been using for years). Dragged out my box of Whitworth tools and sure enough, that’s what it is.
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Just had a similar experience. Just finished putting in a Tufflift parking hoist in the house garage. The nuts on the bolts that attach the hydraulic unit to the post had weird nuts that wouldn't quite work with 12/13mm or 1/2". A Whitworth spanner was perfect. I also went to Bunnings to buy some threaded rod to make some brackets up, and I needed studs so had to drill and tap some flat steel to a UNC thread, put some rod in and weld it at the back. All they had was Whitworth thread rod in 5/16", yet the only sold UNC nuts? They work but they aren't correct especially for high strength joints. Luckily as I was welding the threaded rod to the steel I didn't need it to be a strong hold with the Whitworth threaded rod in the UNC tapped hole, so it was OK. But still strange! |
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Protip: flare spanners are not great but theyre better if you lock them on with vice grips.
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Stubborn flare nuts on master cylinder
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