This is a stock Fiat 600/850 pressure relief valve. We know this type of relief valve are mounted in the blocks of Fiat 600/850. Abarth modified this valve to incorporate a longer spring to increase the oil pressure relief opening point, as the stock relief opening pressure was in the 50 psi range, Abarth was higher. The Abarth Top Hat valve had an extension hood brazed on to the head of the modified hex headed bolt that retained both the plunger and spring - the hex head has a hole drilled in it to allow the taller relief spring to pass through into the new brazed on TOP HAT extension cover - hence the name of the Abarth style relief valve “Top Hat”. Now this little bugger has been known to move from its original block location and find itself adapted on to the oil pump itself, allowing the now vacated oil relief fitting port in the block, where the oil relief valve once was, to become a free port for hooking on an oil line to redirect oil to a full flow filter and/or cooler. This is the basic concept and use of the oil pressure relief valve, modifications done to it, and why it was relocated to allow for oil filtering and cooling purposes. As Abarth, PBS and many others have modified these relief valves into Top Hat versions - what are the specifications for the spring and for the extended top hat length, a combination that seems to be constant between most versions in use? The "Top Hat" shown was made by PBS It has a 20mm extension "hat" braised on to the oil pressure relief valve's top most bolt hex face. Spring has a 9.37mm OD and a 6.73mm ID, spring is 50mm long, 1.32mm spring coil diameter, 18 spring twists, I have not measured or compared the spring to a stock oil pressure relief valve's spring. - the spring looks used, probably taken from another vehicle's relief valve and repurposed ? Anyone know more? I bet many are interested to know the exact design details of a modified oil pressure relief valve called a Top Hat - please discuss.