Valve stem diameters and uses
For most "old car" applications the valve stem hole in the wheel rims come in 3 sizes, which are nominally:
- 8mm diameter, observed on some small and Voiturette cars (as well as motorcycles which often had the same tyre sizes). This 8mm size hole will have an inner tube with what is called either a "TR4" or "TR6" metal stem, both of these stems being virtually identical. At Blockley we use the TR6 stem because the patch that attaches to the tube has a larger surface area, and we like this.
- 11.5 mm (0.453") diameter. The most common size we all know on our cars. This hole size is for use with rubber stem ("TR13") or the metal stem ("TR11") valved inner tubes.
- 16 mm (0.635") diameter , the largest of the sizes. This fits a rubber TR15 stemmed inner tube. Popular on tractors and some post war cars of the 1940's and 1950's. We supply a well fitting adapter / grommet / collar that slips over the TR13 stem to increase it to the bigger TR15 size making it a snug fit in the hole in the wheel, for use in these wheels that have the 16mm hole. This plastic adapter also has the advantage of additional protection for the rubber stem from any potential sharpness at the juncture of where the hole exits the rim.
The valve stem terms mentioned above TR4, TR6, TR11, TR13, TR15 etc were the numbers assigned to the different valve stems by the company Schrader, who started up in the 1890's with their invention that is still going strong!
So when there is a choice with the Blockley inner tubes, why use a metal stemmed TR11 tube over the TR13 rubber stem variety?
Historically valve stems from the dawn of motoring were always metal, bolted into the inner tube. As time went on the metal valve stem became vulcanised onto the tube itself as part of a patch.
The metal "TR11" stem has two distinct advantages over the rubber "TR13" stem, which is why a metal stem is always preferred in arduous conditions or applications such as motor racing.
The first bonus of a metal "TR11" stem is that there is no risk of this stem chafing or being cut against the hole of the wheel rim in the case of any movement, particularly at the base where an annular brass nut remains on the tube stem.
The second advantage is that very very occasionally a fault can occur on any rubber stem where air can leak between the rubber and the inserted brass housing into which the valve is fixed, causing a slow deflation even though the valve itself is not leaking - something which just cannot ever occur with a solid TR11 stem.
All metal valve "TR11" valve stems used on our thick butyl Blockley Superior tubes are nickel plated, just as they were in period. We have not yet found an unplated original stem on an original period tube. And this is also the case with early veteran tubes. The nickel plating avoids the brass tarnishing and is just another bit of attention to detail, the way we like to do things at Blockley!
And finally mention should be made of the screw on valve cap, which should always be used to stop the ingress of dirt. At Blockley we supply all inner tubes with metal valve caps with a washer inside that will keep air in the tube, even if the valve fails. Having this good seal prevents the dirt as well as moisture contaminating the valve core.
As a postscript, there is also another popular valve stem size, used on Veteran and Edwardian cars and into the 1920's using beaded edge tyres, or as they say in the USA "Clincher" tires, which carried on to the Straight Sided tyres that came after and used up to the later 1920's, all of which had an "on centre" valve stem position. These long stems had a diameter of 12mm with a outside thread of 26 tpi and Blockley uses this stem today (Nickel plated as they were all originally in period) on most of the beaded edge range of inner tubes it produces.