Changing shaft size

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30wn78
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Changing shaft size

Post by 30wn78 »

This might seem a silly question but thought I'd better ask before spending my dosh. My centaur currently has a 1" shaft and I'd like to change to 25mm. If I'm also changing my shaft seal, prop, cutlass bearing and coupling is there any reason why this would be an issue. I suppose the only thing left would be the stern tube but from what I gather there is no contact in there?

Mike
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Re: Changing shaft size

Post by TyroSailor »

I've just had all new stern gear when replacing the engine in my Centaur (out with the old VP MD2b, in with the new Vetus M2.18). The engineer said (and who am I to doubt him?) that the Cutless bearing is non-standard and he had to turn it down from the next size up. A pain. The stern tube itself is unaffected but the shaft seal goes on one end and the Cutless bearing on the other, so....

I'd be interested to hear how you get on.
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Re: Changing shaft size

Post by philipstevens »

If you have a Volvo rubber shaft seal, you will have to replace it with a metric size one. Although there is only .4mm difference in size upwards, the 25mm would probably overheat a 1" seal.
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Re: Changing shaft size

Post by 30wn78 »

Hi folks and thanks for the response. Yes the centaur has a strange O.D of 34mm for the cutlass but trafalgar supply them. For a bit extra than what an off the shelf one cost but then no need for a lathe. I used to have a normal type stuffing gland but have almost decided on fitting Radice lip seal. I suppose there would only be a tiny bit more water around the shaft inside the stern tube so I guess it will be okay.

Tyro what type of shaft seal are you having on your new install? I like the pss but £230 is steep and I live on the mud.

Mike
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Re: Changing shaft size

Post by TyroSailor »

Good question. It's not stated on the invoice and any paperwork is in the boat which is 300M away. I suspect the engineer got all the bits from Vetus, judging by the prices - I wasn't savvy enough to get them elsewhere, Still, at least they should all work together well, and if they go wrong it should be easy to sort it out.

Mine lives on the mud too - well, for a few hours twice a day - does that matter? :(
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Re: Changing shaft size

Post by 30wn78 »

From what I read a volvo seal needs burping to equalise so if you dry you would need to "burp" every time before starting the engine. That sound like hard work lifting the floor every time so I'd prefer a self venting system. If what you have is a vetus seal then yours will also self vent so nothing to worry about. From what I gather only the Volvo does it.
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Re: Changing shaft size

Post by mikebuggy »

for what its worth, I changed over from a volvo seal last year to the Radice/Sillette equivalent rubber lip seal. Looks identical but has a vent tube, which I extended with a bit of clear hose to about 2 ft above WL. Works perfectly. No noise. No burping!
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Re: Changing shaft size

Post by 30wn78 »

Thanks Mike that has sealed it!
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Re: Changing shaft size

Post by Chris H »

Yes you may need to change the Volvo Seal.

Not because it may overheat but because it may leak 25mm is 0.4 mm smaller than 1".

Reminds me of a time when a friend said he could not tell whether his shaft was 25mm or 1". So Climbed under his boat in the long wet grass in the middle of winter with my vernier calliper - I was able to let him know that his shaft was ¾ ".

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Re: Changing shaft size

Post by Gary-Cottam »

I was told my Centaur must have a 1" prop shaft due to it's build date.

After I had bought a new split coupling, R&D flexi-coupling, and a PSS seal - When I went to fit everything I found out it was actually 25mm.

Lesson learned, believe nothing and measure yourself!

I do not want to go to the expense of a new shaft and a new prop, so my solution was to have the prop shaft turned down to 25mm with the exception of the part where the prop attaches to it. Probably a strange work around, bit it worked for me!
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Re: Changing shaft size

Post by 30wn78 »

If it works then why not. The only reason for changing to 25mm from one inch is that some of the parts I'm sourcing are easier to get in 25mm.
Still struggling on where to get a shaft. I'd like to buy a prop and shaft from the same place so that taper and keying is done to match if you catch my drift but can't seem to find a source that does both at sensible prices.
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Re: Changing shaft size

Post by 30wn78 »

So a bit of an update. Got my shaft and cutless bearing.
Both 25mm. Seems there's a small amount of play between the shaft and rubber. Is this normal?

I was expecting a tight fit, but that wasn't based on anything just a theory.
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Re: Changing shaft size

Post by Gary-Cottam »

No, for a new bearing I do not believe there should be any play.

When new, it should be a snug fit (not tight), and play only occurs after some wear has taken place.

My understanding is that 1-2mm of play is acceptable, any more and the bearing is due for replacement
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Re: Changing shaft size

Post by aquaplane »

Almost sounds like the cutless bearing is 1" (25.4 mm).
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Re: Changing shaft size

Post by 30wn78 »

Yes I thought it sounded wrong. I rang the supplier of the cutless bearing and they said 10thou is normal in new bearings. It's stamped 25x34x60 which should be perfect for my new 25mm vetus shaft. Annoyingly the bearing is now in the holder so I am going to have to retrieve it some how as they want me to return it to them. TBH I am a little dissatisfied with there customer care.
I will get an accurate measurement with feelers when I get home but the was enough play to rock the bearing on the shaft. Once I have an accurate measurement I can be a bit firmer with them
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