Cracked Tyre Valve Stem

Last week my SS passed its MOT test but with an advisory that I wasn’t expecting. That said, I think it’s well worth checking now and again. The fault was cracking around the rubber tyre valve stem, visible when you put side pressure on it. I replaced the tube with a new Michelin heavy duty tube that has a plastic rather than rubber stem. The photo shows the new stem and the inset shows the old cracked one.


I don’t think I’ve ever seen a plastic stem.
I checked all of mine.
How long do you suppose tubes last on a car that is garage kept and rarely driven?
I’ve seen cracking on very old tubes.
Also on more recent tubeless stem inserts, but that was a car that was outside all the time.

The problem is there are two different diameter tyre valves to suit the diameter of the tyre valve hole in the wheel rim.
The larger diameter - cant recall exactly, but about 15mm was the original hole size as found in Mark IV/V and XK wheels. Sometime later, in about the 1970s, the hole diameter and thus the tyre valve diameter was reduced to about 12mm. So there was a period where owners of cars with 15mm tyre valve holes found when buying new tubes, many vendors would offload tubes with 12mm valves rather than the correct 15mm valves, and if they know what they were doing also sell you a set of plastic valve collars that fitted over your 12mm valves, to make them up to 15mm overall diameter, so they then fitted properly in the 15mm holes in the wheel. If you fitted a 12mm valve into a 15mm hole without these collars, after a period of use, you would invariably get the cracking/breakage at the base of the 12mm tyre valve as per the photo. These days this isn’t a problem, as most specialist Tyre/Tube vendors can offer you correct size tubes, with the correct 15mm tyre valve stems, and if not at least sell you a set of plastic valve collars if you want to still use 12mm valve tubes…
Sorry re vagary re 12mm and 15mm, but both valves have an industry code/terminology and these dimensions are approximate, not accurate - but if you just insist on the ‘larger’ valve stem any reputable vendor will immediately know what you are talking about. Note, you also want a 15-degree off-set valve.

Peters inset photo of cracked valve stem, is immediately and obviously the smaller diameter (12mm) stem, and the photo of valve stem in wheel shows stem as being to small for hole in wheel. I have never heard of plastic tyre valves, so I am sure what is meant is a plastic collar - I hate using these but better that using a 12mm stem straight into a 15mm hole… see pic of a plastic Collar


1 Like


I will admit that I don’t know the age of the failed tube but when I first put the car on the road 30 years ago I had a lot of punctures due to basting grit escaping from the wheel rims and also the use of light weight tubes. I fitted heavy duty tubes all round so the oldest that the tube could be is about 29 years.

Thanks Roger,

I was completely unaware of the stem size issue although I was conscious of the diameter being smaller. I don’t do much mileage now but I’ll keep an eye on my stems. When I described the new tube stem as plastic I may be mistaken but my impression is that it is a harder material with a shiny surface.


Thanks Roger for that tutorial.
On my Mark V and XK120 the valve stems are 5/8" diameter.
On my '38 SS they are 1/2" diameter.
I had the opposite problem on my '37 MGTA; the holes were 1/2" and the new tubes were 9/16" so I had to drill out the holes. And of course de-burring the inside and outside of the holes before painting the wheels and putting the tires on.