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Thread: omc 70/75 Differances???

  1. #11
    Team Member sheuninck's Avatar
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    Default OK: want to modify a 70hp block to a be a 75hp block

    Fred,
    Found a nitro gearcase, thanks, but now I need to re-build the powerhead.
    Can you help with the questions below?
    1.] I can chamfer the top of the exhaust ports
    2.] Where do you cut the notch in the exhaust plate?
    3.] I have the skinny rods
    4.] I need to bore 20 over, .008" diameter clearance, 400 or 600 grit hone?
    5.] what year / engine size do I order the new pistons for?

  2. #12
    Burgess/Evinrude F1 V8 Lars Strom's Avatar
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    Quote Originally Posted by Fast Fred View Post
    76 is said to be the Best year

    My experience is that the 1976 years 75hp short shaft was the best
    production 814cc 3 cyl.
    Lars Strom

    Life is good





    Check my own racing history at BRF...http://www.boatracingfacts.com/forum...ead.php?t=6727

    My racing web site SVERA.se....http://svera.se/blogg/paris-6-hours/

  3. #13
    FFX-61
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    hey Lars, hows it goin.

    ya sheuninck you can do the ports.
    think i got a shot of the exhaust for ya.
    the chicken stick rods are good bein lighter it will rev up quicker ( i'd top it off with a 73 65hp flywheel)

    it's the diamiter of the piston + .008 for wiscos, finish hone, let them do it at the machine shop on the machine hone, then you have to chamfur the port windows.

    wisco part no. 3115P2 is the pistons you want, you'll need three bearin kits part no. B1029

  4. #14
    Team Member sheuninck's Avatar
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    Default what goes first

    Fred, thanks for the feedback.
    Shouldn't I chamfer the ports before I get it bored/honed.
    Do you have one of those flywheels?
    I may not be able to afford the wisecos, there are stock sierras on ebay for 1/3 the price. Will these suffice? If so, what year/hp should I order?

  5. #15
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    i would not buy them pistons, you get what ya pay for, most of the time
    the bore will 90deg all the port windows, they will be sharp, they will trip and or hook the rings and she will fail. no extra fly wheel at this time, but that can change at any time.

    if you do run them pistons, they are for sure cast, the piston +.005 for cast bullets

  6. #16
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    finger ports, you need them.

  7. #17
    Team Member sheuninck's Avatar
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    Default finger ports

    Fred,
    I found some wisicos for $84. It seems like .008 clearance is alot. How low can I go?
    I just send the block/head in for the work:
    - 20, 30 or 40 over depending how it cleans up
    - exhaust port chamfers
    - bring deck height and head down to minimum spec (old APBA SE/EP rules).

    I did see that you mentioned to move the ring end gaps away from them. I am not sure I am ready to attempt finger ports. Do the wiseco pistons have the holes for them? What is the advantage of the finger ports?

    With the short SLT courses, it looks like we mainly need to be able to get off the dock and out of the corners vs top speed.

    Does the exhuast plate notch go hand-in-hand with the chamfers?

  8. #18
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    wisco's are forged, forged Will grow when it gets up to temp, cast does not grow as much. the piston diameter + .008 on a forged piston, or it Will stick.

    get your pistons first, you can't bore it with out the pistons. if they say they
    can, i'd go some ware else


    you can run the ring gaps in the finger ports, you got to get them centered up on the gap nice.

    you got to drill your own pistons, no holes from wisco.

    you need finger ports.

    180 psi compression.

  9. #19
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    i'd call Dave at Sea-way with that one David-L6, he has a Service manual.
    i am not familiar with the make up of them Bullits, wisco's, set up hundereds, roasted a bunch

  10. #20
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    Default differences

    If I remember correctly the 70 hp was 56 cubic inches and the 75 was 49.9 cubic inches. Also, I always liked the 1975 model 75 best.

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