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Problems with the Cobra
Cobras have a reputation for eating their gears. This is usually due to a failed shift cable, and is easily prevented. Cobras are routinely maladjusted, compounding the problem. When the distraught owner of a failed Cobra wrote to Trailer Boats magazine, they brushed him off. I wrote a follow-up letter to Trailer Boats to explain what happened. My letter suggested the dealer was responsible, but it was never published; I suppose it was too long. You may read it below.
Jim Barron, Editor, Tech Letters Trailer Boat Magazine 20700 Belshaw Avenue Carson, CA 90746-3510 Cc: Dan Gust and Tom Ski, OMC Technical Support Mr. Barron: I read the letter from Paul Elder ("Sorry," March 1998) with interest, out of sympathy for Mr. Elder, and because he shares a common misperception of the OMC Cobra "gearset problem." I've learned something about this since I bought a boat last summer with a similar Cobra drive. Trailer Boats Magazine can do a significant public service by recounting the correct story; the truth is less embarrassing for OMC, and some Cobra owners may avoid a similar failure. Early OMC Cobra drives occasionally destroyed their gearsets, with symptoms just like Mr. Elder's. In 1988, OMC decided the teeth on the sliding clutch dog were not inclined steeply enough. Early Cobra gearsets had teeth cut at 2°; subsequent Cobra gearsets have been cut at 5°. Presumably the sharper "bite" would help the clutch dog stay engaged. When Mr. Elder's OMC dealer says "old gears," the dealer is probably referring to an early-production 2° clutch dog. Anyway, the problem persisted, and in 1989 OMC decided the problem was really in the "transom shift cable." This is the lower cable visible to the starboard of the carburetor; it snakes through the transom, and connects with the gearchange mechanism in the vertical drive. OMC redesigned this cable, sent letters to all registered Cobra owners, and OMC dealers replaced them for free. If a cable failure destroyed a gearset, as apparently happened to Mr. Elder, OMC would also replace the gearset without charge. OMC says their cable-replacement letter went out in November of 1989. This was the letter that Mr. Elder's predecessor didn't get. OMC ended this extended warranty in 1994. OMC now says those early-production 2° gearsets were really OK. Contrary to what Mr. Elder was told, as well as OMC's original diagnosis, OMC now claims there were never any "bad gears." OMC says the real problem was the transom shift cable. I believe OMC is correct this time. Current Cobra owners can find out if their cable or gearset was replaced under warranty; call OMC at 847/689-6200, and ask for Technical Support. Please write down your questions before you call, along with the serial numbers from your engine, transom bracket, and vertical drive. OMC Technical Support is pretty good, and free; please don't waste their time. The current OMC transom shift cable has a red plastic cover; if yours doesn't, get it checked immediately. OMC recommends the Cobra transom shift cable be adjusted every fifty hours, with the usual annual maintenance. Replacement is a chore; the vertical drive must be removed for access. Cobra owners capable of re-greasing their own U-joints can change their own cables, but they'll need the OMC shop manual, OMC's streamlined adjustment procedure ("kw-shift.wps", dated 10/17/96), and possibly service bulletin #4052. Remove the exhaust bellows for access; a set of flare wrenches are handy, or you can cut the old cable and use a box-end wrench to remove it. OMC does not have a recommended replacement interval for the cable; four or five years seems prudent to me. OMC technical support claims you need a $50 special tool to adjust the cable; I bought the tool, and found it nearly useless; I suppose it would save some time for a professional tech. The OMC shop manual clearly explains how to adjust the cable without any special tools. Please disregard the section concerning "Core Wire Replacement"; OMC has repudiated this practice. A genuine OMC cable cost me $83, and it took few hours to install. I recommend replacing the "cable jacket" (a length of generic automotive plastic wire-loom flex pipe) at the same time. Non-do-it-yourselfers can expect to pay about $300 to replace this cable. While it is regrettable that Cobra drives had faulty cables, I believe OMC has acted honorably, and their redesigned cable has truly fixed the problem. I know OMC tried to contact Cobra owners about this problem because I have the form letter OMC sent to the original owner of my boat (copy enclosed). OMC can't really be faulted for ending their extended warranty; if they could be held liable for