Mark 55 Block

[jim-spencer]

Does the block from a 1960 Merc 300 interchange with a 1955 mark 55? The bore and stroke are the same, but the 300 is rated at 40 c.i. whereas the mark 55 is rated at 39.6 c.i.

Thanks, Jim

[billw]

The very short answer I have is, "I don’t know." I will tell you this, though….There will probably be SOMETHING different. It may be as important as that the base design is different, to something as small as the hole for the starter is off by an eight inch. Carb mounting flanges are a biggie, to possibly be different. You will generally find much less success swapping around parts on Mercs, than you do with OMCs. The only real way to know for sure, without laboriously looking at both blocks side by side, is to do a part number search. If the original Mark 55 part number happens to superseded to the 1960 300 (which IS entirely possible but not likely) then you’re good to go. If needed, I have some old Parts Price History books that are good with tracking supersessions. I also have the "Where used record," which is sometimes helpful but not as much as the history, when doing what you’re trying to do.

Long live American manufacturing!

[dave-bernard]

a complete short block will interchange.it is a bolt in .use the 300 carbs for best results. starter and accy’s fit.

[jim-spencer]

Thanks for your help. My goal was to use a Merc 300 block, pistons, and crank, and the reed cages and carbs from a mark 55. I bought a mark 55 with a stuck power head, and after disassembling it, I found the block and pistons were in poor condition. I have a useable Merc 300 short block. I noticed the casting numbers on the 2 blocks were different, but the numbers on the crank shaft covers were a match. I want to produce the same 40 horses as the mark 55.

Thanks again for the help, Jim.

[dave-bernard]

the 300 will.

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