Drill W933 tie down stock before tapping

Is it required to drill the stock to 5/16 before running the tap into it? I measure my current holes around 0.296 inches which is less than the 0.3125 a 5/16 drill bit would give you. I have not found any mention of this here or other builder web sites. My concern is the threads will be too deep and cause weakness.

You can use this chart...

http://www.boltdepot.com/fastener-information/US-Tap-Drill-Size.aspx

I don't recall predrilling this before tapping. Should be able to just get the tap into the hole and go to town with lots of cutting oil.

I've done this both ways... drill to correct final size, you will end up with a better fit as the undrill turned out with sloppy threads. Just clamp the part in a vise with protection and drilled it up, just takes a minute.

You will also find it much easier to tap the block.

I did it undrilled. I wanted the full depth of the threads because I was tapping aluminum. It took a generous amount of oil plus I removed the tap when the channels started to clog and cleaned them out, otherwise the chips would have ruined the threads. As it was, I got nice threads and a very close fit on the rings I got from Avery.

Print yourself out some drill/tap charts for your shop. Most give you various thread depth choices,,,50% to 75% thread depth. I always use the 75% for tapping aluminum. Spend the money and buy high quality taps from an industrial supplier such as http://metalworking.mscdirect.com/c...6904&005=3069718812&006=7741756624&007=Search along with 2 cans of Magic Tap ,,,,,,,,,,,,,, one for steel and the other for aluminum. Clean out the chips often. Discard the taps when they get dull.

Update

I was at this "tapping the tie down stock" point in my build yesterday and found some of these question/answer posts quite helpful.
First the question of "do you need to drill the stock out to 5/16" prior to tapping?
I couldn't get the tap to dig into the hole provided, so I drilled the hole to 5/16" for about a quarter inch just to get the tap to go, then completed the tapping at the original hole diameter for the rest of the 1".
The part of the threaded hole that I opened up to 5/16" was less than desirable.
I ran out to the airport and grabbed another tap set that I noticed had a tapered tip.



The picture doesn't quite do it, but both of those taps are the same size and I think it was the tap on the upper left that had a taper to it.
With that I screwed it into a fresh unmolested hole and it went right in. I then grabbed the bad one that I messed up and tapped the other end (luckily I hadn't drilled any holes in the flange to spar areas yet) which came out great as well.



And not to let one wing be heavier than the other, I tapped a fourth hole, so I have threads on both upper and lower ends of my tie down stock pieces.
Secondly, people had asked what is the best lubricant to use. I haven't used a tap since HS shop class, so I don't have much experience pass along. I only had 30 wt. oil for lube in my garage. But half way through my tapping I remembered I have boelube for cutting/drilling so I tried it. Much cleaner, and the chips didn't seem to grind so much when I backed the tap out for cleaning.
So if your getting close to this step in your build and wondering if the hole Van's put in your tie down stock is big enough, mine was and I received my wing kit this last July.
I hope this is helpful.
Tom