As seen in Speedboat Magazine’s
Nov/Dec 2022 Issue.
Pro Magnum Rocker Arms
Q. I have a pair of Mercury 500 EFI engines and have had one of the aluminum rocker arms fail. I am considering upgrading to the Comp Cams Ultra Pro Magnum rocker arms (part number 1820-16). Will these rocker arms fit under the Mercury Racing Valve covers used on the 500 EFI engines?
Fountain Valley, CA
The rocker arms that were used on the 500 EFI engines were manufactured by Crane Cams for Mercury Racing. They were a good choice for many years until some changes were made in the company which is now absorbed into Competition Cams. Mercury Racing switched to Scorpion to make their rocker arms for the later model GM based V8 engine offerings.
We have experienced failures of aluminum rocker arms in more severe applications with increased rpm and valve spring pressures. On engines that we use T&D shaft mounted rocker arms, we have switched exclusively to their steel rocker arm system with great results. And on the engines with stud mounted rocker arms, we have switched to the Comp Cams Ultra Pro Magnum steel rocker arms which also has proven to increase reliability. When possible, we also install a Jomar stud girdle for increased stability.
If you are installing the Comp Cams Pro Magnum Rocker arms on any of the Mercury Racing GM V8 engines including the 500 HP, 500 EFI, 525 EFI, 520 EFI, 540 EFI, 565 EFI,
600 SCI, 662 SCI, and 700 SCI, the rocker arms will fit. But you need to re-use the original posi-locks on the engine which measure .970″ in height. The posi-locks that come with the Comp Cams1820-16 rocker arms are slightly taller and sometimes hit the underside of the valve cover which could crack the cover or prevent it for sealing properly.
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