Let’s untangle the mystery (and the myths) surrounding the timing of the Lotus Esprit camshafts. And while we’re at it, let’s tackle the chaos of camshaft pulleys too. You’ve heard it all—100 MOP, 110 MOP, 104 MOP, flipping pulleys, timing dots vs. pulley dimples, and the rainbow brigade of red, green, blue, and yellow markings. Confusing? Sure. Impossible? Not even close. Let’s dive in.

To get your Esprit, Eclat, Elite, or Jensen Healey’s camshaft timing dialed in like a pro, you need to answer three fundamental questions:

1. Which camshafts do I have?
2. Which camshaft pulleys are on my engine?
3. What specific engine am I working with?

I’ll walk you through each of these, step by step, so you can finally crack the code on timing the 9XX series engines and unlock their full potential. Spoiler alert: it’s more straightforward than it seems—no magic wrenches required. Let’s get to work!

Which cams do I have?

All Lotus factory 9XX series camshafts (yes, even the ones on Jensen Healeys) are conveniently marked on the front of the camshaft, nestled between the pulley and the cam carrier.

These markings might play hide-and-seek, but trust me—they’re there. You might need a flashlight, a bit of patience, and maybe even some creative contortions to spot them, but once you do, they’ll tell you exactly what you’re working with.

Identification marks are as follows:

• No marking at all – Designation is “C” Cam.
  • 272° duration with 0.340″ Lift.
  • Optimal performance is achieved with 110 MOP pulleys.
  • This is the most common camshaft found on all USA S1 and S2 cars.

• One Groove around camshaft – Designation is “D” Cam.
  • 270° Duration with 0.350″ Lift.
  • Optimal performance is achieved with 110 MOP pulleys.
  • This is a very early production (1974) camshaft. Not found on the Lotus Esprit.

• Two Grooves around camshaft – Designation is “E” Cam.
  • 260° Duration with 0.344″ Lift.
  • Optimal performance is achieved with 102.5 MOP pulleys.
  • Provides better low-end torque at the expense of top end power.
  • Found on the intake side of European models and 1980 California models.

• 7777777 around the camshaft – Designation is “107” Cam.
  • 252° Duration with 0.378″ Lift.
  • Optimal performance is achieved with 104 MOP pulleys.
  • Provides more lower end torque than the “E” cam. Power runs out around 5000 rpm.
  • 912LC & 910 cam

• 4444444 around the camshaft – Designation is 104 Cam.
  •  272° Duration with 0.410-0.420″ Lift.
  • Optimal performance is achieved with 104 MOP pulleys.
  • 104 cams were only used on the intake side of select S3 cars (912 HC).

A quick word of caution: “C” cams can be reground to mimic the more desirable 107 and 104 cams. And as the old saying goes, “assume nothing.” The difference between a C cam lobe cross-section and that of a 107/104 is easy to spot—if you know what to look for. Trust your eyes, but when in doubt, let the numbers do the talking.

Here’s how: measure the cross-section of the cam lobe heel, then subtract that from the cross-sectional measurement of the lobe itself. The difference is your lift, and it doesn’t lie. If you discover you’re dealing with a reground cam and have no additional specs or data, well… that’s a rough day in the garage. At that point, you might want to consider investing in new cams. Your engine—and your sanity—will thank you.

Lift = Lobe – Heel

Now that we have the camshafts straightened out, let’s tackle the dreaded pulleys.

Which Pulleys do I have?

Believe it or not, all Lotus pulleys start life from the same mold—or to be precise, two molds: the original trapezoidal gear design and the improved HTD design. The real magic happens later, with the precise placement of the keyway in the center bore and the charmingly cryptic array of colored dots around the outer rim.

We’re not about to dive into the brain-draining depths of engine geometry and theoretical math here—because, honestly, who needs that kind of headache? Instead, let’s get practical. We’ll focus on how to identify the different pulleys, understand their applications, and even experiment with variations to squeeze every last ounce of performance from your badass Lotus.

