Countersink/Bore Angle: 82 or 102 Degrees

[KAIROS]

This is a heads-up for those Americans restoring European boats or vice-versa. Or, maybe this is unique to my boat since I have'nt seen any reference to it here on the Forum (even when I use the advanced google search, Thorne, I swear). It must be here somewhere though.....I can always erase the thread after I'm humiliated for not knowing this before it happened to me. Anyway:

After we'd stripped all the deck hardware off ABLE (built in Scotland in 1963) we wanted to replace most of the fasteners because many were bent from stresses, others coroded. Most deck fasteners were bronze or stainless machine (bolts) or wood screws. Some galvanized.

Most reasonably-priced flathead fasteners sold in the USA have heads which are shaped with a standard 82-degree angle for the countersink head. Most of our deck hardware was custom fabricated by the Scottish builder and had 102-degree angle counterbore made for the builders custom-machined 'European' flathead machine screws, or wood screws with the 102-degree head.

[edit: The exact angle (102) is not terribly important in this discussion. The point is that my boat had fasteners with counterbore angles greater than 100 degrees and sometimes 120]

Using 82-degree countersink fasteners (US) in 102-degree counterbored fittings (European) doesn't give the best result. Obviously the fastener head does not mate quite right with the fitting counterbore. In the case of METAL fittings, the fastening can't perform as well as designed. Some robustness is lost when the forces are focused rather than spread on the mating surfaces. And, the gap around the head is less weatherproof even with bedding compound. In the case of WOOD fittings such as wood cleats, the additional problem with the 82-degree fastener head is that it becomes a wedge which can split the wood more easily than when the angles match.

To minimize problems we chose to use an 82-degree counterbore (Fuller bits) to modify the counterbores of the existing wood fittings and ordered the new metal deck hardware with 82-degree counterbores (which is the default for the fabricators in the US). In some cases we modified the counterbore on metal fittings we re-used.

So, watch out for this. As usual with damn old wood boats, they don't come with an owners manual. This 82 vs. 102 issue becomes serious particularly if you ignore it and start splitting your wood cleats and rails, etc.

Jack

[Canoeyawl]

Re: Countersink/Bore Angle: 82 or 102 Degrees

I wouldn't have noticed it right off the bat, thanks for the heads-up.
It's not to much of a job to regrind the cutters on a Fuller bit to the correct angle.
It can be done by hand well enough for most work, or any machine shop with a tool and cutter grinder should be able to do it.

[Lew Barrett]

Re: Countersink/Bore Angle: 82 or 102 Degrees

Makes sense...but only now that you mention it! Just curious Jack: do you know if the British have/had a standard for this but the continental countries another and the US a third as sometimes happens with tooling?

[donald branscom]

Re: Countersink/Bore Angle: 82 or 102 Degrees

I would just stick with the 82º standard bits drill and countersink.

With a metal screw in wood that last 18-19º is just going to get crushed down anyway.

But if you are worried about a counter sunk screw splitting the wood then I would just use a bit made to put a plug in the hole with a shoulder.
Like this.

[KAIROS]

Re: Countersink/Bore Angle: 82 or 102 Degrees

Makes sense...but only now that you mention it! Just curious Jack: do you know if the British have/had a standard for this but the continental countries another and the US a third as sometimes happens with tooling?

Don't know. I'll bet somebody here with vast knowledge will chime in (I am tempted to say, a person whose head has a 360-degree understanding of this fastener head counterbore angle issue).

Wikipedia says: Countersink cutters are manufactured with six common angles, which are 60°, 82°, 90°, 100°, 110°, or 120°, with the two most common of those being 82° and 90°. Countersunk-head screws that follow the Unified Thread Standard very often have an 82° angle, and screws that follow the ISO standard very often have a 90° angle. Throughout the aerospace industry, countersunk fasteners typically have an angle of 100°. The ideal countersink angle for holes tapped with 60° threads, when no countersunk fastener head will sit in the countersunk area, is often 60°; but often another angle is used if that is the cutter that is at hand, and the difference usually doesn't matter. [I assure you that it does make a difference, especially for wood fittings/components.]

Note the way the angle is specified:



Note that I am using the number 102 here, and it is not shown. It is manufactured though. Mine might be 100 or 110 in reality. The 102-degree and greater angle head fasteners are marketed as an option because they spread the load over a greater surface. So for high tech engineered items, they may have an advantage.....especially for thin materials.

