Cotter Pins--all you wanted to know about them

[jimh]

To properly install a propeller on an Evinrude engine you need a cotter pin. The general thinking is that once a cotter pin has been bent to retain it in place, you should not unbend it, remove it, then reinstall it and re-bend it. Or at least not more than once.

When changing propellers recently on my Evinrude E-TEC 3.3-liter V6 225-HP engine, I was out of new BRP OEM unused cotter pins. I resorted to re-using a cotter pin for a short term test run. Normally I just just buy a few more OEM cotter pins the next time I visit my favorite Evinrude dealer, LOCKEMAN'S HARDWARE and BOAT in Detroit. But the problem was I was about 290-miles northwest of Lockeman's, and even the closest local Evinrude dealer was going to be a 70-mile round-trip. Driving 70-miles to buy a cotter-pin for $1.05 (that is the OEM price for Evinrude part number 0306394 for a stainless steel cotter pin, the specified cotter pin for my application) just did not make sense.

Fig. 1. A typical cotter pin or split pin, as used with Evinrude propeller installation.

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A trip to the local hardware store, less than a mile away, and carrying a sample of the correct cotter pin, allowed me to find that a typical hardware store cotter pin of dimensions 1/8-inch x 1-1/2-inch was a good match for the OEM part, other than it was not stainless steel but just galvanized steel. For freshwater boating, particularly cold, northern Great Lakes freshwater boating, I think galvanized steel in the cotter pin will probably endure long enough for me--which will be until the next propeller test comes along.

Regarding cotter pin dimensions, the 1/8-inch value is the distance across the pins themselves when the pins are held tightly together. The 1-1/2-inch value is the distance of the shorter pin, measured from the end of the pin to beginning of the bend for the eye at the other end. I carefully measured an OEM cotter pin and determined to my own satisfaction that it was, indeed, a 1/8-inch x 1-1/2-inch cotter pin.

Fig. 2. A dimensional drawing for measurements of cotter pins.

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My local hardware store had a drawer of these in stock at the outrageous price of $0.19 each. For one dollar I got five new cotter pins.

For purist that wants a stainless steel cotter pin, there are three options:

• find a hardware store bigger than the local shop in my village of 500 residents, a store that sells stainless steel cotterpins;
• get the OEM cotter pin from an Evinrude dealer for $1.05; or
• buy a lifetime supply of 50 to 100 from a big supplier like GRAINGER or HUYETT (see below).

Here is a link to HUYETT's on-line store:

https://www.huyett.com/products/fasteners/pins/cotter-pins?fs_material=Stainless%20Steel&fs_finish=Plain&fs_type=Cotter%20Pin&fs_inchmetric=Inch&fs_effectivelength=1.500%22%20(1-1%2F2)&fs_outsidediameter=.125%22%20(1%2F8)

Note that they are out of stock until about November 2021.

A fella with a bag of 100 cotter pins would have nearly a lifetime supply for just $16. That quantity ought to cover a lot of propeller changing and testing.

GRAINGER sells these same cotter pins but with a price mark-up.

[Jefecinco]

During Army training as a heavy equipment mechanic, a very long time ago, we were taught to bend only one leg when installing cotter pins. We were not taught to bend the short or long leg. Probably because many cotter pins had equal length legs. You would be amazed how many cotter pins were used on 1950s era bulldozers. We were also taught not to reuse the pins, advise that was often ignored.

[jimh]

BUTCH--your comment about only bending one pin of a cotter pin is most interesting. At Lockeman's Boat and Hardware they install the cotter pin on an Evinrude propeller in that manner. Perhaps that is from advice from the factory given during master-mechanic training.

[Jefecinco]

I never understood the "why" of the method. Being a good soldier I just did it that way.

