OMC Cobra Sterndrive Tech Info


This website and its author are not affiliated in any way with OMC. See complete disclaimer below.

Introduction and History

When some critical Volvo patents expired, Outboard Marine Corp. redesigned their sterndrive product and named it the "Cobra." This sterndrive was mated to many different engines, and was routinely installed on a wide variety of boats. The first Cobra shipped in 1985.

Everyone agreed the new Cobra drive was an improvement over the predecessor "stringer mount" drive, including many existing boat manufacturers that owned large inventories of the now-obsolete drives. Since these manufacturers didn't want to get stuck with un-salable sterndrives, they pressured OMC into limiting Cobra shipments until they could clear out their existing inventories. Thus, the Cobra didn't ship in volume until the 1986 model year. Thanks to Carl G. Craver for this tidbit.

Some early Cobras ate their gearsets. OMC initially thought the clutch dogs were disengaging due to insufficient "bite" (interference cut) on the clutch dog teeth. OMC increased the bite angle from two to five degrees, and all subsequent Cobra clutch dogs have five-degree teeth. All known Cobra owners got a letter (133k) promising warranty coverage if any gearsets failed.

Unfortunately, the five-degree clutch dogs did not solve the problem, and in the following year, OMC re-diagnosed the trouble as sticky shift cables. Cobra owners got another letter, inviting them to deliver their boats to an OMC dealer for a free redesigned cable. OMC would replace any cable that failed for free, and if a cable failure destoryed a gearset, OMC would replace the gearset too. The last time I checked (1997), a Cobra gearset was almost US$1000 (labor extra).

OMC finally ended this extended warranty in early 1994.

I belive the redesigned cable has truly fixed the problem. Alas, many people in the marine business clearly rememberd the gearset troubles, and forgot (or never heard of) the subsequent cable recall and resolution. Many of these folks are "still around," and they have contributed to the Cobra's (undeserved) troubled reputation today.

The Cobra was produced until 1993, when OMC redesigned it to use a cone-clutch shifting mechanism originated by Volvo-Penta. I don't know if OMC purchased gears and etc. made by Volvo, or just copied Volvo's design. OMC was having financial difficulties at that time, and sold a 50% stake in their sterndrive business to Volvo-Penta. This new sterndrive is available with one prop ("SX") or dual props ("DP"). The new SX/DP drive retains many of the original Cobra castings.

OMC has since been taken private by another company, and sold their remaining interest in the sterndrive business to Volvo. OMC and Volvo have agreed to supply parts to each other, but OMC has left the sterndrive business. OMC continues to purchase SX and DP drives for boats produced by OMC subsidiary companies. OMC has a website at I'm unaware of any pertinent Cobra information on OMCs' website.

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 followup 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 here.

Current Status of the Cobra

Original and aftermarket parts seem to be readily available. Comments on 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 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 (128k), 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 (cut-away view, 198k) (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 accomodates 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 renouned 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 waterpump 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. See them all here (141k). This page was scanned from my 1998 Owners' Manual, and does not show where OMC hid the serial number for smallblock Ford V8s (302, 351), or for GM big-block V8s (454, 502).

Whats a Dog Clutch?

Clumsily explained here.

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 suprise; 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, coumpounded 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 deliberatly 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 here). 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.

Adjusting your Cobra Shift Linkage

Dave Brown (marina owner and frequent contributor to once said that a properly adjusted Cobra will "shift with a finger." I urge the reader to demand as much from your own sterndrive. If your sterndrive requires more than one finger to shift, get it fixed soon, because your gearset depends upon it.

If you're serious about working on your sterndrive, I urge you buy all of the appropriate documentation, especially the "shop manual." I strongly recommend you get the original documentation from the people that built your equipment. I'm not impressed by the aftermarket manuals (Clymer, Seloc, &etc). The manuals should be available from your local dealer, or you can call the manufacturer directly and pay with a credit card. The price will be similar; locally, you'll pay sales tax, and if you order from the manufacturer, you'll pay shipping. Even if your shop manual is out-of-date in some respects (as mine is), it is a wealth of information, and very useful when diagnosing an obscure problem.

I'm not listing part numbers for the documentation here because OMC may have different manuals that apply to different years of drives, and I'm too lazy to confirm or disprove this with OMC. However, here are the names of the documents I recommend you get:

OMC Cobra Owner-Operator's Manual

OMC Cobra Service Manual

OMC Cobra Parts Catalog

The third item may surprise you; it's a booklet of exploded parts diagrams for an entire sterndrive, with part numbers. This makes it very convenient to order parts from a dealer; you can order by part number, avoiding the usual colorful "thingy that connects the whatsit to the thingamgig" discussions.

Below is a link to my by-hand transcription of an OMC document that explains how to adjust the shift linkage on the Clutch-Dog Cobra. I believe this procedure supercedes the procedure outlined in the OMC Cobra Service Manual. My copy of the OMC document is dated "10/17/96", and I believe it to be the latest and final word OMC will ever utter on this topic. Fortunately, it is reasonably clear and easy to follow, and I am very satisifed with my results. I hope this information is as helpful to others.

