question for the thread tapping gurus

i am building custom steering for my rig i have obtained a nice length of dom tube with 1/4 inch wall with an inside diameter of .818 thousandths near as i can tell, i want to tap this with a 7/8 inch by 14 threads per in tap, i aquired all the neccesary taps and material dirt cheap, what im worrid about is the materials ID too large for tapping with the above threads, i found some info on the web and it looks like the proper dimension for 7/8 by 14 is more like .787 thousandths, which is a varience of about .030 or 3 hundreths of an inch, if i tap the above material will it hold up or is the ID too large for the thread size i want to use?
Thanks in advance,
joe

1/4" inch is .250", or two hundred fifty thousandths. Given the depth of the 7/8-14 thread, it should be no trouble for you.

Only problem I can really see is that you'll need to ream out the tubing BEFORE you thread it as well - as I can recall, 7/8" + .875", and you'll want .880-.890 basic ID before you get a thread that screws easily, and you won't be "reaming with the tap" - tapping an undersize hole. If you've ever broken a tap, you'll know what I mean (I've dealt with a few...)

5-90

im going to say the tubing is 1.3125 OD and if its .250 wall its .8125 ID and youll be fine with the 7/8 x 14 TPI tap

im not sure where you got what size the hole should be, and because i dont remember off the top of my head, what thread depth does that give? 75-100% TD yeilds the same strength, just a tighter fit......

You need an exact answer. Just look in a chart or call someone to find the hole size that is needed for that size tap. I know that for 3/4-16 you need an 11/16 hole, but I don't know for a 7/8-14. It most likely is 13/16, but you'll want to know for sure. Like 5-90 said, you don't want to try and tap an undersized whole. Anyplace that sells taps should be able to look up what drill size goes with that tap, or you should be able to find a chart somewhere on the internet.

Again, you need an exact answer.........it should be fine just won't cut it.

Are you planning on running heims or TRE's on this setup?

TRE's (at least the 1-ton chevy ones) use 7/8-18 thread, which is even finer than NF thread.

Here's a reference chart that I use a lot:
http://www.korit.com/tapndrill.htm

According to that, for a 7/8-14 hole, you want 13/16", or .8125 for this. I think your original post was mis-understood in that you're actually wondering about the effects of tapping an OVERSIZED hole vs. an undersized one.

Problem there is your minor diameter on the threads is going to be slightly larger than the thread was designed for. This can mess with the thread fitment, giving you slightly more play in the threaded interface, which can cause premature wear and failure. You said the ID of your tube is .818", which means you're only .006" over anyway, which means the minor diameter is only going to be off by .003" on each side...I'd probably be fine with that, as far as tensile strength goes. Just make sure to have a decent amount of thread engagement, and I'd also run a jam nut to tighten the threads and reduce the slop.

To cut that, you'll probably want to chuck the DOM up in a lathe, and use a tap with a larger chamfer on it - a starting tap would work well. As long as you get the tap started true to center it should thread pretty easily.

For TREs, using the 18-pitch 7/8 thread, the desired hole diameter is .828", or 53/64". This is slightly bigger than your tube, so I'd recommend using a boring bar on a lathe to achieve the proper ID. Only go in as far as you absolutely need to in order to keep the ID as true as possible, and if you have access to a ream of that size, that'd be even better. Use a starting tap for the first few threads, then change to a bottoming tap so that the change in ID won't mess up the leading edge of the tap, and you'll be set.