Dark Blocks

[BJuby]

So curious, what's the story behind these. Are they special? Rare? Or are they just a different color due to casting? Just got two Mark 55s and one is an early model and appears to be a dark block. Just want to know what the deal is with these.

Thanks

[Gene East]

I don't really know the answer, however I do know Mercury used different alloys at times with more zinc to reduce corrosion. That could be an answer.

I can tell you the high zinc alloys could be a bear to weld at times.

[Krazy Karl]

There was a pretty extensive thread about this on John's Old Mercury Site. I don't remember the answer but it did have something to do with the alloy used. You could do a search to find it. It was very interesting and a lot of history brought up by some of the very knowledgeable members. I also have a 55H that has a black block but I also had/have a couple of fishing motors with dark blocks. Not real unusual.
kk

[Aeroliner]

Started working on Mercury engines in 1955 and owned a Mercury dealership for 35 years. In the early years Mercury used a high zinc allow for what they thought was needed to provide the required strength. The high strength allow was utilized to control the engine weight. This high zinc alloy was very prone to corrosion as anyone that worked on Merc's used in salt water. Growing up on Long Island I can point to lots of issues taking things apart. A benz-o-matic was nearly always needed to get a lower unit apart. Over time it became an extreame issue with the engines, so Mercury changed the casting allow to somthing closer to a marine grade of aluminum. To also protect the engine Mercury introduced the trim tab made out of zinc and added berried zincs other places. This in combination with the use of stainless fasteners greatly reduced the corrosion on their engines. So if you have a dark freshwater block it will be great but one used in salt water could be an issue when you look under the rear water jacket cover. I have seen cylinder heads completly corroded through.

Alan

[matt1us]

Black block was a slang term we used for the fishing version of the Mark 55h................using the fishing version of the Mark 55 was approved and soon became the 'block' of choice. Even with lower ports the 'black blocks' quickly obsoleted the 55h. Our best engine was being used as a anchor at a New England marina when we found it. We paid $25 dollars for it and it went on to win several titles in the day!..........i still remember my dad pulling it out of 10 feet of water on the end of a chain! Geesh, the things we did back then to find that 'killer' motor.

[BJuby]

How could an engine without the altered porting of the 55H powerhead perform better than it? No porting was done to these blocks whatsoever? Or were different modifications made.