The Dots

First up, the Dots. let’s unravel the colorful mystery of timing dots and figure out what they’re trying to tell us:

• Red dot: 110 MOP (Optimal for C-cam & D cam)
• Blue dot: 100 MOP (Emissions timing for C-cam & D cam)
• Yellow dot: 102.5 MOP (Optimal for E cam)
• Green dot: 104 MOP (Optimal for 104 & 107 cams)

Now, here’s where it gets interesting. Many pulleys have two dots or lines stamped on each side. Why? Likely because Lotus had a “kill two birds with one stone” approach to manufacturing costs. These dual-dot pulleys typically fall into one of two categories:

• 110/115: Separated by approximately 3 teeth—commonly found on many Jensen Healeys.
• 100/110: Separated by approximately 4 teeth—frequently seen on S1 and S2 cars.

Occasionally, you’ll spot other random dots, sometimes white, scattered across the pulley like a misplaced sprinkle on a cupcake. Ignore them. They’re meaningless.

You might also encounter pulleys marked with both “IN” and “EX” on either side but sporting only a single dot per side. Only pay attention to the mark with the dot. Again, this is the result of a single mold design followed by keyway placement and dot marking. Efficiency at its finest, Lotus style.

If the Color is No Longer Visible

Single Dot Pulleys

This can be a bit tricky, but don’t worry—all is not lost. I’ve done some homework to make things easier. Using three pulleys (100 MOP, 104 MOP, and 110 MOP), I stacked them together with their keyways perfectly aligned.

Here’s what I discovered: even though the keyways line up, the pulley teeth don’t. This confirms that the keyways are machined in different locations for each pulley type.

To make identification simpler:

1. I positioned the keyways at 12:00 and projected a theoretical centerline across the pulleys.
   
2. From this line, I projected down the sides of the pulleys to create a reference that clearly shows the difference in timing marks.
   
   

While this method isn’t as precise as breaking out a protractor, it’s a helpful visual reference for identifying your pulley.

Dual Dot Pulleys

This one’s much simpler. If the dots are:

• Separated by 3 teeth: The pulley is 110/115 MOP.
• Separated by 4 teeth: The pulley is 100/110 MOP.

No need to overthink it—just count the teeth between the dots, and you’re good to go!

Now, The Dimples.

Take a close look at the images below. This is a 100/110 MOP pulley. The left image shows the side without the dimple, while the right image shows the side with the dimple facing forward.

Let’s say we’re installing this pulley on the intake camshaft. The most common mistake? Assuming you can just slap the pulley onto the intake camshaft, align the “IN” dot with the “EX” dot on the exhaust camshaft, and call it a day. Spoiler alert: you’re not good to go just yet.

Here’s why: notice that on one side of the pulley, the “IN” mark has a red dot, and on the other side, it has a blue dot. What’s the big deal? Well, now we know that:

• A red dot is 110 MOP.
• A blue dot is 100 MOP.

Two completely different timings—and your engine will absolutely care which one you choose. Getting this wrong could mean your engine isn’t running optimally—or worse, not running at all. Moral of the story? Pay close attention to those dots and dimples before locking things down.

The Dimple Decoded

Intake Camshaft

• Raised dimple forward: 100 MOP
• Raised dimple backward: 110 MOP

Exhaust Camshaft

• Raised dimple forward: 110 MOP
• Raised dimple backward: 100 MOP

And here’s the golden rule: if those colored dots have seen better days (or have disappeared entirely), the dimple is your key to success. Think of it as your trusty guide in the often confusing world of Lotus camshaft timing. With the dimple leading the way, you’ll be well on your way to nailing the correct MOP settings—no guesswork required.

Now What? Let’s Time the Cams

Turn your motor clockwise until you have it on TDC on the flywheel as seen through the cutout in the transmission bell housing. The motor needs to be in this position when lining up the dots of the camshaft pulleys as seen from the front of the motor.  Hard to see while in the car. Use a mirror if necessary.  You cannot line up the dots by looking at the pulleys from the back of the car.

