Pentagonal bolts seem redundant and unnecessary but they are actually more common than you’d think. It is not a “Russian” bolt used on military equipment but a type of security bolt.
The pentagonal type bolt can typically be found in city, municipal and government infrastructure. Applications include for example applications such as manhole covers, valve boxes, meter boxes, ground vaults and electrical junction boxes. The unique 5 side head has no cavities which would fill with dirt or debris. At ground level applications the pentagon bolt is typically recessed to prevent it becoming a tripping hazard. There is even a special key which is called "Trumbull Meter Box Hand Key".
On cars, they can sometimes also be found. For example the Opel Astra, Zafira and the Smart have 14mm pentagonal bolts on the brake callipers to prevent laymen and hobby mechanics working on them. Also, some French cars such as the Citroen, Peugeot and Renault utilise pentagonal 19mm bolts on the brakes.
A pentagonal bolt is a great solution if you want to make a bolt more secure and prevent other people loosening them.
If you want to learn more about classic car mechanics consider my book: "The Ultimate Classic Car Guide - How to Buy, Maintain and Repair Classic Cars".
Nils is a Swiss-German engineer who is obsessed with old cars and engines. He is the author of "The Ultimate Classic Car Guide - How to Buy, Maintain & Repair Classic Cars" and the founder of EVC. His passion has always been with old cars and everything with wheels and engines.