Redline Automotive Accessories
CAS O-Ring Change

by Jim Creer

Note: This article includes photos of a 1.6l engine. Additional photos of a 1.8 provided by Phil Ethier

The objective here is to change the Cam Angle Sensor (CAS) O ring in about 10 minutes without removing the cam cover.  A timing light is not needed.    Have done 5 like this and found it works just as easily on both the 1.6 and 1.8 NA Miatas.  Only variation is the CAS is on the back of the intake cam on the 1.6, and on the back of the exhaust cam on the 1.8.  Both are just as easy to do.  Started doing this around Y2K, after doing several others using by the book.  If not comfortable trying it this way then please follow instructions in your manuals.

 Have a spare CAS, so we can look at one to see what we are trying to do.  The arrow points to the O Ring we will be replacing.  The area in the box is the tip that rotates inside the rear of the cam.  The elongated hole is for the bolt that holds the CAS in place.  This bolt is loosened for the CAS to rotate when changing the ignition timing.

A different view shows the two little “tangs” at the end of the rotating tip.  They are slightly off center.  There is only one way it will fit, so no chance of it being 180’ out of phase after installation.  Also note one end has a little groove running down the end, which is a bit clearer in the previous picture.  The opposing tang has a little dot stamped near the end and no groove.  Found some that did not have this dot and just a flat end.

The size of O Ring needed is 1/8” – 1 ˝” x 1 3/4”.  They run around 59 cents each at about any local auto parts store, or can get this little pack of 4 each for $1.79.  As far as I can tell it is made from the same material as the Dealership replacement units.  BUNA-N.  If you are not sure of this, go ahead and get one from the Dealership.  Expect to pay at least $6, even with some sort of discount.

Another option is a Viton ring, rated for higher temperatures.

To prep for removal, have unplugged the O2 sensor (1.6) and both plugs.  I tucked them back out of the way so have some working space.  Used a soft lead pencil to mark position of the CAS.  When finished, will align the marks again so the ignition timing will not change.  Will probably be some grime and oil on the unit, but the pencil will mark through it.

As a side note for those that frequently prefer to change ignition timing, scribe a timing gauge here for different ignition settings.  Will only take a moment to change it later and no need for a timing light or engine to warm up.  Just remember as the timing belt stretches with age, or belt is replaced, this extra gauge will probably not be accurate.

Remove the 12 mm bolt.  Does not matter if the CAS gets rotated slightly since you have the ignition timing marked.  Just need to be careful we don’t slide the CAS out of the back of the head during this stage.   If the CAS was leaking oil, it will probably be flat, hard, and well lubricated.  Will need both hands and the pencil again for the next step.


Dave let me use this picture of his 1.8 CAS bolt.  Little different shape and has a longer bolt head.  May not be able to get some ratchets behind the head, but a 12 mm box wrench works very well on either the 1.6 or 1.8.

Now for a bit of dexterity!  Slide the CAS straight back toward the firewall.  Do not twist it during removal and try to avoid hitting the tip on anything.  When clear, lower the back and rotate the front upward.  I made a second pencil mark to show orientation where the little tang fits inside the cam.  If you disturb the end slightly, make your best correction guess and mark it anyway.  At best will only be a few degrees off, but may take a few extra light pokes when installing it back together.  Will be a bit clearer as we progress.  In this instance, the position marked was not the tang with the groove.  That should be easy to remember!

Used pocketknife to cut the old ring off, or when you pry up on it a little, it may shatter and fall behind the engine.  Have a new ring handy to fumble in place.  The barrel should have enough oil remaining for lubrication.  If you need a little more oil to smear, just poke finger inside back of cam.  Does not matter if the tip was rotated, since will need to orient it again prior to installing the CAS.  Caution is needed at these stages that pencil marks are not erased.  For the next step will line up our pencil marks, starting with the little tang on the tip, orient it with the cam and ignition timing pencil mark and slide it in back in place. 

 Note.  Many instances it is difficult to remove the CAS from behind the head, but still easy enough to replace ring as shown.

If you see this little space on the barrel between the new O ring and yoke, that means the little tangs are not properly aligned with the cam.  Back the CAS out slightly and rotate a hair.  Try another poke.  Repeat if needed until it slides in.  If you try rotating the unit with pressure on the cam, will just be twisting the nose out of position and be at it for a long time!  I have only hit one exactly with the first poke, but could expect about 3 to 5 little jabs.  Most will be pending on how well you eyeballed both pencil marks.

 In this position the new O ring is flush against the yoke.  That means it is properly aligned inside the cam.  The fatter new O ring will be a little harder to squeeze inside the yoke.  It should have some oil on it, can push harder on it and twist the CAS around a bit until it snaps back in place.

