View Full Version : Silicone Bronze Screw Breakage

Peter Jacobs
07-02-2001, 09:49 PM
WHile building the Acorn I've managed to break off a few silicone bronze screws. At the time, I put this down to maybe being a bit heavy handed with the screw driver. I noticed they all seem to break where the thread meets the shank.

Low and behold, on page 24 of the latest issue of Wooden Boat, Rollin Thurlow is having the same problem.

Is anyone else having this problem?

(BTW ... the floors are done http://media5.hypernet.com/~dick/ubb/smile.gif

Ed Harrow
07-02-2001, 10:19 PM
It is, quite obviously, a conspiracy by the united forces of fiber, glass, and resin... Are you able to get them out? Have you tried? Left-handed drill bits might be an aid tho a bit hard to get. McMaster-Carr would be a source.

You are, of course, lubing the screws...

07-02-2001, 10:52 PM
I had the same problem when I began to use the square drive screws in smaller sizes (#6 , #8). The square hole just leaves very little material at the top of the shank. I simply had to re-adjust my thinking on how much torque to apply in tightening the screw. Also, I never use a power driver for the final torqueing. Lubrication can help, but not prevent breakage. All kinds of screw extractors are available at machine supply houses.

Cecil Borel
07-03-2001, 08:51 AM
Peter: I had good luck installing silicon bronze screws if I prefitted a blue steel screw of the same size first. When I was worried about breakage, I would drill for the standard size steel screw, tighten it down, then remove it and install the silicon bronze screw. It was more trouble to install, but less trouble than removing broken silicon bronze screws, especially in critical locations.

Alan Peck
07-03-2001, 09:10 AM
I don't know if this is part of anyone's breakage problems, but I think it is best to use Bronze screws that have cut threads with a full-body shank. Many modern screws use rolled threads with a reduced shank.

In addition, I have found that when using tapered drills, that the catalog listed size is not necessarily the best size. I experimented with various sizes until I found what I thought gave the best fit for various size screws.

07-03-2001, 09:21 AM
Alan is 100% on the money, there are bogus screws out there. As he describes, you need to use the screws that have the deep cut threads and the large root diameter. When placed side by side the bogus screws are easy to identify, shallow threads with a tapering root diameter. As Alan mentioned, experiment with drill sizes, and shapes. For some applications you may want to use a straight drill with the preferred full root diameter screws.

[This message has been edited by RGM (edited 07-03-2001).]

Ed Harrow
07-03-2001, 09:24 AM
There is so much crap hitting the shelves of our hardware suppliers these days, largely from the PRC. I bought a couple of shackles and the damn things close up when the pin is tightened! I wouldn't be surprised if these screws come from the same source.

07-03-2001, 12:54 PM
I was getting ready to order a batch of McFeely's "boatbuilding" screws, which are made in China. McFeely's says: "Designed specifically for boat building, these Silicon Bronze screws have cut threads and the full-body-diameter shank preferred by boat builders. Unlike our rolled thread steel screws with their reduced shank, the shank diameter below the head is the same diameter as the outside of the threads." Any experience with these?

H Downey
07-03-2001, 01:12 PM
It just doesn't pay to be cheap on your screws. Jamestown Distributors has very good prices (and service) on high quality cut thread, full shank, square drive silicon bronze screws. Last time I looked, McFeely's wasn't even close on price!

I have used a few hundred #6 and #8 over the past year with no problems. I use the three-step drills, which have a straight drill for the root, a wider section for the shank, and a countersink/counterbore. I had two cordless drills, one to drill, one to drive. No breakage, no stripped holes, no sweat. Working against epoxy setup time, the last thing you want is a lot of fussing around.

07-03-2001, 06:01 PM
Last fall I bought a box of McFeely's "Made in China, Boatbuilding, Flat Head, Square Drive, Silicon Bronze"... #8 x 1". Paid $71.57 for a thousand, have used over half and have had no problem. I do predrill, but use a power driver and can't recall busting more than two or three, and that was into locust.


07-03-2001, 06:09 PM
Anybody good enough with computer pix to draw what you mean by root diameter, all that? Diagrams of the good the bad and the ooogly? I have run into what look like cheapo SB square drive #6 -- the threads go right up to the head, and they look kind of like sheet metal screws -- not like the old stuff.

