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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 07-30-2011
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Cutless bearing question

I have a question about how much play there should be between the cutless bearing and the shaft. I have a boat with a full keel so there is no strut. The cutless bearing sits in the shaft tube and it is held in place by two set screws. I hauled the boat and it is on a trailer so I can work on it easily. There is about .5 mm or slightly less than a 1/32 of an inch play/movement between the shaft and the cutless bearing. Should there be any movement or is this amount OK?

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Old 07-30-2011
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That amount of 'play' in your shaft at the bearing sounds acceptable to me with replacement due when the 'play' increases to more like 1/16". The snugger the fit the better.
Does your shaft bearing cause any vibrations or noise that you notice while operating the engine? If not then enjoy your boat and periodically check the bearing and shaft for looseness.
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Old 07-30-2011
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Standard accepted practice for 'perfect' cutless (stave) bearings clearance** is 0,001-0,002 inches per inch of shaft diameter, assuming perfect shaft alignment.

At less than the above the necessary 'film thickness' of the water will not develop to be sufficient to support the shafting side-loads and the result will be rapid wear of the rubber faces of the bearing surface. 1/32" (0,030") is where I usually change out to a new bearing (but I engineered such plain bearing for a living for many many years so I have wee bit more of the needed info - cheaper than refacing or replacing a shaft).

That stated, it all depends on how accurately the engine is aligned with the shaft and how accurately the center of rotation of the shaft is relative to the bearing length.

Simple speak: if you are encountering no knocks (shaft whip) nor undue vibrations due to shaft side loading (bad engine alignment), the shaft is looooong in relation to the distance to the strut/bearing housing then 1/32" tends to be the max. technically agreed clearance of a cutless in normal marine applications. If you are planning a looong trip/voyage then now would be a good time to consider replacement; if daysailing etc. then 1/32" will probably be OK (short term).

BTW/FWIW ... that bearing is NOT held in place by those set screws. Those set screws are there ONLY as a 'backup,' as what holds such bearing in place is an *interference fit* wherein the OD of the bearing is larger than the bore of the 'housing' .... by approx. 0,001" per inch of housing ID ... The bearing being 'larger' in OD than the bore of the 'housing' is PRESSED (forced) into the bore of the 'housing'.

Last edited by RichH; 07-30-2011 at 10:41 PM.
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OK. Thank you for your comments and information. I think it seems best to replace the cutless bearing now while the boat is out of the water even though there is not much play between the bearing and the shaft.

I have to drop the rudder and remove the prop but that is not very complicated.

As I recall, when the yard replaced the bearing there was not a very tight fit between the shaft log and the bearing and it is not a pressure fit now. There is no play between the log and the bearing. The play is between the shaft and the bearing. There is no strut on my boat as you can see in the photos and the log (what the bearing fits in) is fiberglass.
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Old 07-30-2011
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You guys should go with what RichH commented above.
I'm planning on doing a similar Cutlass bearing replacement this fall.
I've got all the parts on hand though (I hope).
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