 Brings us back to this picture.  Replace the bolt and ensure your first pencil mark is aligned prior to tightening.  Now the ignition timing is back in the same setting before the CAS was removed.  Plug the 3 plugs back in and go for a drive!  Can also pat your self on the back for saving a $100 from having a Dealership do the job.  Should also be able to do the job in about 10 minutes rather than dropping the Miata off somewhere for a day.

Why do it like this?  For other reasons than stated above, the O ring and the cam gasket may not fail at the same time.  Many owners are unaware of this ring when replacing the cam gasket.  The 1.8 can leak oil down and ruin the heater hose..  The 1.6 can leak oil down on the small unused water cap nipple.  Not sure if this will hurt the cap but will guess it does not do it much good!  We could pull the cam cover to do it, but will probably be spending at least 5 times longer.  Even if the cam gasket is still serviceable, no need to move the coil bracket or clean the old gasket sealer from the 6 points and apply fresh sealant.

Note from Boyce Rigdon:

Great help, Would suggest Changing the Return Heater Hose at the same time as it is the problem created by the leak and about the only way to get to the heater hose is to remove this part. I found the moulded heater hose at the local Parks auto, the dealers didn't have this part, manugactured by Gates Stock Number 18937 Small Coolant Hose, for $8.06. Total cost of repairs, O-Ring. 58 cents, Hose $8.06, Sweat, swearing, bloody knuckles, Satisfaction Priceless.

Note from Joe Newman:

If you find yourself having trouble aligning the dog ears when reinstalling the sensor, I found this small hint to be very helpful. With the sensor still out of the car, use a small pocket mirror and a flashlight to peek at the end of the camshaft before you get started. This allowed me to quickly position the dog ears to a good starting point and saved me the effort of having to remove the back cover of the sensor.

Note from Rob Goldberg:

Just wanted to suggest the following additions to your excellent CAS O-ring article.

Note from Chris Dobbins

My wife and I just changed the CAS o-ring on her '97 miata. Your articles helped tremendously. Thanks for posting them. Could I get you to add a couple of notes to them?

When we started, I looked at the oil dipstick and told my wife that we'd better remove it or I'd probably break off the top. Then, I didn't remove it and, during the removal, I broke it right off. So, could I suggest that anyone performing this job remove the oil dipstick and cover the hole with a piece of tape?

Also, on her 1.8l I wouldn't have been able to remove the CAS adjuster bolt without first sliding the coil pack to one side. Too much was in the way. There are three 12mm bolts holding the coil pack on. One long bolt is at the bottom center (remove it from the passenger side of the car) and two shorter ones are at the top. I did not have to remove any of the plug wires to get enough clearance.

Oh, the seal cost me $0.58 at Advance Auto Parts.

Note from Ralph Barker:

Just replaced the CAS o ring on my 96. To help find the slot for the tang, I cut a stip of cardboard the approximate size of the tang slot and bent it flat like the letter L. Put the bottom of the L into the slot inside the cam and using the top of the L as a guide, I scribed two marks on the outside of the bracket. It gave me a better starting point to refit the tang.

I also found it easier to replace the CAS by laying right on the front of the car with my chest on the end of valve cover. Not very comfortable but easier to make the slight adjustments necesary to replace CAS. Make sure you put down a blanket or towel so you dont scratch your paint.

Note From Ilya Ganelin:

I just completed the o-ring replacement on the CAS on a 1997 miata. I have two suggestions.

Overall note: REMOVE THE VALVE COVER! Definitely makes the job much easier.

Technical improvements:
1) If not removing the valve/CAS cover, then in addition to removing the 3 bolts holding the coil packs in place, if you remove the single 10mm bolt holding the brake line to the firewall, then you can move the coil pack over enough to fully extract the CAS which makes replacing the o-ring easier.

2) Installation with a Viton o-ring. The Viton ring is much thicker and getting it in was a major pain. I tried numerous solutions but how I ended up succeeding in the end was with the valve cover off, I ensured I was aligned. Then I looped a seatbelt around the back of it and pulled from the front. This gave me enough leverage to get it to pop in after a mere couple minutes. I spent an hour fighting with it until I took a break and came up with this idea.

3) Lastly, if your CAS isn't coming out after you remove the 12mm bolt holding it in place, I tapped it out gently with a punch + mallet.

Mark Gronowski

Just wanted to update the article with a source for viton o-rings 1-1/2 x 1-3/4 x 1/8 Viton O-Ring: Monster Fastener (on Amazon and on their website The Viton seal offers a much higher temperature range and better resistance to compression setting ("flat spotting").

The temperature ranges and benefits of the Viton over the Buna-N are listed below:

Buna-N: Temperature Range: -35°F to 250°F
- Good Compression set resistance but will deform

Viton: Temperature Range: -15°F to 400°F
- Excellent compression set resistance that resists "flat spotting"


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23 December, 2016

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