Original Author: Phil Barnett (prb) - 2013

Back to Maintenance

CAS O-Ring

How many Miata engines have had a leaking CAS O-Ring? Well, how many Miata's were built?

It seems that this is a maintenance item on every Miata I work on and from reading on miata.net it's the number one oil leak spot on the entire Miata product line. Every one I've replaced was hard as a rock and had a flat cross section instead of being round. It's near the exhaust and it gets pretty hot back there.

With a cold engine and a clean finger, wipe your finger across the back of the head under the CAS. Did your finger come back wet with engine oil? If so, it's the CAS O-Ring leaking. If it didn't, someone already replaced it. Nitrile replacements seem to last around 40k miles before they start to seep. Viton O-Rings will last much longer and that is because Viton has higher heat tolerance.

You can order Viton O-Rings from http://www.allorings.com. Order size 222. The minimum order is 10. These come shipped at just under $1.50 each. This seems like a good deal if you know a lot of Miata owners or are ordering for a club. There is a wealth of information on their web site regarding materials and failure modes of o-rings. Good reading.

Based on my measurements of cross section of the kits of O-Rings from Harbor Freight, I cannot recommend the Harbor Freight O-Ring kit for use in a Miata CAS sealing application. It is a huge a huge assortment that is quite handy to have in your arsenal of tools and bits and pieces, but the cross section of two sets I bought at least 5 years apart were smaller than the specification, the newer ones being .120 in cross section.

The specification for the cross section of a 222 o-ring is .139±.004 (3.53±0.10 mm)

Reference site for o-ring specs.

Another place that has been reported to carry individually packaged size 222 O-Rings is Ace Hardware. NAPA is also a great choice as they usually have a great selection of O-Rings.

A search on eBay for Viton 222 will supply you with plenty of opportunities to get Viton O-Rings.

Amazon.com carries Viton 222 O-Rings in many packaging quantities.

I have some Viton 222 O-Rings here that I bought from oringsandmore.com. They all measured .137 in cross section.

I strongly recommend taking the time to source a Viton O-Ring from a reputable source.

As usual, the Miata Garage on Miata.net comes to the rescue with beautifully illustrated step by step instructions.


Note: If your CAS O-Ring has been leaking, it's been leaking on your heater hoses. Because they are right there and just below where this seal leaks. Oil will ruin those hoses. If you see that those two hoses are oily, you need to order replacements and replace them with fresh hoses. Miata engines are tough as cockroaches but running out of water from a burst hose will kill one in 3 minutes or less. Don't risk your engine for a $20 part. Replace your heater hoses when you replace your leaky o-ring. The engine you save will be your own.

How align your CAS the easy way...

  1. Turn the crank to get cylinder one to TDC, align the timing mark to 0 degrees.
  2. Look in the oil cap opening to see if you are TDC firing or TDC exhaust for the #1 cylinder. If the exhaust cam lobe is pointing out, rotate the mark on the CAS engagement to 12:00. If it is pointing in, rotate the mark on the CAS to 06:00.
This eliminates the guess work on where to turn the engagement.

Cross Reference

Ace Hardware (Buna-n (Nitrile))#99
Dorman (Buna-n (Nitrile))64222
Fel-Pro (probably Nitrile)35608
Fel-Pro (probably Nitrile)427
Generic1.5" ID x 1.75" OD x 1/8" Cross Section
AllOrings.com (nitrile, viton, et al)222
ORingsAndMore.com(nitrile, viton 75, viton 90, et al)222
Mazda (probably Nitrile)9954-10-3801
McMaster Carr (Viton) 25 pack.9464K64
Miata Roadster Viton Quad
Motormite (Buna-n (Nitrile))64222
O-Tite (Buna-n (Nitrile))64222
Victor-Reinz (probably Nitrile)72222