Ken Liden
07-03-2001, 08:43 PM
Look in the archives. We have had this discussion before. There is no doubt that cut threads are much better than rolled threads. I use bees wax to lube the threads and always use a tapered bit.

There are plenty of screw manufacturers here in the US. You don't need mail order folks like Jamestown or Hamilton. Simply call your nearest fastener distributer listed in most yellow pages. You will be amazed at the stuff that is available and in sizes you only dreamed of.

Ed Harrow
07-03-2001, 09:20 PM
WoodenBoat's book, Planking and Fastening has an excellent chapter on screws. Well worth reading, probably more information than one really wants to know (sure was for me!).

George Roberts
07-03-2001, 09:22 PM
I don't know what the design strength standards of the silicone bronze screws are but ...

Let's assume that in times past the strength exceded the design standards by 15% and you torqued them 10% above the design standards. No screws failed.

Let's assume that in times current the strength exceded the design standards by 5% and you torqued them 10% above the design standards. All screws fail.

If this is true, you are at fault not the screws. I don't intend to place blame.

The proper way to use screws (or any materials) is to test each batch that comes in to determine the breaking strength (you can use the design strength if that is easier) and then limit your torque to some fraction of the actual breaking strength.

I understand that is a bit hard to do for the home builder but it is necessary. Several people told you how to do that. I think one way was to prethread the hole with a steel screw (you can purchase actual tools shaped like screw threads) and then set the clutch on your screw gun to a light setting.

A second method is simply to use screws that meet your strength needs. Most screw machine companies will run a batch of 50,000 for you. If you give them the strength requirements, you will be happy with the results. If the size is standard, the cost should be reasonable.

Ross Faneuf
07-03-2001, 10:26 PM
One thing to look at carefully is the way you're drilling for the screws. I got a great bit of advice from a builder years ago: if you're breaking screws (bronze), you're not drilling your pilot hole deep enough. Actually, he was more emphatic - you shouldn't have to lubricate, and if anything breaks you're not drilling deep enough. I always use a setup which drills as deep as the length of the screw (I use tapered drills, except for 3/4" and under), or a dight further. I can't remember when I last broke a screw, and they certainly hold well.

The point here is that force you're puttting into the screw is driving the threads into the wood, which is where you get your holding power - not into forcing apart the fibers at the tip of the screw. If you want to do that, use a screw that was designed for it - like a drywall screw. But now we're not talking bronze any more. A conventional wood screw, particularly in hardwood, should have a pilot hole which relieves the tip of forcing itself through the wood.

I would like to make a bit of a point of this: I don't lubricate screws, and think it's bad practice - at least in the woods I usually use. I don't use oak, so I can't comment on that. But mahogany, cherry, teak, tulip poplar, white pine, and both fir and okume plywood need no lubrication.

Any time you can drive a screw hard enough to begin to crush the wood under the head you've driven hard enough. I try to stop at just that point, but I probably overdrive more often than underdrive.

I'm working mostly in SA mahogany, and driving with either a screwdriver bit in a brace, or (preferably) cordless driver with Frearson bit. Lengths 3/4 to 4", #6 to #16. I actually prefer Frearson to square drive; it's only me, but I can't get along with square drive, and strip them all the time.

[This message has been edited by Ross Faneuf (edited 07-03-2001).]

07-04-2001, 01:28 AM
I was using the square drive with rolled threads and broke a few rather easily, chucked them away and went for cut thread screws and no more breakages!!!
The cut threads have stronger shanks, the rolled threads seem to have a diminished size in the shank and straight thread. Oh yes I also used a tapered drill bit, this made the job easier.

Paul Frederiksen
07-04-2001, 02:24 AM
I think there may be a problem with a batch that Jamestown bought. I have been using their screws for two years with no problems. Then the other day I bought some and they twisted off with almost no torque. I examined them closely and found that the threads were cut too deep high on the shaft and down at the point they were cut too shallow. The whole box was like this. The shaft, not counting the threads, was actually tear drop shaped with a large bulge down about a quarter of the way up from the point. The screws are worthless and I am sending them back with a complaint.

07-04-2001, 07:04 AM
Tried to buy some 20 X 2 carriage bolts from Jamestown the other day. The 2 1/2 were cheaper than other smaller sizes. Jamestown said they were not available because they sent their entire inventory of that and another size carriage bolt from China back to China because of inferior construction causing customer complaints. Maybe your screws were among the Chinese inventory. Paul did they seem a little cheaper than others on the price list?

