Which cam O-ring?

The old O rings that seal the cam covered on the rear end of the cam where squashed badly. The kit I have covers different models and has extra bits in it. I have a choice of 2 O rings and neither are a good fit

I don’t think your old o-rings were “squashed badly.” The original o-rings supplied by the factory had a square profile with a semi-circular lip around one side. These were possibly a custom-made item for the ends of the early Jaguar cam covers. I have three of these unique o-rings in my collection of spare parts that I removed from early XK engines.

The spare parts suppliers now provide generic round-profile o-rings for this application, which probably don’t seal as well as the originals. I suspect a generic square-profile o-ring would seal better, but I’ve never tried to find the best size.

Here in the U.S. we have McMaster Carr who claims they can make custom o-rings for special applications. One of my “to do projects” is to send them several of these original square-profile o-rings supplied by Jaguar to see if they can reproduce them at a decent cost.

Thanks Mike


These people do a range of seals
I measure the components at
29mm inside
36mm outside
3.3mm thick
Theirs are thinner but I could double up, what do you think would be a good size to try?
I don’t yet know their outside diameter.

I’m looking at the semi-circular o-ring cutout in a spare XK120 cylinder head. It’s machined as a right angle - not a round grove. So it was meant for a square-profile o-ring.

Each side of the o-ring cutout is 3.5 mm. However, the special square profile o-rings removed from that cylinder head measure roughly 4 mm on each side. So the factory must have thought that .5 mm “squish” would provide a good seal.

That’s the best I can do. I’m not an o-ring expert.

It wasn’t necessarily designed for a square o-ring; it’s just that a square groove is easier to cut.
I measure the ID of the head/cover groove at 1.450", the OD of the plug at 1.140", and the depth of the head groove at .146", the cover being slightly less.
Based on that it looks like inch size #217 is pretty close at 1.449 OD x 1.109 ID x .139 thick.
The next smaller size #216 is 1.387 x 1.109 x .139 which may be a bit small.
There are square o-rings on the market in those sizes.
I used a round o-ring and glopped it up with Permatex No.2 gasket sealant.
If we convert the groove measurements to metric, 36.83 x 28.96 x 3.71; so a metric o-ring 37 x 29 x 4mm might work.

Here are some better photos of what I believe Jaguar originally supplied as the C.2312 o-ring, which provided a precise seal at the interface between the rear of the cylinder head, the cam cover, and the (intake side) Flanged Sealing Plug or the (exhaust side) Rev. Counter Adaptor. As I noted earlier, it was a square o-ring with a semi-circular lip around one side.

The first photo shows the external side of the o-ring. The semi-circular lip surrounds the outside end of the cam cover.


The second photo shows the internal side of the o-ring.

The third photo shows the o-ring inserted into the machined cutout at the rear of an XK120 cylinder head. It’s a precise fit unlike the fit provided by a generic round-profile o-ring. Little if any sealant is needed to make a dripless seal.


The fourth photo shows the fit when the cam cover is in place. Note the seni-circular lip.

I’ve found three of these special o-rings on long-dormant XK120 cylinder heads. Big Jim appears to have found them on his XK140 cylinder head. It would interesting to know if Jaguar continued supplying these special o-rings for later cylinder heads.

That makes sense. In the Mark IX parts book there is also an oversize seal C.2312/1 with the remark “To be used only if necessary to prevent oil leak”.

On later engines such as the 3.8 and 4.2 Mark X the part number was C.19044, described as ‘O’ Ring, used only on the exhaust side. The intake side on these later models had an electric rev counter generator so it used a different o-ring and a half seal.