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Shaft cutlass bearing clearance ( new Duramax bearings are loose )

Discussion in 'Props, Shafts & Seals' started by dogsharks, Apr 27, 2007.

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  1. dogsharks

    dogsharks Guest

    I received my Duramax bearing yesterday. That was very fast shipping!

    Last night I slid the bearing onto the shaft and I was pretty surprised at the rather loose fit. I was so surprised I decided to call the guys at the marine supply house where I purchased the unit.

    They were very cordial, and pretty adament about the quality of the Duramax products, said they had a great reputation, and they were selling a ton of them all the time. I asked about the clearance issue, and told them I was used to Morse, which was a tight fit out of the box. The guy then said I might want to talk to the tech department at Duramax. I called Duramax and they were very cordial and very informative as well. I got to talk to their tech guy.

    He said the "BACK" (that's the name of the particular bearing I purchased) bearing is a 1" shaft designation, and the outer shell is 1-1/4" outside diameter, 4" long. This is correct. He then said there should be no tight fit with the Duramax brand, as they machine their products for an initial fit with between 3 and 8 thousandths of an inch.

    I said "the darn thing was loose on the shaft, and I've replaced bearings with that much tolerance". I told him I was accustomed to Morse, which had a rather tight fit, and I have lubricated these with soapy water in the past to install them. Naturally, those would have a period of wear-in when used.

    He said it's actually better to have an even machined tolerance because the shaft will tend to ride within a water film not unlike a pressurized engine bearing will ride on an oil film around a crankshaft journal. He said with the Morse brand, due to the tight initial fit (snug fit), you are toploading the bearing and this is not as good. I said the Morse bearing would have a "burn in" period where it would adapt to the environment.

    Personally, I would rather have a tight fit. That's the way I've been conditioned, but I'm willing to give this a try. I'm very interested in knowing if anyone else has info on this.

    In any case, I have another 1" shaft at home on another boat, and I'll check that one too. Of particular note, when you order 1" shaft stock, or any shaft stock for that matter, there is a tolerance in it too. They all don't come exactly the same. For this reason, when you have a new prop shaft made, you should also have your shaft hub matched up to the shaft, because if you dont you may discover the shaft is a few thousandths bigger than the hole in your hub, and that presents one heck of a problem when you are standing on your head in the engine bay trying to get things to fit up.

    Therefore, my tolerance scare may be because that shaft is a little on the skinny side. It measured 1", and totally sure of it.

    Like I said, stay tuned, we're going to do a road test on this product and I'll let you know how the loose bearing theory actually works in practice.

    Regards, Paul
  2. storm

    storm Guest

    might help?

    you might want to check metric size, if the o.d shell housing allows and if you shaft is out of spec. it work for me.
  3. dogsharks

    dogsharks Guest

    Thanks for the note. I checked further and decided to order a new shaft. That appeared to make up the difference, and the bearing is not as loose as initially thought. The Morse bearings I've used in the past were firm when installed, and they apparently wear-in; at least that's how my Chris Craft install was.

    This time around with the Duramax, I understand they have a different concept and they don't install with a firm press-on like the Morse, and this is what threw me. WIth a slightly worn shaft, probably more than I realized, along with the Duramax concept of not being a firm fit upon new install, I was alarmed. WIth the new shaft I'm no longer alarmed, thankfully.

    Regards, P
  4. dikikh.dm

    dikikh.dm New Member

    Joined:
    Apr 22, 2015
    Messages:
    22
    Location:
    Malta
    Hello everyone,

    Could anyone advise me a reference for the minimum/maximum clearance of the cutlass bearings?

    Diameter of the shaft is 100 mm.

    Thanks and regards.
  5. Bamboo

    Bamboo Senior Member

    Joined:
    Dec 19, 2008
    Messages:
    935
    Location:
    Palm Beach, FL
    As a general rule if you can see space anywhere between the shaft and the bearing considerably larger than a wooden pencil lead it's either the shaft is not centered or the bearing is worn. The larger the shaft the larger allowed space; a 4 inch shaft is heavy enough to be tighter on the bottom than the top, not so much with a 1 inch shaft. If you (or someone who is strong) can lift/move the shaft up and down then replace the bearings.

    http://www.yachtforums.com/threads/the-sea-deuce-65-donzi-hull-4-sold.10464/page-10

    this is a new bearing with a 4 inch shaft- 100mm = 3.937 nches
    Last edited: Jan 25, 2017
  6. dikikh.dm

    dikikh.dm New Member

    Joined:
    Apr 22, 2015
    Messages:
    22
    Location:
    Malta
    Thanks for your reply and the example on the picture. It's always nice to look at the clean new installation:)

    What would you say about such clearances (all in MM), measured while the boat is still in water:

    upload_2017-1-30_18-56-11.png

    Thanks and regards.
  7. Bamboo

    Bamboo Senior Member

    Joined:
    Dec 19, 2008
    Messages:
    935
    Location:
    Palm Beach, FL
    Those look good for a nearly 4 inch shaft. How did you take those measurements in the water? Is there vibration when running at the transom? Can you turn the shafts by hand? What were the measurements on the blocks and stands after the install?
  8. dikikh.dm

    dikikh.dm New Member

    Joined:
    Apr 22, 2015
    Messages:
    22
    Location:
    Malta
    Thanks for advise!

    The measurements were taken with filler gauges by a diver, while checking/changing the anodes.
    Unfortunately, I don't have a proper history from the previous drydocking, that's why I asked him to check actual clearances to have at least some understanding of the bearings condition. Will check it once again on hard, when we go to the shipyard next time.
    I wouldn't say that we have excessive vibration, probably only on the low revs and nothing beyond acceptable limits.
    I don't think, that it's possible to turn 10 metres long and 100 mm thick shaft by hand, but I will try in the dock:)

    Have you ever used Thordon bearings instead of usual Cutlass? Or may be you know anyone, who had an opportunity to compare their performance?
  9. Bamboo

    Bamboo Senior Member

    Joined:
    Dec 19, 2008
    Messages:
    935
    Location:
    Palm Beach, FL
    The diver should be able to turn the shaft as should a person on land, even when dry.
    The feeler gauge measurements will be different underwater than they will be on the hard; frankly I've never had someone measure them UW.
    When running at cruising speed if you put your hands on the aft end on the port side and then the stbd you can feel the vibrations; they may be slightly different- and that should tell you which side requires more attention- however that attention/vibration may be due to propellers and a vibration analysis specialist may be needed. A simple feeler gauge of cutless bearings should tell you if the bearing are worn and if the shaft is in alignment with the struts.