On the 42 – they started life also as Mercury Racing HP500’s with Gen VI blocks. The previous owner said the engines were gone through once by the original owner but had no documentation to show what was done or when but thought the cams were replaced, top ends done (see a pattern here?)– I bought it with the knowledge that I would have to do the top ends at a minimum over the winter. I was planning for springs and gaskets and maybe a valve job so I was not all too shocked when I saw the broken spring on the starboard engine. The hull has 580 hours, the Merc springs last 2-300 hours so in my head I thought if the engines were gone through at around 250-300 hours, then I’d be pushing it but should be able to make it to close to 600 without failure. I missed it by that much…
Upon teardown both heads on engine one had two broken springs and the corresponding valves and seats were a little hammered. The springs are all duals with a damper between the inner and outer spring. On hole #8 the inner spring got pulverized and ran through the oil system and trashed the bearings. It also took out the valve guide. It looked like someone had dumped a spoonful of silver anti-seize under the valve cover. To my good fortune, all hard parts were perfect – but everything with small passages that couldn’t be easily cleaned was replaced.
I had the local shop rebuild the heads. They were cleaned, decked, and received Inconel exhaust valves, SS intake valves, Isky endurance springs, new guides where appropriate, seals, valve job and springs shimmed to get the right seat pressure. I had the shop do the heads on both engines so everything would match.
They also decked both blocks to clean up the head surface. The bores measured well within limit for size, circularity and taper so I got lucky and didn’t need to increase the bore size, which meant no new pistons ☺
Cranks and pistons measured spot on, so it was as simple matter of new bearings, rings and oil pumps to get the short blocks complete. I did run into an issue with some clevite bearings that were bad out of the box, but the shop swapped them out for me so I could keep the builds going. The cams I pulled out were not the original merc cams, but Crane 731’s which I reused. I put in new crane lifters, and Comp ultra pro magnum rockers. All new oil lines and oil coolers, gaskets etc.
I’m sure I’m forgetting details here – but these should be good for another 300 hours.
Here's a shot that shows the graphics on the top deck - the large surface was a perfect canvas and begging for some color.
look forward to seeing it with some paint on it and back on the road 
Awesome to see your saving this one from the scrap yard!