[R DAVIS]

The FAA recognizes one preferred method of installing cotter pins as covered here. There are some optional methods when space does not allow for the preferred method and those optional methods can be found in AC43.13-1.
First and foremost the cotter pin must be of the correct size. If not called out, a good rule of thumb is the 2/3rds of the hole diameter but in any case the cotter pin should fit neatly into the hole, with very little side play.
If you look closely at a cotter pin you will notice it has a long leg and a shorter leg. The long leg is intended to go over the top of the bolt or fastener and the short leg along-side the nut.

1. Insert the cotter pin with the long leg on top.
2. Seat the bulbed end of the cotter pin securely into the nut and bolt.
3. Using duck-billed or needle nosed pliers, grab hold of the upper leg and in one fluid motion bend to a vertical position.
4. Using side-angle cutters (dikes- to most of us) trim the short leg so as not to cut into the structure when bent.
5. Bend the short leg down vertically along-side the nut.
6. Using your dikes trim the remaining long leg so it will lay 1/2 to 3/4 across the end of the protruding bolt or fastener.
7. Bend the remaining leg over top of the bolt or fastener ensuring it is flush so as not to become a snag.
Here are some general rules that can help you in installing and inspecting your cotter pin installation.
a. The prong leg bent down should not rest against the surface structure or washer.
b. Never reuse a removed cotter pin, use a new cotter pin at each installation.
c. If you mess up on the installation, remove the cotter pin and give it another go until it is installed correctly.
d. The prong leg bent over the bolt end should not extend beyond the bolt diameter. This can cause a snag and injure you or someone else.
e. All prongs should be bent over a reasonable radius. Sharp-angled bends can cause breakage. Lightly tapping with a light mallet can assist you in bending the prong legs on larger diameter cotter pins.

Fig. 3. Cotter pin installation on castle nut as used in aircraft fasteners.

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Note that the upper arm of the pin does not extend beyond the diameter of the bolt, and the lower arm does not touch the washer or surface of the object being fastened. Also, in marine applications (even freshwater) all fasteners should be of a high grade of stainless steel. High grade stainless fasteners are available at many ACE Hardware stores, and on-line at Graingers and Amazon.

[Jefecinco]

The solution used in aviation applications appears to be a good one. However, it seems only appropriate to a castle or crenelated nut. Some, if not all, propeller nuts requiring a cotter pin are probably castle nuts. Many others are not. Thanks for the information.

[jimh]

I like the orientation of the eye of the cotter pin as seen above in Figure 3. The cotter pin eye is oriented in-line with the propeller shaft. I think that is how the master mechanic Doug at my Evinrude dealer installs them.

[dtmackey]

The FAA is very focused on fasteners and preventing their ability to come undone for obvious reasons. Even on bolts they restrain them with stainless wire. I bought one of the tools for doing this and it's come in handy many times.

The FAA recommended procedure for an outboard is excessive and just makes it more difficult to remove.

The FAA frowns on things falling out of the sky.

http://www.boltdepot.com stocks a large assortment of cotter pins and has great pricing on all fasterners. They are my go-to source for restocking my bins.

D-

[Roamad]

I would highly recommended sticking with a stainless cotter pin — based on a bad experience I had.

Let me share the story of the thousand-dollar cotter pin.

Two years ago we had just purchased a houseboat with sterndrive propulsion. We were on our maiden voyage, moving the boat from the marina where we had purchased the boat to her new home marina, a distance of maybe 15 miles downriver in the California Delta.

The trip went smoothly until we had to wait for a drawbridge to open towards the end of the trip. Vehicle traffic over the bridge was heavy, so we had to wait awhile. This required us to do a lot of maneuvering in the narrow channel — circling and constantly shifting from forward to reverse and back to hold our position. Suddenly the transmission stopped shifting and we were adrift. The wind and river current began pushing us into some rocks. So we had to throw out two anchors.

Not knowing what went wrong and having few tools aboard, we decided to call for a tow. Three hours and $700 later, we were towed to a waterfront restaurant downstream that allowed us to leave the boat overnight at no charge.

We lost one of the anchors at the place where we went adrift. We marked it with a float and meant to go back and recover it. But someone else took it by the time we returned. Replacement value: About $150.

