Cummins Marine Diesel Repower Specialists Forums General Discussion Cutlass bearing set screw layout?

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  • #75101

    Scott
    Participant
    Country: United States

    I’m in the process of rebuilding the stern tube in my 25’ Downeast boat and need to drill and tap the tube for set screws to fix the cutlass bearing. Is there a recommended layout for the screws? I’ll be using cone point set screws and can spot drill the cutlass. The cutlass is a light press fit into fiberglass. The page on installing shafts, logs, and struts is really helpfull https://www.sbmar.com/articles/installing-shafts-logs-struts-rudders-with-your-favorite-engine-transmission-part-2/ and it looks like there are three set screws per strut, two located on one side towards the fore/aft ends and the third located 180° opposite and centered between the other two. Is that correct? Right now I’m leaning towards locating two on the same side and in-line about 2” from the either end of the cutlass.

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  • #75239

    Tony Athens
    Moderator
    Vessel Name: Local Banks
    Engines: QSB 6.7 550 HP
    Location: Oxnard, CA
    Country: USA

    This is how we do it and have been for 25 Yrs +……….My own protocol based on “logic” as to what needs to be accomplished…

    1) I never use a press fit on the stern bearing–NEVER means just that.. I always want the bearing to slide in and out by hand…………Sand, grind, machine what is needs to get a “SLIDE FIT”.. .010″-.030″ clearance is a good number, but that all depends on the concentricity of the strut or stern tube ID..

    2) I always put a notch/groove ( what ever) on the aft end on the bearing & make that the TOP so I can “SEE IT”.

    3) set screw pattern–Does not matter other that I personally do not like it with any symmetry so It cannot be mixed up

    4) I never just spot drill, other that for initial marking–I make one mark at a time thru that already tapped holes ( use tap drill size to mark) in the tube, remove the bearing and drill full set screw diameter ( 5/16″ or ??) deep enough to where the drill just about touches the rubber ( if it does, it DOES NOT MATTER) …………………….

    5) Clean off any burrs, and reinstall the bearing. Put the set screw in and locate the bearing to where the set screw Allen wrenches into the hole you just made with NO INTERFERENCE .. Seat it in the hole very LIGHTLY.

    6) now, make / drill Hole #2.. Same way.. put the bearing in and now lightly seat both screws.

    7) if you have a 3rd screw- do it the same.

    8) With the screws in and “seated lightly”, you should be able to back out each screw about a full turn, but the the bearing CANNOT GO “ANYPLACE” as it mechanically “locked” in place, yet, & if done right, the bearing may seem a loose.

    9) Now, coat the bearing with your favorite underwater sealant ( we prefer generic low durometer moisture cured polyurethanes).. Push it in and locate the screws.. Put the same sealant on the screws/in the tapped holes & seat them and then, back, BACK OFF off 1/2~1 full turn.. Off course, you have ground the longer than needed SS set screws to length to where they are even or just lower than the OD of the stern tube.

    10) put grease/soap, or what ever on your shaft and slide it thru. Align / couple it to the other end and let all cure for a day or 2/3.. …………………………The base engineering principle here is NO pressure on the bearing and to let the shaft “semi-float it in place……… Of course, I am assuming the stern tube is in perfect alignment in all planes to what is on the other end ( the ID of the stern tube at the opposite end is most important) ……..If not, then we/you have other issues to deal with

    That’s the “Tony Way” .. To me, anything else is LESS..

    Tony

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