SOE Sabotage – Magnetic Petrol Tank Bomb

During the Second World War Britain’s Special Operations Executive (SOE) developed a whole series of sabotage devices for use behind enemy lines. Using unique archival footage this series of short videos examines some of the weapons developed for use by SOE agents in occupied Europe. In this episode we look at an explosive magnet bomb, designed to be attached to any magnetic surface and detonate to destroy machinery or vehicles. It later evolved into the small pocket-sized ‘Clam mine’.

Today, we’re lucky enough to have some colour footage showing the of testing of a magnetic bomb which could be attached to the petrol tank of vehicles. The footage comes courtesy of the Imperial War Museum.

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A still from the footage showing the charge placed on the body of the car (Imperial War Museum)

From the film we can see that the bomb consisted of a small block of plastic explosive, a pair of strip magnets (or possible a horseshoe-shaped magnet) and a Switch No.10 time pencil delay detonator. The explosive block itself looks to be slightly smaller than the SOE’s standard 1.5lb charge.

In the film we see the bomb placed on the boot (or trunk) of a saloon car before various civilians and a corporal experiment with various ways of covertly attaching the bomb to the underside of the car. At one point the corporal allows himself to be dragged along behind the vehicle before making his escape.

Luckily the 16mm footage, filmed by Major Cecil Clarke, also shows us the effect of the explosive charge mounted on a petrol tank full of fuel. According to the details listed for the film by the Imperial War Museum the footage was filmed in 1940, at SOE Station XVII, located at Brickendonbury House in Hertfordshire.

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A still from the footage showing the bomb’s magnets (Imperial War Museum)

This configuration of the bomb doesn’t appear in the Special Operations Executive’s Descriptive Catalogue of Special Devices and Supplies published in 1944. However, Colonel Leslie Wood, Station XII’s commanding officer, described the demonstration put on during a visit by Brigadier Robert Laycock of the Commandos and William Donovan, the head of the American OSS in June 1942. One of the scheduled demonstrations was the “Effect of small ‘magnet’ charge of explosive on petrol tank of car.”

It appears that this ad hoc magnet charge evolved into ‘the Clam’, which was a smaller, version of the magnetic Limpet mine. The Clam evolved through a number of marks with the MkI having a stamped sheet metal casing and the later MkIII using a bakelite, plastic casing.  Both were made up of a plastic explosive charge inside a rectangular, rounded case with a pair of magnets at either end. They were detonated by either a Time Pencil or an L Delay fuse attached to a No.27 detonator. The MkIII had 8oz (226g) of high explosive filler, such as TNT/Tetryl 55/45.

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MkIII Clam (Imperial War Museum)

While unlike the larger Limpet they weren’t developed for under water use but the Clam could be mount onto any vaguely flat magnetic surface including engine blocks, fuel tanks, crank cases, cylinder blocks, rail tracks and steel plate.

At just 5.75” x 2.75” x 1.5” they were easily concealable, could be carried in a pocket and were non-descript enough not to draw attention. An estimated 68,000 Clams were made under supervision at Aston House according to Des Turner’s book on Station XII.

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Thanks David Sampson of for the use of his photo of the cutaway Clam.


World War II Allied Sabotage Devices and Booby Traps, G.L. Rottman

Technology and the Civil War, S. Mountjoy & T. McNeese

SOE’s Descriptive Catalogue of Special Devices and Supplies, c.1944

SOE’s Secret Weapons Centre: Station 12, D. Turner

SOE: The Scientific Secrets, F. Boyce & D. Everett

The footage is part of the Imperial War Museum‘s collection © IWM MGH 4325 and is used under the Non-commercial Use agreement.