bgilbert

Is there any difference between a 'bottom' or 'blind' tap and a regular ole off the shelf metric tap??? I am installing head studs and need to clean out the threads in the block. I bought a 12mm x 1.75 metric tap from the store. I didn't see anything on the package or the other boxes at the store that mention 'bottom/blind' tap. What gives? Thanks.

Geico266

Yes, the bottom tap "makes" threads all the way to the bottom of the hole. A regular tap is more tappered. If you are just cleaning up threads the regular tap should work if you cannot find a bottom tap. Blow the holes out good with compressed air.

bgilbert

Wow, quick response. Thanks alot. I guess I shoulda asked before buying.

Geico266

You could take an old bolt and grind flutes in it, run it all the way down also. The will clean out the last 3-5 threads the regular tap misses.

Good luck!

edwinsmith

Yes, this is good but use the highest grade bolt you can find. They are harder and will do a better job and won't get dull as fast.

Edwin

bgilbert

Hmm, I might try that with an old head bolt.

Boatnik

Just for educational purposes and to allow me to show off a little, Taps generally come in sets of three. The number one tap is the taper or starter tap. It has a shallow taper at the begining to allow easy tapping of a drilled hole. The second tap is the plug tap. It is used after the first tap to add more thread to the bottom of the hole. It does this by utilizing a steeper taper at the start of the tap. It is used with great success for cleaning up damaged threads. The number three tap is the bottom tap. It was designed with no taper so it would cut threads all the way to the limit of the tap's length. These are used widely for cleaning out holes for reassembly. They can also straighten out damaged threads.
Metric taps have followed the same convention in the last thirty or so years, but in the "old days" they would actually grind the top of the threads off on the first tap, or taper tap, and grind not so much off the threads of the second tap or plug tap. The only tap that would cut a full thread was the third tap. They did that to make it easier tapping a drilled hole in harder steels. It aslo allowed for a much more precise hole in the manufacturing process. Always use some form of coolant or lubrication when using taps. Light oil is a good choice for steel and diesel fuel or kerosene or a light penetrating oil works for cast iron. Carefully turn the tap into the hole to be cleaned for a short distance and then back it completely out and clean it. Continue in this manner until you reach firm resistance and stop. Remove the tap and clean and inspect the hole. Remember that taps are extremely hard and brittle. If you snap one off in a hole it is a bear to get out sometimes. Especially if it gets bound up in the hole because of chip binding or dirt. I hope this helps in the mystery of the tap.
Another tip for tapping. If you are worried about shavings getting where they don't belong, like when you install a EGT pyrometer. Coat the tap with grease. Turn it into the hole a 1/2 turn and remove. Clean and re-grease. Continue in this fashion until you have reached the desired depth. Then vacuum the shavings and excess grease away from the hole. Install a plug and then thoroughly clean the outside area.
I hope I didn't get to long winded. I also ope that this little tidbit of information helps prevent some of the evil things that taps can do. Good Luck.

edwinsmith

The bit about the tops being ground on the metric taps reminded me of when I was using an acme thread tap while making a fancy turnbuckle. The tap was ground in stages to cut the threads successively deeper. Acme threads have the top and bottom flattened which gives a very strong screw for things which have to turn a lot and hold a lot of pressure on the threads. They are very difficult to cut and require the special taps.

Then there's square threads.

Edwin

Geico266

That will work. Grind several "flutes" not just 4 like a tap. Gives it more "clean out" area. Hold the bolt in such a way as to give the "cutting edge" more or less angle as needed. The less angle the better, You'll figure it out. Works great.

Boatnik

And the list goes on. I from time to time manufacture valve stems and bushings to repair the old junk we run at the Electricity Factory. Imagine cutting a double lead, left hand stub acme internal thread. They made me do one out of stainless steel for my job performance skill test when I got the job in 1977. Lucky for me the stem was 2 inch in diameter. Oh, by the way, We single point most threads on an engine lathe. No CNC here.

BearKiller

This conversation reminds me of my rifle-smithing days.

You ain't lived til you snap a 6-48 tap off in a BRNO-98 receiver.

I finally got wise and left them in the hole they broke in, drilling a second hole in the scope mount-base and threading a new hole.

A hundred years later, someone might remove the scope-base and my sins will be exposed.

edwinsmith

I learnt that in machine shop class and never did one since. I do CNC programming though. You haven't lived until you figure out how to do a tapered chamfer around a 1/2 inch inside radius. Try that on a Bridgeport!

Edwin

wannadiesel

Bill, if that tap was cheap enough for you to want to modify it, you can use a grinder to take the tapered part off it and make it into a bottom tap. I've done that before.

BearKiller

A broken tap can also be successfully used as a bottom tap.

bgilbert

Ok, I took your advise. Cut off the tapered end. Pretty good/clean cut with die grinder/cut off wheel. Does this affect the strength/integrity or anything with this tap? It was a Sears Craftsman $5 tap. All the horror stories of taps breaking, I don't want to screw something up .