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  1. #1
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    Merc In-line 6 engine block interchangeability

    I've got a 1978 90hp in-line 6. I was wondering if anyone knew what years this engine block would interchange with? I was also wondering what the difference was between the 90 and the 115? Maybe just carbs and exhaust?

    Thanks for your help!,, rla

  2. #2
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    When you say 115 you cover a lot of ground. The first 115 was in 1970 and was a 135 from the same year choked down with reed stops and restriction in the carb throat. In 1973 the block became a different part number from the 1500 which was the big engine of the year. Later after the engines became rated at the propshaft instead of the crankshaft the previous 150 became the 115 and the 115 became the 90. The 90 carries the same cubic inches and rotating assembly but other internals are different and so is the port timing. The 90 did not have the hole in the piston and the machined area in the cylinder matching it and that in itself makes it difficult to upgrade to the 150 or later model 115 specs.
    bama hama

  3. #3
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    thanks, anyone happen to know what years the 1978 L6 will interchange with? Thanks

  4. #4
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    73-78

  5. #5
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    You can use 79-88 however the ignition system is different. You can use the old cobra head harness however make sure the kill wire is connected internally in the control box and you have the proper engine harness. The 90hp from your vintage has really small ports and is a great pontoon motor, it just can't breathe enuf to get past about 5200rpm.

    To be sure of what you have, check the serial number and verify the year.

    Kirk S.

    P.S. As stated before, the only L6 block that shared the same porting for different horsepower ratings was the 70-72 1150 and 70-71 1350. This block also used the old exhaust patter and will not fit. Look for a post 82 115hp which is basically the same as the pre 82 140hp.
    Last edited by CrayzKirk; 08-07-2012 at 10:05 AM.


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