When you read this document, be aware that it assumes you have a servicable Transom Bracket Shift Cable installed and adjusted correctly. If you are replacing your Transom Shift Cable, you'll need to adjust it with the ponderously-named Transom Bracket Shift Cable Adjustment procedure from the OMC Cobra Service Manual (page 11-16 in my copy). I have the two applicable pages 11-16 (290k) and 11-17 (288k) here. This procedure requires only common wrenches, a straightedge, and a ruler; note that subsequent Service Bulletins from OMC require you to use special Cobra-specific tools to make this adjustment.

Here is the legendary how-to-adjust-your-Cobra-shift-linkage document (what your dealer never told you, or didn't know):

OMC Cobra Shift System Adjustments for "KW-RG" (1986-1991) Clutch Dog Models (Stuart's transcription; 10k HTML)

Here are images of the of the original two-page document, provided so readers may see it, and point out any errors in my transcription:

OMC Cobra Shift System Ajustments for "KW-RG", page 1 (160k)

OMC Cobra Shift System Ajustments for "KW-RG", page 2 (201k)

This Service Bulletin describes how to recognize a defective spring in the Cobra shift linkage, and the p/n of the correct replacement. It cost me about US$2.50 for this part, but the new spring was still too stiff. However, it's straightforward to crunch the spring by hand to get a more appropriate tension:

"Wrong Shift Interrupter Spring" (120k JPEG)

This Service Bulletin just says to install the latest ESA (Electronic Shift Assist) module. When OMC converted from a two-degree clutch dog to a five-degree, the dog was more reluctant to disengage. The new ESA makes the engine idle slower and weaker to make disengagement easier.

If you have the original two-degree gearset, and your ESA works, you probably don't need the new ESA.

"Hard Shifting" (95k)

This is a catch-all of typical problems that make the Cobra hard to shift. My own unit had none of these problems, yet I found these pages very interesting; some very obscure and un-intuitive problems are explained here:

Diagnosis of Cobra shift problems, page 1 (140k)

Diagnosis of Cobra shift problems, page 2 (180k)

Diagnosis of Cobra shift problems, page 3 (197k)

Diagnosis of Cobra shift problems, page 4 (155k)

Diagnosis of Cobra shift problems, page 5 (170k)

This is a page with pictures of some special tools OMC recommends for working on the Cobra shift linkage. According to OMC, you must have these tools in hand before you can adjust your Cobra shift linkage.

My own experience is that none of these tools are necessary, although I'm sure they would make the procedure faster and more convenient.

Recommended Tools for adjusting the Cobra shift linkage (76k)

Just for completeness, here's a letter that came with some of the service bulletins that OMC graciously sent to me:

OMC Letter to Stuart (55k)

Here is the letter informing Cobra owners of the gearset recall. I I was fortunate that the original owner of my boat kept this now-historic document. I believe there was a subsequent Transom Bracket Shift Cable Recall letter, but I do not have a copy; presumably it was given to the dealer when the cable was replaced.

OMC Gearset Recall Letter (133k)

This Web Page

My intent in creating this web page is to spread what I've learned about the Cobra to other Cobra owners, that others may profit from what I've learned. I'm doing this as a Public Service; I'm not selling anything, or making any money with this information. I'm not trying to criticize anyone or force any changes in any corporate policies; I'm fighting ignorance.

I did not create this web page to deprive OMC of documentation sales, or to save a few bucks for Cobra owners too cheap to buy the manuals, or to deprive OMC dealers of service business. If you own a Cobra, and you want to work on it yourself, you already know why you need the shop manuals. I realize, however, there are many people that are considering the purcase of a boat with a Cobra drive, or have been putting off the purchase of some manuals, or were just curious, &etc.

Sadly, there are still many OMC dealers and independent technicians blissfully unaware of this information, and occasionally a Cobra gearset is destroyed due to such ignorance. Such incidents are expensive for Cobra owners, and they help propagate the "bad gears" story. For example, here is a letter (297k) from the March 1998 issue of Trailer Boats Magazine documenting such a case.

If you spot an error, or have any suggestion for improving these pages, please send me a note. I'm reluctant to put my exact email address here, due to spamming concerns, but I'll spell it out: "stuart at hastings period org."

Legal Disclaimer

This information is provided as a Public Service; nobody is making any money from this.

This website and its author are not affiliated in any way with Outboard Marine Corporation (OMC). OMC disavows any responsibility for this website, and the information herein. Neither OMC nor Stuart Hastings warrantee this information, and we accept no responsibility for any consequences if you attempt to apply any of the information stored here.

OMC has looked at this website. When I created this website, I sent OMC an email describing this location and content. An OMC representative responded with this letter, graciously explaining OMC's position. I have already addressed the issues raised in the letter; you're reading the disclaimer.

Please understand, dear reader, that I have reproduced OMC technical documents here, documents that OMC could claim copyright protection for. OMC has graciously agreed to tolerate this practice (at least, for now).

Given the fickle and ever-changing nature of the Web, this site will not exist forever. If you own, or expect to own, a Cobra sterndrive, I humbly suggest you make personal copies of any/all informtion contained here that seems useful to you.

Created 19feb1999

Last Update 23jun1999