Fitting the Oil Temperature Gauge: the Vokes 'Dog's Bone' (Bristol Engine)
On a Bristol-engined AC, the oil-temperature gauge take-off is not where you might expect, and owners refitting a long-dormant gauge often go hunting in the wrong place. It is tied to the car's original Vokes oil filter and a distinctive alloy piece known to everyone as the "Dog's Bone." Here is where the sensor goes, the fallback if that piece is long gone, and how to check the old Smith's gauge before you fit it.
Confirm against your car
This applies to Bristol-engined AC's. Confirm against your own car, originals and later filter conversions differ. Capillary gauges contain a volatile fluid, so handle gently and have repairs done by a specialist. If in doubt, the AC Owners' Club can point you to someone who knows these gauges.
The Vokes filter and the "Dog's Bone"
On Bristol-engined AC's the Vokes oil filter was raised to clear the chassis by an alloy extension piece fitted at Thames Ditton. Its shape, exactly like a dog's bone, gave it the name everyone uses. On top of the Dog's Bone is a tapped hole, 3/8 inch BSP. If the car left the factory without an oil-temperature gauge, that hole carried a brass blanking plug; if the gauge was fitted, the plug was replaced with a male-to-male threaded spigot that accepts the Smith's temperature bulb.
If the Dog's Bone is gone
Many cars lost their Dog's Bone extension when owners gave up on the Vokes filter and fitted a modern spin-on cannister. If yours has, look instead for a spare tapped hole on the port (left) rear corner of the Bristol sump, the same take-off point used on the Bristol 400, and a sound fallback for the temperature sender.
Test the gauge before you fit it
Smith's oil-temperature gauges are the capillary type: a bulb, a fine capillary tube and the dial, filled with a volatile fluid (diethyl ether). Before installing, test the gauge and sensor, gently warming the bulb, the traditional "kitchen" test, to check the capillary and bulb are still sealed and not leaking. A capillary that has lost its fluid will not read.
These gauges are repairable: a capillary specialist can refill and reseal them, and owners have had this done successfully. Careful do-it-yourself repair is possible but fiddly, and the volatile fluid makes a specialist the safer route.
In short
On a Bristol-engined AC the oil-temperature bulb goes in the 3/8 inch BSP hole on top of the Vokes filter's alloy Dog's Bone extension, in place of the blanking plug. If the Dog's Bone has been lost to a spin-on filter conversion, use the spare tapped hole at the port rear corner of the sump. Either way, test the old Smith's capillary gauge for leaks before fitting, and have a specialist refill it if it has gone flat.
Sources. This article draws on the experience of Bristol-engine owners in the AC Owners' Club community.
A general guide to the oil-temperature take-off and gauge on Bristol-engined AC's, not a substitute for your workshop manual or a specialist. Confirm the take-off on your own car, and have capillary gauges repaired professionally.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.