Transmission Mission: Service on the Cortez’s Most Fragile Part



L’s and my biggest worry with our 1965 Clark Cortez motorhome is the transmission. Here’s why …

When Clark set about designing the Cortez, it had a clear goal in mind: to be the first purpose-built front-wheel-drive RV on the market. This design would offer several advantages over the competition — most of which were front-engined and rear wheel drive. First, because the Cortez is front-engined and front wheel drive, it did not need additional components, such as the prop shaft, rear diff, and axles. Second, this meant the Cortez could have a lower floor. Third, a front-wheel-drive layout would also weigh less and be more efficient, giving better gas mileage.

While this all sounds great, there was a major issue: At the time there was no front-wheel-drive transmissions that would work in the Cortez due to space, weight, and capability and work with its original engine (a Chrysler 225 inline-six). To overcome this though, Clark set pencil to paper and designed their own four-speed manual transmission.

“Wonderful!” I hear you say. We even thought so too at first.

Yet, this transmission has a reputation for, well, breaking so much so that — over 50 years later — this unique four-speed manual transmission is the Achilles heel of the Clark Cortez.

Parts are not produced for the transmission because only Clark manufactured them — and Clark stopped making RVs (and their parts) after only sixteen years. It is the reason that so many Clarks have disappeared from the roads! Finding a transmission can be tricky and expensive — Though there is an option to change to a three-speed automatic transmission (which was installed later on the Kent Cortez), but this is not a simple plug-and-play job. In fact, this involves changing the cradle to a Kent one, changing other components (such as the drive shafts), along with getting several custom fabrications.

This leads to the simple fact that we want to make sure our transmission is looked after and in as good of a condition as possible.

With that rather long introduction out of the way, let’s get into our next project!

Our Cortez’s odometer claims to have done a little over 10,000 miles in its 56 years. Because our gauge only reads to 100,000 miles, the real mileage could be anywhere from its displayed 10,000 … to 110,000 … or 210,000 … or even more. Hell, the odometer might not even be the original, in which case the mileage on it is irrelevant! Not only this, but we have no idea when — or even if — the transmission oil was changed because there is no service history. If it never was serviced, the transmission oil would be over 50-years-old — and to give an idea, it could be good to change transmission oil every five years.

Therefore, due to unconfirmed mileage and no service history on our Cortez, L and I decided it would be sensible to change the transmission oil.

It is important to note that when changing any oil, the vehicle should be at operating temperature before the oil is dropped. (This is because when oil is cold, it is more viscous. The word ‘viscous‘ or ‘viscosity‘ is used to describe how thick or free-flowing a liquid is. If a liquid is more viscous, then it is thicker and goopier — which is horrible for changing oil. When a liquid is less viscous, it is faster-flowing and runny — This happens when the liquid is hot.) Before starting the job, we drove around the farm for 20 minutes to allow the engine and its oil to heat up.

Next, we slid under our Cortez.

From here, the actual job of changing the transmission oil is as easy as warming up the vehicle — providing that both the drain and fill plugs unscrew from the transmission casing. Fortunately, they did (because I dread to think of the mission it would have been if one had seized or rounded off).

With a drain pan set under the drain plug, it was time to crawl under the RV and get dirty!

L grabbed a ratchet spanner then crawled under the RV to remove the fill plug before moving onto the drain plug. After unscrewing the plug, hot and stinking gear oil came flooding out. This was a huge relief because it meant that there was at least oil in the transmission. Also, the oil was in a semi-decent condition — It looked semi-transparent and light golden (not black as it did the first time we dropped the engine oil when doing the breaks). And, although gear oil doesn’t smell great to start with, there was no burnt smell.

After allowing the oil to drain, we cleaned and reinstalled the drain plug — but not before we did a little housekeeping. Armed with a wire brush and can of brake cleaner each, I set about cleaning the transmission as much as possible while L cleaned the drain plug. Approximately 0.01 seconds after her first spray of brake cleaner, she started screaming for me to get out from under the RV, which is when I came to find out she had successfully sprayed brake cleaner in her eye! I quickly flushed her eye with plenty of water and made sure she was okay …

so that — like a true hero — she got right back under the RV.

From here, it was time to fill back up the transmission. After a large amount of research, I settled on a transmission oil specifically designed for heavy-duty applications, including “towing, hauling, racing, commercial use or other severe duty” so it should be great for our RV.

Back to the installation: Lying under a 9,000-pound RV can be daunting and uncomfortable — but adding in squeezing fresh oil into the transmission made this job really awkward. Fortunately, this transmission oil is cleverly in squeeze bags, which makes adding it easier.

We had five bags of transmission oil — and fully expected to use nearly all of it — which is why we were confused when the new oil started flowing out of the fill plug hole after only starting the third bag. Time for a bit of on-the-spot research to find our drain plug (Number 112) and fill plug (Number 113) — just for fun, leave a comment letting us know how long it took you to find them …

Another challenge for you: How many drain plugs do you see? The answer is our transmission actually as two drain plugs, which means we filling the transmission early because we needed to take out both plugs — an oversight that was frustrating. Fortunately for us, when we dropped the second drain plug only old oil poured out (so there must be some sort of divide inside the transmission where oil sits in two areas). With the second drain plug cleaned and reinstalled, we resumed adding in new oil. This time the third and fourth bags emptied and during the fifth, oil started coming out of the fill plug.

The key to knowing when your transmission is full of fresh oil is when the oil starts leaking out of the fill plug. Once the oil leaks out, grab your filler plug, squirt in a last glug of oil, and screw in the plug — It’s as simple as that!

After cleaning up and disposing of our transmission oil in the oil-burning heater, I backed our Cortez out into the cold Virginia air. A quick test drive showed we still have issues with initially getting the Cortez into gear but once it is in, the gear changes are smoother. More time and research is needed to resolve this gear-selection issue, but we are certain it has to do with our clutch-release bearing (which works with our transmission) or linkage system (which operates the gear selector), and that project is for another day!

Author: Andy

Ey up! I’m the calm, laidback, English one of the two. If L is the fuel, I’m the engine -- Without her, I’m pretty happy being sat still. That said, when I’m out and moving I make the most of it. I’m super squeamish, a stickler for the rules, and if I’ve not had a cup of coffee in the morning it’s probably best to let me be. I love fishing, hiking, and vehicle mechanics and I’m not scared of learning something new!

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