Gudgeon bolts rusted out: how to extract?

Thread starter #1
I noticed that the rudder on my Sunfish felt loose and when I looked at the gudgeon, I found that the stainless steel bolts on the upper aspect of the fitting had corroded away and broken internally such that although I could see the screw heads on the outside they were no longer connected with the shaft of the bolts.

There are four bolts altogether, and the two bolts on the lower part of the gudgeon are intact.

What apparently happened is that the "stainless" bolts corroded away and broke off so that the remaining portion of the bolts within the hull is now flush with the transom.

Does anyone have any tips or ideas as to how to remove the broken portion of the bolts within the hull? Any suggestions would be welcome since the rudder is obviously a key part of the boat and I don't want to screw it up.


Well-Known Member
Chances are good that the lower fasteners don't have long to go, either. Back out the lower fasteners to see what exactly was installed.

Chances are also good that the wood backing is strong enough to support new fasteners. I'd leave the old rusted fasteners in place, and seal all four fastener holes. Raise the gudgeon a ¼-inch, drill new holes and replace with 2-inch S/S sheet metal screws. Go sailing! :cool:

But when my Clonefish rudder fell off, I cut an offset 6-inch inspection port, replaced all the fasteners with S/S bolts.


From information that member mixmkr posted on this forum, I'd today cut access holes in the bottom of the hull, fix the gudgeon's fasteners, and epoxy the holes closed. The rear upper deck remains strong, and the hull-bottom is stronger-yet.


Active Member
1. Fast Solution - Drill holes below broken bolts and install pop rivets.
2. Good Solution - Install inspection port and replace backing plate or try to remove studs with vice-grips.
3. I don't want no stinking holes in my boat Solution - Pop the rear deck.

beldar boathead

Well-Known Member
Pop rivets seem unwise for holding the rudder on. If it was a good idea, the Sunfish builder would have done it years ago as a cost-cutting measure. Drilling stainless is not fun, but you can drill new clearance holes in the stern bracket, then smaller holes in the backer that you will thread and use new stainless screws to attach the bracket to the backer. Or do as Webfoot says in option 1, and install a port as he says in option 2 to thru-bolt the bracket and plate. This is probably the easiest. If you do this, I would try to get the other 2 screws out before they give way and replace with fresh Stainless items.

Popping the rear deck is excessive work. Having an inspection port back there is handy for drying the boat, storing stuff, etc.


Active Member
A 3/16 pop-rivet has a tensile strength of 320 lbs and shear strength of 260 lbs. If
that's not enough a 1/4 rivet gives 1240 lbs tensile and 1000 lbs shear.


Well-Known Member
Correction: there is no room to raise the gudgeon. Leave the plate where it is, and drill four new holes instead.

A threaded rear plate would mean removing the bad bolts by using a left-hand-twist bit. Note the length, and go a ¼-inch longer. Use new bits to drill stainless—"Cobalt" is best, but any new bit is essential. Trying to drill stainless with a dull bit could "drill-start" a fire!

Then locate (or make) a "self-tapping" replacement bolt. ("catching" the old threads would be difficult—start new threads).


beldar boathead

Well-Known Member
A 3/16 pop-rivet has a tensile strength of 320 lbs and shear strength of 260 lbs. If
that's not enough a 1/4 rivet gives 1240 lbs tensile and 1000 lbs shear.
I'd be more worried about the rivet deforming in the hull as opposed to the thing shearing off or pulling in two. And I would prefer to not have quarter inch holes in my hull. Plus rivets are hollow and in the water a lot. If you try rivets, let us know how it works - it could be a good solution.
Thread starter #10
Foulweather Jack what year is your sunfish?
Thank you for all the suggestions. My Sunfish is relatively new: a 2013 model. I took the remaining stainless steel bolts to West Marine today and found that the bolts are actually metric o.7 threads (3/4" long). I did buy some bolt extractors from Home Depot and I'm considering drilling out the old bolts with a bolt extractor. With luck I should be able then to simply install new stainless bolts.

I sprayed the old bolt remnants that are lodged within the transom with an anti-corrosive/bolt loosening product today. I'll let that work for a day or two before I try to drill out the old bolts.
Thread starter #12
For what it's worth I've posted above a photo of the transom after the gudgeon was removed. The photo is upside down, so the holes at the bottom are closest to the deck: these are the ones with the remnants of the stainless bolts that are embedded within the hull and flush with the transom.


Active Member
Pop rivets are not hollow, the pull-shank remains in the rivet hence they to not
leak water. No they do not deform the material being used and this includes riveting
together sheet metal. We use the same concept to to make repairs to aircraft except
they are Aircraft grade and called Cherry-Lock rivets. Pull rivets are used in blind holes
when you can't get at the back of the rivet to buck it.