Syd MacDonald
07-04-2001, 09:39 PM
See wb no. 161, page 24. It seems Rollin Thurlow has been having the same problems.

Bob Cleek
07-05-2001, 03:41 PM
Shows to go ya... I haven't seen anything from the People's Republic of China that wasn't junk. Time was, the same could be said of Japan, until they started building consumer electronics and cars, so maybe there's hope. Bottom line... don't buy PRC exports. What are we sending them that we really need to sell? Pepsi and Marlboros? I don't think so. Only when their trade ministries get smart and realize that the US market demands QUALITY, will it be worth out time to consider imports. Meanwhile, look for the Union label! LOL

07-05-2001, 05:27 PM
To "pwilling": Look in the Jamestown Distributors catalog on p6 -- they have a drawing of a wood screw with terminology. The shank is the part of the screw next to the head, with no threading. The root diameter is how thick the screw is in the threaded part after the threads are cut.

07-07-2001, 12:04 AM
Allen is dead on.If you bought the square drives from Mcfeelys,be sure they are the boat screws.Secondly,be sure to torque them carefully as the square drive system allows for alot of torque because the screw will not easily cam out like so many others.

Don Danenberg
07-07-2001, 09:34 PM
The quality of the Chinese Silicon Bronze has suffered of late, and should for the forseeable future. Clark Craft Fasteners has recently pulled 92,000 of these from their shelves, after purchasing a 'hardness tester', after I and associates complained about some 20,000 fasteners.
The problem with these properly-cut fasteners was the softness of the wire used in their construction.
Its just too d@m=d bad that Jamestown Distributors so recently tripled their prices with the new ownership.

07-11-2001, 05:58 PM
Yea! I bought a tapered drill bit set
imported from China. Put them in the drill
and they wabbled. They were bent? How the
heck do you bend a drill bit? Must have been
tapered asymetrically.

Scott Rosen
07-12-2001, 12:47 PM
Speaking of tapered drill bits . . .

Am I the only one here who gets better results with straight bits? It seems that the taper in the bit never quite matches the taper of the screw and either results in a hole that is too tight or too loose. I find that if I use a straight bit just slightly narrower than the shank diameter, I get the best results. The only thing worse than breaking off the head of the screw is driving the screw home and finding that it just keeps spinning and spinning and spinning. Or so I've heard. http://media5.hypernet.com/~dick/ubb/wink.gif

Ed Harrow
07-12-2001, 04:15 PM
Scott, the WoodenBoat book on planking has quite a dissertation on that very subject. I forget who wrote the article, but it is exceedingly well done.

Stan Derelian
07-12-2001, 10:59 PM
Throw my two cents in here. Just having the same problem. First 1500 screws had what seemed like a lot of breakage, but thought it was just my amateurish technique. Re-order of 300 from same place (who had themselves reordered from their supplier) were worse. Finally ordered some from a supplier in Toronto, and their was no breakage. They are the same size (8's 1.5in), but look different. The good ones seem to have a thicker core diameter that doesn't taper quite as quickly. Also, the finish is different. The bad screws have a smoother, shinier surface finish.

Is anyone getting a refund?

Mike Keers
07-12-2001, 11:21 PM
I have been experiencing about 10-15% breakage, in both #8 and #12 out of a lot of 600 screws. I was quite interested when I saw the letter in WoodenBoat, then the thread here.

After it had played itself out a bit, I contacted Hamiltom Marine, who I'd bought from. I presented them with "the evidence", and they said they had recieved one other complaint, plus a WB editor had been in the store and mentioned the problems many of us were having out here.

David Normann, from Hamilton, has replaced the broken screws, on my word, at no cost. He also confirmed that it seems the problem is in a bad batch of Chinese screws, and there's little the retailers can do but pressure the importer to pressure the manufacturers.

It's hard to "buy American" when all we do is push burgers in this country these days, eh?

07-16-2001, 12:40 AM
Hey Scott, have you tried the three step drill/countersinks described earlier in this topic by H Downey? If you are a straight drill fan, as I also tend to be then you will probably like using these if you haven't already. They're a another good thing to have rattling around in your tool box. A number of outfits manufacture them.