How biga hole should I drill???

[Boomyal]

Is there a correct ratio of pilot hole size to lag bolt diameter when drilling holes in fresh timber? I'm getting ready to re-drill the motor mount holes in my pretty new motor mount stringers. I've run many a lag bolt into timber before and never really given a lot of thought to this aspect. But with the critical nature of anchoring a 550 lb motor and the difficulty of redoing it later, I thought it prudent to inquire.<br /><br />My thought was to use the vernier calipers to determine the base diameter of the lag bolt. (that is at the base of the threads) and choose a drill bit a teeny bit smaller. Then use a drill to match the upper unthreaded portion of the bolt to start the hole.<br /><br />Do you think this will provide the most effective bite of the lag, considering a physical dynamics of ramming a piece of steel into wood.<br /><br />Whatever size I end up using, I will run the bolt down the hole,(pre-thread the hole, if you will) remove it and fill the hole with as much CPES as it will swallow.

 

[DJ]

Re: How biga hole should I drill???

I do not know if there is anything "scientific" regarding your question.<br /><br />Some woods will "displace" more than others.<br /><br />Your theory sounds like exactly what I would do.

 

[Dunaruna]

Re: How biga hole should I drill???

Drilling to small and you risk cracking the timber, to big and the thread won't bite. I've just done mine and used the same method you have described except......... I didn't use a drill bit a 'teeny' bit smaller. I used one a bees d**k smaller (sorry, couldn't resist)<br /><br />Aldo

 

[Boomyal]

Re: How biga hole should I drill???

Originally posted by Dunaruna:<br /> Drilling to small and you risk cracking the timber, to big and the thread won't bite. I've just done mine and used the same method you have described except......... I didn't use a drill bit a 'teeny' bit smaller. I used one a bees d**k smaller (sorry, couldn't resist)<br /><br />Aldo

Originally posted by Dunaruna:<br /> Drilling to small and you risk cracking the timber, to big and the thread won't bite. I've just done mine and used the same method you have described except......... I didn't use a drill bit a 'teeny' bit smaller. I used one a bees d**k smaller (sorry, couldn't resist)<br /><br />Aldo

and btw, was that the turgid or flacid size & also thanks for the validation.<br />You also, DJ.

 

[18rabbit]

Re: How biga hole should I drill???

I’m seeing “lag bolt” mentioned in your query. If so, there should not be any threads in the base material. Note the smooth portion of the shank, above the threads. For a lag bolt you want a hole the diameter of the smooth shank/threads. And if you have any wood in there, get out your Dixie cups and the CPES... <br /><br />EDIT: if you’re using “lag screws” the hole should be the same size as the shank, below the screws, not the smooth shank, if any. There are tables that indicate the proper drill bit size for the shank of any give lag screw. Remember, Google is your friend. <br /><br /> http://www.boltdepot.com/fastener-information/Wood-Screw-Pilot-Hole-Size.htm

 

[crab bait]

Re: How biga hole should I drill???

well you should see the bee's in texas.. .

 

[ED21]

Re: How biga hole should I drill???

Shouldn't an outboard be fastened w/ through bolts? I would concerned w/ vibrations & any eventual water getting in & causing rot pulling out lag bolts.

 

[Boomyal]

Re: How biga hole should I drill???

Originally posted by 18rabbit:<br /> I’m seeing “lag bolt” mentioned in your query. If so, there should not be any threads in the base material. Note the smooth portion of the shank, above the threads. For a lag bolt you want a hole the diameter of the smooth shank/threads. And if you have any wood in there, get out your Dixie cups and the CPES... <br /><br />EDIT: if you’re using “lag screws” the hole should be the same size as the shank, below the screws, not the smooth shank, if any. There are tables that indicate the proper drill bit size for the shank of any give lag screw. Remember, Google is your friend. <br /><br /> http://www.boltdepot.com/fastener-information/Wood-Screw-Pilot-Hole-Size.htm

18R, I'm gonna call it a lag bolt til the day I die. The stupid things have a bolt head, soooooooo therefore it is a BOLT! <br /><br /><br />Ed, these are holes for motor mount feet for an I/O.

 

[Boomyal]

Re: How biga hole should I drill???

Originally posted by crab bait:<br /> well you should see the bee's in texas.. .

Oh, I should have surmized that ALL things Texan are bigger.

 

[qaztwo]

Re: How biga hole should I drill???

When I redid my motor mount which was an i/o, I drilled the hole at an inch. Placed the bolt in and fill with epoxy and milled glass fibers. Coated the bolt with soap incase i ever want to take it out. <br />Good luck.

 

[Mark42]

Re: How biga hole should I drill???

How about using a threaded insert and stainless bolts? The threaded inserts have deep cut threads to get an extra bite in the wood. They also will tell you what size hole to drill and how to set them. Might want to put some epoxy on the inserts before threading into the wood. The inserts will be much stronger than a regular lag bolt.

 

[BoatBuoy]

Re: How biga hole should I drill???

Boomyall,<br /> After that statement, I hope you have your asbestos suit on.

 

[Boomyal]

Re: How biga hole should I drill???

Originally posted by BoatBuoy:<br /> Boomyall,<br /> After that statement, I hope you have your asbestos suit on.

 

[Boomyal]

Re: How biga hole should I drill???

Thanks for the insert info Mark, but I'm all thru now. I think you'd need to be a machinist to make every thing line up perfectly.<br /><br />qaztwo, I think you're set for life. With those one inch holes did you drill a few smaller holes out lateraly to fill with epoxy? <br /><br /><br />FWIIW, I was hesitant to drill thru my new multi-layered fiberglass covering on my MM stringers. They are so pretty and I didn't want to splinter up the glass as I drilled into it. Soooooo, I got out my Forstner bits to drill the counter sink hole. They cut a very clean hole thru the glass. I then used a standerd drill bit to drill inside the counter sink hole, to the appropriate depth for the lag BOLT. I then threaded in all the bolts, without further splintering up the glass, removed them and filled all the holes up with CPES. I just finished the job and am confident that I'll never have to be concerned with my motor mounts again. Oh, and I did use SS lag BOLTS.

 

[ED21]

Re: How biga hole should I drill???

I have outboards on the brain. <br />Sounds like you did it right.

 

[Lou C]

Re: How biga hole should I drill???

That CPES is some good stuff isn't it??<br />I used it on everything from my floor repair on the FW last year, to building a mount for wheels on the transom of my dinghy, to fixing the rot on our back porch. Good (but smelly) stuff. Every piece of wood I use in an outdoors project gets some of it. Just keep the fume respirator handy!

 

[Boomyal]

Re: How biga hole should I drill???

Originally posted by Lou C:<br /> That CPES is some good stuff isn't it??<br />I used it on everything from my floor repair on the FW last year, to building a mount for wheels on the transom of my dinghy, to fixing the rot on our back porch. Good (but smelly) stuff. Every piece of wood I use in an outdoors project gets some of it. Just keep the fume respirator handy!

I'm hooked, Lou. Liquid gold it is but I will have to always keep some in inventory. Like you, I use it for everything.

 

[qaztwo]

Re: How biga hole should I drill???

Hey Boomyal I did not drill any cross holes, thought about it though. I did a test on a 6x6 and the same bolt. Drilled a hole in the timber palced the bolt and epoxy mix in and left it alone for a week. Then placed the timber on some blocks and using a chain and washers hung about 400lbs on it, ran out of weights or I would of seen how much it would have taken to destroy it. After that figured no cross needed. By the way I also soaked evrything in cpes also. And it does stink.