YachtForums.Com - View Single Post - Shaft cutlass bearing clearance ( new Duramax bearings are loose )

dogsharks · 04-27-2007, 11:56 AM

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

04-27-2007, 11:56 AM	#1
dogsharks Registered User Join Date: Dec 2005 Location: Tennessee Posts: 70	Shaft cutlass bearing clearance ( new Duramax bearings are loose ) 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