8 replies to this topic

[mk=john]

Hi
I am currently working on my 850cc engine, out of my 1960 mini minor. I have removed the heater tap from the cylinder head, but one stud was seized in the cylinder head. after all attempts to remove it using the two nut method (along with WD40), I accidentally stripped the thread.

Anyway, my only alternative was to cut off the exposed bit, and drill through the remainder of the stud as close as possible the the thread in the cylinder head. Anyway, i managed to drill close to the threa, but there is a bit of the stud left which is filling the thread (The thread grooves in the cylinder head still have metal from the stud stuck there)

How can I remove this? I don't want to have to spend more money getting a thread tapping set if there is an alternative.
If tapping it out is the only way, what is the size and type of thread I need look for when buying a threading tap?

Thanks

Edited by mk=john, 12 March 2008 - 09:58 PM.

[ginigwunkle]

if there is only a little left in the threads only then you may be able too, with a fine screwdriver / something with small pointy end, poke the remnants ouit of the thread otherwise im gunna say itll be an english sized tap...... do you have any taps - cant you tap it out with a metric and use all new nuts etc...

[dklawson]

Sorry to hear this John.

I'm not sure I follow you. Are you saying that you have drilled out the center of the broken stud but tiny bits of the stud threads are still left in the head? If so, and if your pilot hole is fairly true to center, drill out as much of the stud as possible. The studs should be 1/4-28 (UNF). The tap drill for this would be 0.214" but I would not use anything larger than say... a 5mm drill bit (which would be 0.197"). Once you've done that, take a pick or probe and see if you can get the broken bits of thread out of the hole. You may get lucky.

If this doesn't work, you don't have to buy a complete set of taps. However, in this case what you might benefit from is a 3-piece set for the 1/4 UNF size. This set would include a taper tap, a plug tap, and a bottoming tap. As it's name implies, the taper tap has a long lead in angle along its length for starting a tapped hole. Once started... and the tap bottoms in the hole) you switch sequentially to the other two taps in turn until you've tapped to the bottom of the hole. The bottoming tap will have almost no lead-in taper so it's the tool to let you cut threads to the bottom of a blind hole once you've got the threads started.

With any taps, use oil to lubricate the hole. Line the tap up perpendicular to the surface of the head... angles are bad. Turn the tap in about 1/2 turn while applying slight downward pressure, then back it out 1/4 turn. Keep moving forward 1/2 and back 1/4 until you feel the tap bottom in the hole (turning backwards breaks the chips that the tap teeth cut). DO NOT apply excessive torque and ALWAYS use a sharp tap. If you have to buy taps, buy alloy steel, not tungsten steel. Examine the teeth on the tap and if you can't easily scratch a fingernail with them... use a better tap.

[Angusdog]

With any taps, use oil to lubricate the hole. Line the tap up perpendicular to the surface of the head... angles are bad.

In NZ, there is this stuff called Rocor, which is specialised thread cutting grease. It sounds counter intuitive, but makes an enormous difference. There may be a similar product in the UK. Also, to ensure the tap goes in vertically, mount the tap in a drill press with the head underneath. Then turn the chuck by hand with pressure from the drill press hand wheel.

[CraigyBoy]

If all else fails, have it Spark Eroded, it is a very accurate way to remove the unwanted material.

I had some done last year and only cost a couple of quid and doesnt touch the internal thread.

[dklawson]

If all else fails, have it Spark Eroded.

That sounds like it's the process that over here we call "Electro-Discharge Machining" (EDM). If John takes the head to a shop for this work does he ask them "Do you Spark Erode broken bolts?" or is there a different term/name that he should use for the process?

Regardless, this process can remove anything metal stuck in a metal part (as long as it fits inside the tank they submerge the part in). It's not cheap but it's highly effective.

[CraigyBoy]

There are two types of EDM, Spark Erosion and Wire Erosion, wire erosion is what parts such as turbo compressor blades are made from, as it can produce tolerances between components that can hardly be seen by the eye.

What you need is Spark erosion, where your head will be in a bath of Dielectric fluid and an electrode will have a current flowing through it, which will be lowered to the work height and the electrode will arc on the work, eroding the material.

This is reasonably accurate and not too expensive.

To find a plce that does this, Spark Erosion or EDM will throw the right results, people in that business will know what he's on about.

Edited by CraigyBoy, 13 March 2008 - 12:47 PM.

[Ethel]

It is only the heater tap, you would probably get away with soldering the stud in if you want a quick fix - or drill and tap a size up.

[mk=john]

Hi
I managed to solve the problem. I drilled a tapping size drill (5.5mm). Then just got hold of 1/4-28 UNF taps and now its fine. The stud is slightly off to the left, but only a tiny amount and the heater tap still fits on. Torqued down to 8 lb. New studs were added with copper grease, new nuts, looks a treat!

Thanks for the help anyway

Edited by mk=john, 16 March 2008 - 09:29 AM.