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Tech Tip...holes for flatheat/countersunk screws

lbhsbz

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As I'm de-rigging the back of my new-to-me Lavey 18'10", I'm finding that most of pieces fastened using flat allen head screws are fucked up. The tunnel tabs for one...and I've noticed on a lot of boats I've played with, the tab braces always seem to be loose due to wallered out holes.

Here's why. SAE flathead screws typically use a 82° angle on the underside of the head...not 90°, not 100°. I've measured a few of the holes that are not fucked up, and they were all countersunk to something other than 82° by whoever made these parts. If the angles don't match, we will have effectively a point load situation which...especially in aluminum, will become loose over time and under load. Use the right cutting tool, unless you're dealing with wood, then it doesn't matter.

I'm making some new side plates for the outboard bracket today (to shorten it so I can run a hyd. jack plate). I have a couple 82° countersinks, but of course, they are nowhere to be found...so we have to wait for McMaster to show up tomorrow.
 
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wsuwrhr

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Yeap, machine screws(flat and phillips) are usually 100deg, flat head "allens" 82deg.

Its only 8deg difference, what is the big deal? (insert roll eyes here.)

Just give it an 82 deg kiss, done. :)

Brian
 
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lbhsbz

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Yeap, machine screws(flat and phillips) are usually 100deg, flat head "allens" 82deg.

Its only 8deg difference, what is the big deal?

Just give it an 82 deg kiss, done. :)

Brian

Big deal is that if one expects an assembly to stay together correctly with dynamic loads on it, that weeee little ring of area where the underside of the bolt head contacts the thing it's supposed to hold against another thing will get larger and larger as the joint becomes looser and looser...until it's just all fucked up. Perfect example is tunnel tabs on small outboards (like mine)...about 1/2 I've seen have the little turnbuckle mounts loose on the tabs, even though the bolts/screws are tight. It's because the hole is so wallered out that the underside of the bolt head is bottoming against the mount, rather than squeezing the plate against the mount, because there's not enough hole left.
 

wsuwrhr

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https://www.merriam-webster.com/dictionary/satire

:)


I figured you would "get it" ;)

Brian


Big deal is that if one expects an assembly to stay together correctly with dynamic loads on it, that weeee little ring of area where the underside of the bolt head contacts the thing it's supposed to hold against another thing will get larger and larger as the joint becomes looser and looser...until it's just all fucked up. Perfect example is tunnel tabs on small outboards (like mine)...about 1/2 I've seen have the little turnbuckle mounts loose on the tabs, even though the bolts/screws are tight. It's because the hole is so wallered out that the underside of the bolt head is bottoming against the mount, rather than squeezing the plate against the mount, because there's not enough hole left.
 

ROY123

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This essentially turns your security screw into a flathead building screw. Then you canremove the screw with a wide-blade flathead screwdriver [source: Klimek]. Creating a Grip An easy way to remove a one way screw is to create a strong grip on the head of the screw. This can be done by using locking pliers around the screw.
 
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