Upside down?
Staff member
Interesting (?): there's a second boat ID (under the gudgeon) on newer boats such as the one shown above (built year 2012).
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Well-Known Member
Unless these were in subjected to severe saltwater use, nothing here tells me the old bolts are stainless—which would not react to a magnet—or only very slightly.

In stainless, should a bolt extractor break off, removing that would be much trouble. :(

My new extractor set:

Fullscreen capture 412017 55004 PM.bmp.jpg

I wouldn't trust this quality of extractor (those on the left) to work effectively, and there are much poorer quality extractors out there.

Even a new left-hand drill bit will dull its cutting edges drilling out one S/S bolt. With any luck, the new LH bit will "catch" and extract the S/S bolt from the plate.

You'd be back to "making holes" in the deck (or bottom) if new holes are drilled in the gudgeon. It'd be best to clamp the gudgeon into a drill-press and use oil to cool the (new) drill bit.

Interesting (?): there's a second boat ID (under the gudgeon) on newer boats.
'Wonder when that started? I'd guess that'd be for authenticating a stolen/recovery Sunfish claim.
Thread starter #16
I think the old bolts are stainless steel, since the gudgeon itself is also stainless. I live on a saltwater river, and the Sunfish is kept on a floating dock by the river so there's fairly constant salt spray on the boat.

The screws are metric self threading machine screws. At the suggestion of my local hardware store guy, I bought some new titanium drill bits, as well as some new metric stainless steel machine screws, and I've been experimenting with one of the old broken screws to try to drill a pilot hole into it that would accommodate an extractor tool.

Unfortunately the stainless steel is so tough that I'm unable to drill into it even with a brand new titanium 1/16" drill bit.

On the Sunfish, I sprayed the old bolts with some PB Catalyst spray in the hopes of loosening them up for future removal.

Does anyone have a suggestion for a type of drill bit that could drill a pilot hole into the old bolts? I suspect that if I can get a pilot hole within the old bolts I can remove them with a spiral type extractor.
It's really finicky to try to drill out small broken bolts or screws, even if the part holding the screw can be clamped on a drill press table. Stainless steel can be an especially tough material to drill.

I wouldn't bother trying to drill out/remove the old screws/bolts from the transom. I would plug the existing holes in the transom with your choice of catalyzed goop and sand flat, then drill new holes in the rudder gudgeon close to the old holes. Mark new matching holes on the transom and drill through the transom and ss backing plate. Just to be OCD I would add ss washers under the heads of the new machine screws/bolts to more-or-less cover the original holes in the gudgeon. If the inside of the transom is accessible through an inspection port I'd use ss #10 or #12 bolts and ss Nyloc nuts with ss washers under the bolt heads and lock nuts.

A cobalt drill bit (M35 or M42) at slow speed w/ lube is the machinist's preferred way to drill stainless steel. Note: there are quantities of cheap, fake, "cobalt" drill bits on the market. A drill bit with "cobalt nitrate" coating is not the same as an M35 or M42 cobalt bit. A good quality high speed steel (HSS) bit at slow speed w/ lube should work for this job.

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After re-reading your posts more closely I think you were able to remove the bottom gudgeon screws successfully. This obviously makes the repair even easier. You only need two new holes in the top of the gudgeon and through the transom & backing plate. Fastening through the existing bottom gudgeon/transom holes locates the gudgeon for the two new holes through the top of the transom.

The corrosion stains (rust??) behind the gudgeon and the fact that the old fastenings corroded to failure are surprising. Were the old fastenings in fact stainless steel? (i.e. non-magnetic or barely magnetic)?

From the internet:
The most common type of stainless steel is the "300 series", and the most common grades of 300 series are 304 and 316. The big difference between 304 grade & 316 grade is that 316 contains molybdenum, and the addition of molybdenum "... drastically enhances corrosion resistance, especially for more saline or chloride-exposed environments." 316 grade is commonly marketed as "marine grade stainless steel."

It's difficult to impossible to know, but I expect most big box/hardware store stainless steel is 304 grade at best, and might even be "200 series" which is less expensive than 300 series (200 replaces nickel with manganese), but which is also less corrosion resistant than any of the 300s.

It seems safest to use 316 (316L is even better) whenever possible. More ss info at: Stainless steel - Wikipedia
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Well-Known Member

S/S 316 is the recommended grade for radiator hose clamps near salt water.

Anyone know what is keeping the S/S interior backing plate from falling off? :confused:



Active Member
I got a Sunfish that I'm going to mess with soon, I let you know what the
backing plate is made of but I can almost bet with perfect confidence the factory
did not invest in Stainless Steel. I do believe it will just be a piece of scrap
metal of roughly the needed size. Personally, the easy way is to just put
in a inspection port, knock the plate out and buy or make a new one. Use bolt and locknuts,
pop-rivets, a metal tap or whatever you preference is. It's all good.