The 907 Engine:

 The 907 was designed by Lotus to deliver optimal performance with 110 MOP intake and 110 MOP exhaust. The other timing variations? Those were purely for helping Lotus meet increasingly strict emissions regulations. Unfortunately, these adjustments also helped reduce performance and torque—something the 907 engine wasn’t exactly overflowing with to begin with.

For the best performance, set your camshaft timing to 110 IN / 110 EX. On dual-dot pulleys, this means:

• Raised dimple backward on the intake camshaft
• Raised dimple forward on the exhaust camshaft

Simple, right?

910, 912, and 920 Engines:

The Lotus Esprit 910, 912, and 920 engines were designed for peak performance with 104 MOP intake and 104 MOP exhaust. Luckily, most 912 engines from 1980 through 1987 came correctly timed from the factory at 104/104.

By the way:

• The 912-series engines are naturally aspirated.
• The 910-series engines are turbocharged.

After 1983, Lotus started playing with cam timing again to comply with ever-tightening emissions standards. Many engines were timed at 110 IN / 104 EX or even 110 IN / 100 EX—neither of which did much for performance.

For 1983–1985 Turbo engines timed at 110 IN / 104 EX, flipping of the pulleys won’t get you there. Here’s how to get back to 104 IN / 104 EX:

1. Remove the intake pulley and replace it with a 104 green dot pulley.
2. Lotus thoughtfully placed an extra 104 green dot pulley on the auxiliary shaft. Remove it and install it on the intake camshaft.
3. Reinstall the old 110 pulley on the auxiliary shaft.
4. Align the timing dots as seen from the front of the motor.

And voilà, you’re good to go!

For 1985 onward Turbo 910 engines timed at 110 IN / 100 EX, again flipping the pulleys won’t get you there. Follow the same procedure for the intake camshaft to switch it to 104 MOP.

To get the exhaust camshaft to 104 MOP, you’ll need another 104 green dot pulley. If you don’t have one lying around, it’s time to go shopping. But here’s the good news: even if you leave the exhaust camshaft at 100 MOP, you’ll still see a noticeable performance improvement just by swapping the intake pulley to 104 MOP.

Installation Best Practices:

When installing camshaft pulleys, keep these tips in mind:

• Apply a small amount of anti-seize to the pulley bore before sliding it onto the camshaft.
• Use a dab of blue Loctite on the pulley bore when installing the auxiliary pulley.
• Always refer to the workshop manual for the correct torque specs.

Tweaking Timing? Not with Stock Pulleys

Here’s the deal: Lotus camshaft pulleys aren’t designed for “tweaking” timing. They’re fixed—you get what you get. If you’re planning to experiment with alternative timings, like 103 IN / 106 EX, you’ll need adjustable camshaft pulleys. There’s no way around it.

But what about advancing the intake cam by rotating it one tooth? Sure, you can do that… but should you? Let’s do the math:

• The pulleys have 40 teeth, so each tooth equals 9 degrees of camshaft rotation.
• Now, factor in that the crankshaft turns twice for every camshaft rotation. That means advancing one tooth isn’t 9 degrees—it’s a whopping 18 degrees of crankshaft rotation.

That’s not “tweaking”—that’s throwing precision out the window and inviting chaos. Unless you’re aiming to turn your engine into a science experiment, it’s best to use the right tools for proper fine-tuning.

Final Note

If your engine is properly set at TDC and you install the camshaft pulleys and belt only to find the timing dots won’t line up—maybe off by half a tooth or so—there’s a good chance your engine geometry has been altered.

This could happen if:

• The cylinder head has been surfaced.
• The block deck has been surfaced.
• The camshaft carriers have been surfaced.

Whatever the cause, if you’re facing this situation, you’ll need to invest in adjustable pulleys to dial the timing back into spec. It’s the only way to ensure your engine runs as it should.

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