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If i have to replace my water pipe ports rusted out on a manifold and have to use 1/8" taper pipe threads - I don't unnerstand the taper hole making in practice.

If the taper tap is tapered and it s, how do you know how deep to make your tap go ?

You do not use a taper drill to drill hole to correct size first do you! Taper drill? what sort of drugs am I on ?

I have done just one taper thread job before and struggled with the concept of it working

If I can get a 6.35mm barb with a standard thread like M8 1.25m I'll be going that way

Anyone explain the taper thread making business ?

ta, Jono

The beauty of a tapered thread is that you can determine how deep you want the fitting to sit by how deep you tap the thread. Plus it always pulls up nice and tight and seals well. With a parrellel thread, sometimes you can screw the fitting all the way through the material and out the other side without it getting tight.
Is that too basic? Or did you want some engineering type terms?

Like Brumbyrunner said the advantage of using a tapered tap is that you have a choice of how far you "screw" the tap into the hole .

It works like this , for every tap out there in a multitude of British Metric etc sizes there is a set tapping drill size to ensure the correct thread depth .
When you drill the correct sized hole for a tapered thread naturally the hole will be parallel , couldn't imagine the flutes on the drill not jamming if the drill was tapered . Usually if you want a smooth tapered hole or bore you would drill it normally and use a tapered reamer to get the desired form . Reamers have a far lower rate of twist than a drill bit (twist drill is the correct term) and more flutes - more cutting edges as well . A twist drill is a higher speed tool than a reamer and used generally to rough out a hole or where tolerances are not as critical , surface finish is also better in reamed holes .

Back to tapered threads also known as pipe or gas threads .
The allies had no end of trouble with tapered threads in WW2 because there were no real standards on the depth of the tapped threads in the given hole size .

This shows up often on Japanese cars with gas threads in inlet manifolds for vaccum barb fittings . I had issues years ago trying to fit Autronic air temp sensors into Nissan FJ20ET inlet manifolds - throttle bodies actually . The tapped hole was not tapped deep enough for the sensors 1/8 gas thread union so we used to run a 1/8 gas thread die nut down the sensors brass thread to make it fit . Really the thread on the throttle body should have been tapped deeper but it was always quicker and easier to modify the sensor and screw it into the throttlebody in situ .

I'm not sure which fittings you mean , generally you tap holes to suit the most easily sourced suitable fitting available . If the holes have corroded to the point where the gas fitting hole cant be repaired go up a size if possible because lots of engines use large fittings with small barbes on them .
A neat trick we used to do was find a cheap male gas/pipe thread bung , drill it to the correct size and braze a piece of cut off fuel line tube into it with its existing barb . Just depends whats available at the time .

Cheers A .

I thought as much. Gee's , I never reemed the hole before, but makes good sense to do so.

On the EA81 inlet manifold is a press fit mild steel tube of 6.35mm OD. Water hose connects to the same tube type sticking out the top of drver side block. The fix is to replace is to drill out and then tap the hole to suit a screw in barbed fitting made of bras and easily available ( i hope in right size) with the 1/8 BSPT male end. An option if no other is to tap an easy thread like M8 1.25 to suit a bolt drilled out in its centre and braze a correct size tube.

My 'problem' here is currently how to make the holes thread taper correct. Time to sift through some bitsnpieces

ta

Hint: Ensure the fitting you want to use is available (or just buy it) BEFORE you go tapping threads for some wonderous fitting that would do the job perfectly but does not exist

I made the fitting from a good metal hose thing cut off from another manifold, cut the unnecessary length from a brake booster manifold fitting with the 1/8 NPT 27 male thread already on it, drilled out to suit and pressed the 6.6mm OD pipe into it. Borrowed a 1/8 NPT tap, drilled an 8.5mm then 9.0mm hole where the rusted steel pipe was and tapped away. DONE

Good hint too , coz the so called brass 1/8 NPT 27 with 1/4" barb fitting looks a little bit too small