Grub screw for cutless bearing – how?

[BabaYaga]

I am changing the cutless bearing in my long keeled GRP boat and would be grateful for some advice.
The present bearing (brass shell, 100mm long) was quite a tight fit in the GRP stern tube, requiring a large hammer to install 16 years ago and subsequently now very difficult to remove.
To make replacement easier in the future, I am planning to slightly widen the stern tube to give the bearing a somewhat looser fit. To hold it in place and stop it from rotating I will instal a grub screw or two.

My questions:
Does this sound like a good idea?
Do I simply drill a hole and tap a thread through the GRP deadwood into the stern tube (about where the blue dot is in the photo?)
Should I use two grub screws, one on either side?
Would M5 or M6 be a suitable screw size for the 38mm diameter bearing (25mm shaft)?
Anything else to be aware of?

 

[emmalina]

Personaly I stopped using brass encapsulated cutlass bearings about 12 years ago when I discovered the resin encapsulated ones. A lot easier to remove ! Also easier to fit if you have a belt sander , Easier to make the bearing smaller than the hole bigger.

 

[Tranona]

Not a good idea. It is meant to be a tight fit. Suggest you have a composite bearing made of something like Maritex and epoxy it in. Not only will it likely outlast a cutless but when your children (or next owner!) need to replace it they can heat it up to soften the epoxy and remove it easily.

 

[emmalina]

Tried Thordon and Vesconite they both transmit noise to the hull but are very hard wearing.

 

[LadyInBed]

Was it the correct diameter bearing in the first place?

 

[BabaYaga]

Many thanks for the replies so far.

Personaly I stopped using brass encapsulated cutlass bearings about 12 years ago when I discovered the resin encapsulated ones. A lot easier to remove ! Also easier to fit if you have a belt sander , Easier to make the bearing smaller than the hole bigger.

Not a good idea. It is meant to be a tight fit. Suggest you have a composite bearing made of something like Maritex and epoxy it in. Not only will it likely outlast a cutless but when your children (or next owner!) need to replace it they can heat it up to soften the epoxy and remove it easily.

In my experience (limited, but still...) removing a bearing that has been epoxied in (phenolic shell) is also quite difficult. Maritex looks fine and may last a very long time, but my replacement cutless with brass shell has already arrived.
I still intend to give it a tight fit, only not super tight. The grub screw idea is more of a belt and braces approach.
When you say 'not a good idea', is it the shell material, the grub screw or something else that you do not like. Please expand if you wish!

Was it the correct diameter bearing in the first place?

Definitely the right size. As far as I have found out, bearings for 25mm shafts mostly come in diameters 1 1/4" (31.75mm), 35mm, 1 1/2 (38.1mm) and 40mm.
Problem is my stern tube is a .1 or .2mm too tight.

 

[john_morris_uk]

I wouldn't make the hole much bigger. They're meant to be a tight fit. As it happens I replaced our cutless bearing in our P bracket only yesterday. A mallet and drift got it out and a mallet put the new one in. The P Bracket does have the advantage of having two grub screws. (I thought that there was only one and managed to press the old one half way out before I noticed the score line from the grubscrew I missed.

It it REALLY is too tight, some carbide paper on a suitable length of dowel will take a fraction of a mm off, but I'd be very wary.

 

[Daydream believer]

Mine has a single grub screw. I can push it in with a piece of wood held against it & tap lightly with a hammer. The grub screw does go through some metal though. Cannot recall exact detail, but there is one hex screw of 6mm diam SS The bearing itself is a casing with some sort of grooved plastic inside that. It is on my launch & I removed it when I was correcting the poor engine alignment by a certain local "marine" firm

 

[Blueboatman]

Long keel set up like yours :
Two grub screws in the stern tube itself at the aft end , one each side . Cutlass bearings removed and reinserted not with hammers but threaded bar , large washers and nuts .
Cutlass ‘bedded’ in Sikafkex not epoxy
No issues

 

[Poignard]

When I replaced my cutless bearing ca 20 years ago the old one did not want to come out and the replacement was a press fit.

I couldn't see the need for grubscrews so I didn't fit any. The bearing has never moved.

 

[dansaskip]

As others have said not a good idea to enlarge the stern tube.
I removed a stubborn tight fitting cutlets bearing by very very carefully cutting it longwise almost through but not quite with a hacksaw blade. perhaps two cuts then it should collapse a little to make removal a doodle. But I stress you need to be very careful and not cut into the stern tube itself.

 

[BabaYaga]

Thank you all for the further replies.

I wouldn't make the hole much bigger. They're meant to be a tight fit. As it happens I replaced our cutless bearing in our P bracket only yesterday. A mallet and drift got it out and a mallet put the new one in. The P Bracket does have the advantage of having two grub screws. (I thought that there was only one and managed to press the old one half way out before I noticed the score line from the grubscrew I missed.

It it REALLY is too tight, some carbide paper on a suitable length of dowel will take a fraction of a mm off, but I'd be very wary.

I do intend to be very careful. When I installed the present one I did not manage to get it fully in, but had to cut off the last 10mm or so of the bearing. So the case might be that the stern tube is not quite uniform in inner diameter.

Long keel set up like yours :
Two grub screws in the stern tube itself at the aft end , one each side . Cutlass bearings removed and reinserted not with hammers but threaded bar , large washers and nuts .
Cutlass ‘bedded’ in Sikafkex not epoxy
No issues

Removal by threaded bar, nuts and washer sounds like a great idea. I doubt it would have worked in my case, though. Maybe it will next time.

As others have said not a good idea to enlarge the stern tube.
I removed a stubborn tight fitting cutlets bearing by very very carefully cutting it longwise almost through but not quite with a hacksaw blade. perhaps two cuts then it should collapse a little to make removal a doodle. But I stress you need to be very careful and not cut into the stern tube itself.

That is also how I got mine out in the end.

 

[Tranona]

I still intend to give it a tight fit, only not super tight. The grub screw idea is more of a belt and braces approach.
When you say 'not a good idea', is it the shell material, the grub screw or something else that you do not like. Please expand if you wish!

Probably answered by now - the enlarging the tube and relying on grub screws was my concern. Tricky to relieve the tube enough to make it a bit easier but still be an interference fit.

 

[KAM]

Mine has 2 m6 set screws at right angles. Seems to work ok.

 

[BabaYaga]

Just to wrap this up:
I did manage to take a tiny amount of material off the inner part of the stern tube wall (one or two tenths of a mm). The new bearing was installed successfully, still requiring a fair bit of force. Enough to convince me that grub screws will not really be necessary.
In case anyone has similar problems, this is the tool I used: A length of 38mm OD aluminium tube with 3/4 of the circumference cut off, patch of emery paper glued to top section.