such failures forever, marine drives would be very expensive, like aircraft are today. This cable problem is insidious because the only likely symptom before gearset failure is stiff shifting. As the cable progressively binds up, shifting effort will gradually increase, and most owners won't notice because they only drive their own boats. Dave Brown of Brown's Marina (Canada) suggested that a properly-adjusted Cobra will "shift with a finger." If any reader's Cobra drive requires more than one finger to shift, I urge them to have their shift mechanism checked promptly. I learned this cable story by pursuing the original "bad gears" story that I heard about my own sterndrive. I purchased my boat last summer, and my gearshift was very stiff, but neither I nor the seller realized anything was wrong. So far, I've cleaned and re-greased my gearshift, replaced both shift cables (too long), replaced my transom shift cable (damaged and near failure), and I've replaced the centering spring in my electronic shift assist linkage (OMC had a defective batch of these). My Cobra will now "shift with a finger," at least in my driveway. I can offer only sympathy for Mr. Elder; he cannot be faulted for his ignorance of this "shifting timebomb" in his sterndrive. Indeed, if OMC is correct, neither the original owner nor the local OMC dealer understood the real problem, and Mr. Elder is paying for their collective ignorance. OMC dealers are "supposed to know" about this cable, and educate their customers. If all Cobra owners knew that an $83 cable could easily destroy some $1000 gears, all those cables would get replaced, and the Cobra's reputation would improve. Now you know, Mr. Elder. Perhaps OMC is at fault for not communicating this cable story to their dealers more effectively. If it's any consolation, Mr. Elder, I suspect the original owner spent enough money on depreciation to buy several gearsets. Boating is not a cheap hobby. I predict Mr. Elder's rebuilt Cobra will be trouble-free so long as his transom shift cable is OK (I assume that his dealer changed the cable when they installed his new gearset!). In my experience, the Cobra shift mechanism is the only weak point in an otherwise robust and intelligently-designed sterndrive. I also predict that many more Cobras will commit "cablecide" in the coming years. I urge all current Cobra owners to replace their "transom shift cable" if there is any doubt about its current condition. Happy Boating, Stuart E. Hastings stuart at hastings.org P.S. I got most of this story from Dan Gust and Tom Ski in OMC technical support. Trailer Boats should probably verify my version with OMC, and perhaps Mr. Elder's OMC dealer, before printing this letter. (copy of OMC form letter enclosed) (copy of Trailer Boats column enclosed)
Current Status of the Cobra
Original and aftermarket parts seem to be readily available. Comments on rec.boats suggest that many dealers and technicians are still misunderstanding Cobras, and I expect that Cobra expertise will diminish over time.
If you're buying a used boat, you're stuck with the drive already installed. If it's a Cobra, you can use the information here to get it shifting perfectly; properly maintained; it is just as good as or better than the comparable MerCruiser products.
The terrible reputation of the Cobra may depress the price of Cobra-equipped boats slightly. If you know how to adjust a Cobra, this is not an entirely bad thing.
Identifying Your Cobra
I'm aware of three varieties of Cobra:
1. Dog-clutch, prop exhaust (labelled "OMC Cobra")
The most common Cobra, and the subject of this website. The F-N-R gears are located in the bottom, in-line with the propeller. Note the top of the outdrive (portion exposed to water outside the boat) is relatively flat.
2. Dog-clutch, through-hull exhaust (labelled "King Cobra")
Mechanically the same as #1, but louder.
3. Cone-clutch (also labelled "King Cobra")
This Cobra is very rare; I have never personally seen one. The F-N-R gears are located in the top of the outdrive, directly behind the engine. Note the hump on top of the outdrive case that accommodates these bulkier gears.
The Volvo-style "cone clutch" in the 1993-on "King Cobra" is extremely durable, and it is not subject to the "bad gears" failure the others are famous for. The 1993-on "King Cobra" is also not otherwise addressed on this webpage; if you have one, buy the manuals, and take it to a qualified
Volvo technician. Rejoice! Volvo-style gears are renowned for near-indestructibility.
Note the water-pump located on the back of the F-N-R gearcase on the 1993-on OMC "King Cobra." I've been told that the Volvo SX and DP drives use a similar water pump attached to the engine crank pulley (inside the boat).
