I had typed out the little note below in a PM to a couple of Forum members and it just occurred to me that there may be people who are interested in this topic, but who are not clear on the various types of taps. Taps are the tool used to form internal (female) threads in holes. For each thread size (usually referred to as the “nominal” diameter) and thread form (metric, inch, fine or coarse, pipe thread etc.) you need a specific tap.
The corresponding tool to form external or “male” threads is called a die. These things are sold in large sets and amazingly, considering what they can do, they are not really very expensive. Here is a picture of the three most commonly used types of taps:
The taper and plug taps are used to start threading a hole. The tapered end helps to centre and align the tool and ensure that the axis of the threads is along the axis of the hole itself. The bottoming tap doesn’t have that tapered end and so it is very difficult to achieve that alignment and very easy to get the tool “cocked” in the hole - which will definitely result in a mess and quite possibly, a ruined workpiece.
Unfortunately, bottoming taps are usually not included in tap & die sets because they are sort of specialized tools normally used by machine shops. However, you can make a bottoming tap by grinding the end of a tapered tap - if you are careful. The steel used to make taps is very very hard and so grinding the end of a tap to eliminate the tapered portion may not be as easy as it sounds.
Bottoming taps are used almost exclusively for getting blind holes threaded right to the bottom or at least, as deep as possible. Usually, they are needed in the sort of situation Bob has with his oil pump where the first few threads in the workpiece are stripped or damaged or there is some other difficulty with the adequacy of the thread engagement (ie. the length of the hole in the workpiece which has threads that are engaged by the fastener).
FYI - threaded joints develop their full strength within a certain number of “thread diameters” (the nominal size of the thread). Going in with a longer bolt into a deeper threaded hole is simply not necessary.
The key variable here is the hardness and strength of the material into which the fastener is threaded (i.e. the workpiece). Most fasteners are made of steel and so if you assume that all steel has the same strength (which is absolutely NOT correct - but please bear with me) then a commonly used rule of thumb is that a steel fastener going into a steel part needs to engage only about one nominal diameter of thread in the hole to develop a full strength joint. Thus, a 10mm bolt threaded into a steel workpiece needs to engage about 10mm of threads in the hole for full strength.
For softer materials like brass, copper and aluminium, you need 2-3 diameters of thread engagement to develop a full strength joint.
So - a 6mm bolt threaded into an aluminium engine case needs a hole that has about 12-18mm of threading for full strength. Less than that amount of thread in the hole can lead to stripping or pull-outs. That’s why a bottoming tap is so useful in working on motorcycles (and aircraft as well) which almost always use a lot of aluminium parts in critical areas.
The key things about bottoming taps are to get the end ground square and not mess up the flutes (cutting surfaces) on the end of the tap. That’s why I’d like to get some conventional taps and then use our machine shop at school to do the grinding properly.
If you want to use a bottoming tap, you should always begin with a starting “tapered” tap (this is the “conventional” one with the tapered nose on it) to start cutting the threads in the hole. This will ensure that axis of the thread is along the same axis as the hole itself. Be sure to use cutting fluid when using the tap and be sure also to reverse direction of rotation occasionally to break the chip thatnthe tap flutes are forming. You should turn the tap in several revolutions and then back the tap right out to remove the chips from the flutes and ensure a good clean cutting surface.
After you have cut the thread as far down the hole as possible with the tapered tap, you must clear out the chips and then go in with the bottoming tap (very gently) to cut the final few threads down at the bottom of the blind hole.
Then you clean out those chips and then measure your fastener very carefully to ensure that it does NOT bottom in the hole.
NOTE: clearing out the chips usually involves the use of compressed air which is fun to play with but can be dangerous. Put a rag over the mouth of the hole and for goodness sakes, WEAR SAFETY GLASSES. Then just blow the air through the cloth and as the chips come flying back up the hole, they are normally trapped in the cloth and don’t go all over your shop. BUT DO NOT FORGET THE SAFETY GLASSES PLEASE!!!
OK now, what the heck do you do if a bottoming tap does not fix your problem. Well, the mechanical engineer has more solutions in his/her arsenal which brings us to....
HELICOILS:
A helicoil is a special type of insert that can be installed in a hole which has stripped or damaged threads to restore a part to service. The helicoil looks like a little spring but the inside is actually a female thread and the outside is a male thread. Installing a helicoil involves drilling out the stripped hole and tapping it to a larger diameter with a special tap - and then screwing in the helicoil “spring” with a special tool - and then breaking off a little tang or segment of the spring wire to open up the hole for insertion of the normal fastener. Helicoils are sold in kits which will repair a specific size of thread (see photo below).
Note the little notch on the wire at the 10 o’clock position on the right hand photo. That is where the tang will break off.
The only potential downside of a helicoil thread repair is that there may not be sufficient space for the larger hole required to install a helicoil of a certain nominal size. On the other hand, since the helicoil is made of steel, you can get a very secure joint with a shorter length of thread engagement on the bolt and you can return an expensive aluminium engine case to service.
I think I’m correct in saying that some high-end engines come from the factory with helicoils in place for critical threaded holes such as spark plugs and cylinder studs. I installed helicoils in my 1975 XS650B spark plug holes back in the late 1970s when I found that my P/O had stripped them and used cigarette package foil to keep the plugs (sort of) in place.
Here endeth the lesson.
Pete
