Lag screw in plywood - what size pilot hole?

[Matthew N. Masail]

I just bought my first bench vise, it's one similar to this http://www.leevalley.com/US/wood/page.aspx?p=49980&cat=1,41659, the big one.
I've never used lag screw before so I'm not sure how to do it. if the lag screw is 12mm then should I drill a 10mm pilot hole? or less? I'm assuming the more the thread grabs the better, but it's
going to be about 10cm long so I don't want to have a hard time screwing it all the way, or have it split the plywood.
Thanks.

[pat warner]

Vise work is heavy work. Lag screws will loosen in time from all the stress.
Ply does not hold threads as well as wood, metal or plastic.
Screws/bolts and nuts, a better choice.
Lag screws the only fastener available?! Then drill 1/16 - 1/8 under bolt diameter.
Test in scrap.

[Matthew N. Masail]

I can use bolts.... but then I'd have 4 ugly holes on my bench top....

[keith micinski]

I'm not sure now using bolts would leave a different hole then a lag screw would? If you are going to use lag screws though I like to drill my hole about half the size of the screw. I don't know if it is true or not but it feels like it gives me a little more strength in the threads on the wood because you are compressing them more. It goes in not much harder then if your hole was bigger and I have yet to have anything split on me using this technique.

[Cary Falk]

The proper size of the pilot is the size of the screw minus the threads. The easiest way to size them is to hold the screw up to the light. Hold the drill bit up in front of the screw. When the shank disappears behind the drill bit and all you see is threads then you have the right pilot hole size.

[Dan Hintz]

The proper size of the pilot is the size of the screw minus the threads.
Yeppers... in other words, match up the drill bit with the screw shank size.

[Matt Meiser]

And you can just do it by eye with good results.

[glenn bradley]

I have two vises mounted with 5/16" lags that are rock solid after years of use. I expected them to fail but, others here told me to forge ahead. I am as surprised as anyone that they are still tight as the day they went in. Drill the hole the size of the shank (between the threads) and drill it a little longer than the fastener. A screw that size has no business cutting its own hole by being forced into solid material for the last little bit like many of us do for smaller fasteners. I use this same method (picked up somewhere along the way) for all screws going into MDF as well; no splits, no failures ;-)

[Bill White]

Shank size is called the "root" of the thread. That's the drill size to use as was stated before. Holes are holes. I would use bolts and locking nuts/washers to secure the vice(s).
Bill

[Matthew N. Masail]

Thanks guys, I'll get the lags, worst case scenario it will work loose and I'll have to bolt it.