The rest of you with "dog-clutch" Cobras can continue reading. You will need your serial numbers when querying OMC about your drive. OMC hid several different serial numbers in various places.
This page was scanned from my 1998 Owners' Manual, and does not show where OMC hid the serial number for small block Ford V8s (302, 351), or for GM big-block V8s (454, 502).
What’s a Dog Clutch?
From Webster's Ninth New Collegiate Dictionary:
dog n. ... 3 a : any of various usually simple mechanical devices for holding, gripping, or fastening that consist of a spike, rod, or bar
Alas, that definition doesn't help much.
I suppose a dog clutch is a "clutching mechanism that employs a clutch dog. That's no better, as it seems to be self-referential.
The better answer is "the mechanism composed of item #38 and the adjacent gears #32 and #33 in this exploded parts diagram below." (This page scanned from the OMC Cobra Parts Catalog.)
Item #38 is the "clutch dog" inside a Cobra stern drive. It's a stout, hollow, sliding cylinder splined to the propeller shaft, with teeth cut into each face for engaging the adjacent gears.
The teeth on the clutch dog resemble the decoration on top of a chess-piece called the "rook" (or "castle").
Immediately adjacent to the clutch dog on the propeller shaft are gears that spin freely on the shaft. These gears mate with a third gear that is driven by the engine, thus these gears are always turning while the engine is running. These gears turn in opposite directions.
The stern drive is in neutral when the clutch dog is between the nearby gears; the drive engages forward or reverses by sliding the clutch dog into the appropriate gear that turns with the engine.
There is no "clutch," as you find in an automobile. Gears engage with a jolt. Some say "the water is your clutch," but I always thought a clutch had to be between the engine and the load to be useful ;-) .
#39 is the pin that forces the clutch dog to turn with the propeller shaft. #18 is the propeller shaft and #15 slides fore/aft to drive the clutch dog.
Two different gears are marked #32; the one on the propeller shaft is the forward gear, and the other turns with vertical driveshaft #35. Gear #33 is reverse.
Shifting is accomplished by raising and lowering rod #36; it connects with detented arm #11. Ball bearing #6 (pressured by spring #12) falls into the detents milled into #11. Arm #11 rocks bellcrank #14 to and fro, transforming the up/down of #11 into fore/aft on cradle #13 and shifting shaft #15.
My Own Cobra Experience
My Cobra has a 2bbl 4.3L V6, and is fitted to a 1988 Four Winns Horizon 180, a typical 18-foot bowrider. This is my first boat, so it took me a while to realize what a serious problem I was facing.
My Cobra gearshift was very stiff, requiring about three times as much force as it should to engage/disengage gears. I made many phone calls to OMC, accumulated documentation, talked to many people, and replaced parts until it improved. The main problem turned out to be the Transom Bracket Shift Cable; it seems that the cable OMC installed in 1989 during the recall was still on my boat, and it was failing. In retrospect, this should not have been a surprise; the cable was approximately nine years old! Today, my Cobra "shifts with a finger," as should any properly adjusted Cobra.
My Cobra also had a stalling problem when cold; if you engaged a gear before the engine was fully warmed up (a ten-minute process), it would stall. This was an annoyance, as well as an embarrassment when there is a line of folks waiting to launch their boats.
The stalling problem turned out to be a vacuum leak, compounded by a complete carburetor maladjustment. Of course, I didn't figure this out until I removed the carburetor for a rebuild (one of the hold-down nuts was loose). It appeared that a previous owner/mechanic deliberately maladjusted everything on the carburetor in a vain attempt to compensate for the vacuum leak. Today, I can engage reverse and back off the trailer as soon as the engine is running.
When reviving my boat from winter storage in 1999, I carefully re-visited the adjustment procedure, and I actually measured the cable drag of my transom bracket shift cable. It was too high; another hour of exploration led to a defective "retainer" see part "R" in picture 11 below
I thought this was very subtle problem, but it's known and documented in the OMC service bulletins.
I have some hints for Cobra owners that defy classification. If you have any of your own to share, please send me an e-mail.
Recreated with the permission of the Author for CBOC