We returned the next day with a mechanic, who quickly determined the cause was a rusted cotter pin that broke on the shift linkage inside the outdrive. After I got over feeling stupid for not figuring this out myself, it was a quick and simple fix. The mechanic’s work cost me $150.

Moral to the story: Paying $1 for a stainless cotter pin is worth every penny. Also: Carry spares.

[fno]

Hi guys, for what it's worth, Graingers and others are a crap resource compared to McMaster-Carr. JimH's 1/8" x 1-1/2" cotter pin is available as part number

https://www.mcmaster.com/98355A150

for the extravagant price of $10.36 for a package of 25 pieces. Keep in mind that these are not zinc plated, nor are they galvanized, and for the purists they aren't even 304 SS. These are 316 SS (which is the most corrosion resistant except for 316L) that can be had for a real world price. Keep in mind that McMaster-Carr has warehouses in Atlanta, New Jersey, and Chicago and normally can ship your order to you within a day or two. Shipping might be $9.00 or less.

Aside: I tried to keep my post above light hearted in my recommendation of McMaster-Carr. As most of us are Boston Whaler owners, we tend to be particular about the quality and reliability of our efforts to keep our vessels shipshape. Generally speaking, the local hardware, Home Depot, and Lowes have less than suitable hardware (304 SS, 308 SS from china) for real world use on our boats. Yes, McMaster will cost more but what you get in return will be more than satisfactory. I say this from 38 years of using them as a sometimes daily supplier of many things in my work and leisure pursuits. Take a stroll thru their website and I can almost guarantee that you will find something that you need or needed that nobody seemed to have anymore, in stock or available to order. That's the way McMaster-Carr rolls.

[jimh]

Hi guys, for what it's worth, Graingers and others are a crap resource compared to McMaster-Carr...

Hi Frank--thanks for the endorsement of McMaster-Carr. Although I have mentioned McMaster-Carr before as a source for some unusual components (like special quad-ring circular rubber seals for hydraulic steering helm pumps), they were not on my primary radar as a source of cotter pins.

I usually just buy the cotter pins from my Evinrude dealer. But these days the notion of making a 50-mile (or longer) round-trip to buy just a few cotter pins seems to be a bit crazy.

My local hardware store in our little village of 500 residents is less than a mile away. Things are rather laid-back up here, as they're only open Monday through Friday, 8 a.m. to 5 p.m. But in a pinch, I'd rather put in a new cotter pin than un-bend, remove, straighten, re-install, and re-bend a used cotter pin.

[jimh]

Let me share the story of the thousand-dollar cotter pin.

Your account of the problems you encountered on the initial voyage with a used boat with sterndrive propulsion is more or less an adaptation of the old proverb that begins, "For the want of a nail..."

Now, somewhat ironically, a bent nail could be an acceptable temporary substitute for a cotter pin.

Paying $1 for a stainless cotter pin is worth every penny.

I have no objection to paying $1 for the OEM cotter pin for my propeller retainer cap, and I have bought many that way. What prompted my inquiry on the cotter pin was I was down to my last one and was looking for a local source that did not require a long drive.

The government allows a business deduction of about $0.57-per-mile for travel. So driving 70-miles to get a new OEM cotter pin would represent a reasonable cost of about $39.90. That is a lot of overhead cost to get a cotter pin that I may only use for about a week or two before I put on a different propeller.

Carry spares.

I have a bag of about 20 cotter pins of various sizes in a waterproof bag in a waterproof container on the boat.

[jimh]

I recently installed a new propeller on the engine propeller shaft, and I used an alternate method for the cotter pin installation. I illustrate the technique below in Figure 4.

Fig. 4. Cotter pin installation with the orientation of the cotter pin bends in-line with the propeller shaft.

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As shown above, the shorter pin was bent about 180-degrees and folded over the end of the propeller shaft. The longer pin was then cut-to-length so that it would make a nice 90-degree bend and be a flush fit onto the end of the spacer. This method produced a better fit of the keeper washer onto the propeller nut. There is no looseness in the fit of the keeper washer, and the cotter pin head is nicely wedged into the opening in the propeller shaft.

