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View Full Version : What pilot hole size for lag bolt



Brian Brown
01-18-2010, 3:32 PM
I'm bolting my lathe to the top of a cabinet. The top is made of particle board. and the lags are 5/16. What size drill bit will give me a pilot hole that still leaves the most bolt gripping power, without crumbling the particle board?

Steve Schlumpf
01-18-2010, 3:36 PM
Brian - a regular 5/16" wood screw requires a 7/64" pilot in hardwood and a 3/32" in softwood. A lot depends on the threads of the lag - are they standard or are they the deep threads found in specialty screws?

I would probably go with the smaller hole size and keep an eye on how easy it screws in place.

David Walser
01-18-2010, 3:49 PM
Brian,

Here's a link to a handy chart that should help to answer your question:
http://www.portlandbolt.com/technicalinformation/lead-hole-diameter-lag-bolt-chart.html

However, I'd be very reluctant to use lag bolts into particleboard to anchor a lathe. The vibration is apt to crumble the edges of the hole in the particleboard. I would be very surprised if the lag bolts didn't strip-out fairly quickly. Instead, I'd try to glue some 4/4 hardwood stock to the underside of the particleboard top and put my lag bolts into that. Otherwise, I'd glue the 4/4 hardwood to the top of the particleboard and run the lag bolts through both the hardwood and the particleboard. (I both cases, I'd drill the particleboard so that the threads of the lag bolt did not cut into the particleboard.)

Good luck!

Steve Mawson
01-18-2010, 5:17 PM
Just my preference but I would use bolts. Back up with some hardwood underside and put the bolt through everything. That way you can remove and replace if you ever need to. My $.05 worth.

Dick Sowa
01-18-2010, 7:55 PM
I agree that without a solid wood backing, I would go with bolt/nut combination with large flat washers. Even then, unless you double nut (jamb nut), they will eventually work loose.

Allen Neighbors
01-18-2010, 8:14 PM
Just as a thought: I have used screws in particle board that I soaked the pre-drilled hole with Super Glue, then used a screw gun to run the screws in fairly fast. It is still possible to get the screws out, also using a screw gun. If you spray the screws with silicone, ahead of time, and let it dry, they'll come out really easy, but the super glue in the holes, strengthen the pb. Don't know if it would work to hold down a lathe, though.