COUNTERSINK CARRIAGE BOLT HEADS?

What kind, options, etc.

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bobinpowayca
Posts: 400
Joined: Thu Jun 16, 2011 1:40 pm
Location: Poway, CA

COUNTERSINK CARRIAGE BOLT HEADS?

Postby bobinpowayca » Mon Apr 01, 2013 4:00 pm

Hi folks, I'm building the Geronimo - I'm at the point where I'm ready to bolt the transom knee to the transom and keel. Plans call for four 5/16" carriage bolts, nuts on the inside and bolt head on the outside.
I know the idea of the carriage bolt is to sink the square underside of the bolthead into the plywood while tightening the nut. So if you then tighten it up the flat underside of the rounded head would be flush with the plywood, leaving the rounded head (1/8" or so?) protruding above the plywood, which would then leave a big bump when you fiberglass over the bolt head. Also been reading to never countersink plywood.
So, would you just grind down the bolt head to nearly flush with the plywood?
Or, would you first do a flat countersink say with a forstner bit the diameter of the round bolthead, and the depth of the rounded part of the head? That way the top of the bolthead would be flush with the plywood.
First way (grinding) would leave less bolthead, second way (forstner countersink) would leave a little less plywood but would look better. What do y' think shipmates?
Bob
_______________
Built the Glen-L 17 (1988), Geronimo (2018)
PBR support (1968)

Moeregaard
Posts: 351
Joined: Mon Dec 03, 2007 11:31 pm
Location: Thousand Oaks, California

Re: COUNTERSINK CARRIAGE BOLT HEADS?

Postby Moeregaard » Mon Apr 01, 2013 4:28 pm

Bob, on our Zip build we used a Forstner bit to counterbore the transom just enough to keep the bolt heads slightly below the surface. We went a little deeper with the counterbore on the keel batten, so that we wouldn't "rediscover" the bolt head during the fairing process. This isn't much of a problem aft, but when fairing the keel to the stem, you may remove a significant amount of wood, and getting those bolt heads well below the surface will prevent having your plane iron discovering said bolt heads. I think we counterbored about 50% of the way through the batten and this gave us no trouble when fairing everything. Put your counterbores in first, so that your Forstner bit won't be trying to center itself in a 5/16" hole.

-Mark Shipley
A boat is just a wooden box with no right angles.

bobinpowayca
Posts: 400
Joined: Thu Jun 16, 2011 1:40 pm
Location: Poway, CA

Re: COUNTERSINK CARRIAGE BOLT HEADS?

Postby bobinpowayca » Mon Apr 01, 2013 7:06 pm

Thanks Mark. Yeah, good tip on doing the counterbore first. Bob
Bob
_______________
Built the Glen-L 17 (1988), Geronimo (2018)
PBR support (1968)


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