The cotter pin installation method I had been using is illustrated below in Figure 5.

Fig. 5. Cotter pin installation with the orientation of the cotter pin bends at 90-degrees to the propeller shaft.

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The method illustrated above in Figure 5 is what I would describe as the more common technique.

[Jefecinco]

The method you used certainly looks very professional. It may not do the job better but it's always satisfying to do professional looking work. I like it.

[jimh]

FOLLOW UP on Cotter Pin Installation

In the process of doing some work on my E-TEC 225-HP engine, I took a careful look at the cotter pin that holds the "keeper" fitting in place over the propeller nut. I installed this particular cotter pin in 2021, and after five years in service it was looking a bit suspect. As seen in Figure 6, the keeper was cutting a notch into the cotter pin. Not wanting to loose any parts from the propeller shaft, including possibly a $600 propeller, I decided to replace the cotter pin.

Fig. 6. The cotter pin at age 5, after removing soot. There is wear on the pin and some rust forming.

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As I mention in the initial article in this thread, the source of the existing cotter pin was from a local hardware store, and it was NOT stainless steel. There was some rust forming on it. The boat has only been in freshwater, and really only a ridiculously small number of days each year, but in the summer it sits outdoors, where the propeller can get wet from morning dew and from me spraying off all the damn bugs that land on the boat and die every night. Typically I am rinsing off the stern of the boat almost every morning. The boat then sits in the sun all day. So you have steel, water, air, and some heat, perhaps a good recipe to promote rust.

Also, in my article above I commented on how firmly the keeper and the pin were being retained, but five years later this was no longer true. The keeper had quite a range of wiggle room, probably associated with how it was cutting a groove into the cotter pin.

As part of that process or servicing the cotter pin, I scrubbed off a lot of soot from the keeper, the propeller nut, and the brass propeller adaptor fitting. As you can see, the part number embossed on the brass fitting returned to be visible again. I also removed the propeller and re-greased the propeller shaft splines and the thrust bearing.

Then I reinstalled the propeller, and torqued the propeller retaining nut to about 65 to 70-ft-lbs, (the service manual specification is 70 to 80-ft-lb) using my quite ancient Sears Craftsman torque wrench, the type with a long pointer indicator that gives the torque reading from a companion ruler-scale and a long handle to produce the necessary torque, (and thus not the more modern digit torque wrench in common use today).

Then the keeper gizmo was put into position and a new cotter pin installed. I am quite sure the new cotter pin was one of several in a small plastic bag that I bought a few years ago from my Evinrude dealer using the official Evinrude part number, and I hope were all stainless steel.

The new installation is shown in Figure 7.

Fig. 7. The replacement cotter pin, installed September 22, 2026. I also scrubbed the soot off the brass adaptor fitting and the keeper.

cotterPin2026.jpeg (81.91 KiB) Viewed 109 times

Again, at the time of installation the cotter pin was holding the keeper in a firm position. Perhaps in five more years I can update how this cotter pin survived. If there is a repeat of the keeper cutting into the cotter pin, I think I will revert to the more traditional method of installing the cotter pin as shown in Figure 5 above. Also, I will forever more to be sure to use stainless steel cotter pins.

TOOLS AND PARTS NEEDED
I had all of the parts needed to service the cotter pin already on the boat, including the proper socket for the propeller nut, a suitable ratchet wrench handle for that socket, spare cotter pins, spare keepers, spare retaining nuts, and--an essential part--a block or wood to keep the propeller from turning, needle nose pliers, and diagonal cutters; the only exception was the torque wrench. When performing certain maintenance procedures, I like to verify that all the tools and parts I need are stowed on the boat. I also have the appropriate parts if I need to fit a different style propeller or adaptor, such as an old-fashioned pressed-in rubber mount OEM propeller, or a Mercury propeller using a FLO-TORQ II adaptor for the Evinrude.