The bolts that held my chainplates down to the deck had an even greater angle, probably the 120-degrees. My chainplates are flat bars running fore & aft bolted down to the deck (through below-deck timbers), so the upward force is all on the fastener heads. The builder (McGruers) was noted for this kind of thing......the flatter angle (120) spreads the load out the most. They had contracts with the Royal Air Force and seem to have let their engineering run wild sometimes.....or maybe they had a bunch of RAF fasteners left over and used them on my boat? It's rather high tech for a 28' cruiser/racer...but they did also build some highly competitive racing machines too. It seems ridiculous to me, and in fact they were ridiculously meticulous builders/designers. [They built all the laminated hollow ROUND wood spars used in RAF planes]

It is fairly obvious when you just look at the fasteners side-by-side.

Worst case for me was when I had some fittings regalvanized before realizing the issue. Just had to change the counterbore and re-galvanize again. Wood and bronze no problem. Could have just bought the 102 or 120-degree-head fasteners, but I didn't want to perpetuate the issue and have a mix (replaced every stink'n fastener except planking/structural).

Also note that we have it easier than the Europeans for a change. If I were there and restoring a US-built boat with 82-degree counterbores, machining them to the greater angle of my country's fasteners would not be an option (imagine it in the diagram). You can only go from higher to lower angle easily.

[KAIROS]

Re: Countersink/Bore Angle: 82 or 102 Degrees

I would just stick with the 82º standard bits drill and countersink.

With a metal screw in wood that last 18-19º is just going to get crushed down anyway.....

Yes, the fastener issues on a boat are maddening enough. Keep it simpler with one standard. Wish the builder wasn't so anal on this issue (but I appreciate their obsessions in other parts of the boat).

Mahogany did seem to crush enough for it not to be an issue except in small parts with larger fasteners. Teak will definitely split though.

[donald branscom]

Re: Countersink/Bore Angle: 82 or 102 Degrees

Just use the standard 82º drill.
I have never heard of the 102º angle. Maybe a bit that was over sharpened???

If you are worried about countersunk screws splitting the wood, then use a forstner bit and make the head of the screw against a square shoulder. Makes more sense to me.

A 82º angle screw would probably just crush down that other 18º angle countersink anyway. Its wood and wood crushes. Metal is a different story.


Also some of those other screws with different angles may be made , but WHERE are you going to get them???

[KAIROS]

Re: Countersink/Bore Angle: 82 or 102 Degrees

.....A 82º angle screw would probably just crush down that other 18º angle countersink anyway. Its wood and wood crushes. Metal is a different story.

Just to reiterate, since our mssgs. must have passed each other in cyberspace, a harder wood like teak will definitely split in cases where mahogany might not. I quickly decided to just re-bore everything to 82º and not test my luck.

[donald branscom]

Re: Countersink/Bore Angle: 82 or 102 Degrees

Just to reiterate, since our mssgs. must have passed each other in cyberspace, a harder wood like teak will definitely split in cases where mahogany might not. I quickly decided to just re-bore everything to 82º and not test my luck.

Sounds good.
I wonder if you could get allen head wood screws so the screw head would fill up the hole.

[KAIROS]

Re: Countersink/Bore Angle: 82 or 102 Degrees

I should clarify.....too long-winded:

The issue is simply this....if your boat was built in some foreign land, the countersink angle on the boat's flathead fasteners may differ from the typical angle for fasteners commonly available in your country now. When refastening with new fasteners, the fastener head may not match the counterbore of the original metal or wood item to be fastened.

This miss-match causes minor problems for metal items as long as the head/counterbore miss-match is minimal. In the case of softer wood items to be refastened, this mismatch is probably not significant since the wood will deform/crush to accommodate the miss-match. In harder woods, such as a teak rail or cleat, the miss-matched fastener head may cause the wood to split when the fastener is tightened.

One fix is to re-bore the fastener hole to match the new fastener's head. Or, find fasteners that match the existing holes on your boat.

Enough said for me....though glad to debate more.

[Lew Barrett]

Re: Countersink/Bore Angle: 82 or 102 Degrees

I
Enough said for me....though glad to debate more.

That'll do