Shaft size - metric or imperial

superheat6k

superheat6k

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My boat was built in 1990, so this would suggest my shafts are metric 38mm, but the engines are American Cummins, imperial, with Hurth gearboxes. The simple measurement with a vernier is spot on 1.5".

The R&D coupling has bolts with a fine thread and a somewhat odd 16mm AF nuts (or 5/8" AF).

I am replacing the Cutless bearings and fitting PSS seals to replace the original stuffing boxes.

So do I use 38mm (1.496") or 1.500" bearings and seals. Or do I get the micrometer out and measure the shaft properly ?
 
To go down to the last thou or so, I'd have more faith in a micrometer reading than a vernier. Another pointer should be that a metric shaft will usually have a 10:1 taper, 12:1 for imperial. Or does 0.004" really matter on a cutless bearing!
 
For a cutless bearing, you need to get the overall diameter precise as well. Given the age of your boat, I'd expect the measurements to be imperial. From bitter experience I know that a 45mm cutless bearing will not fit in a 1¾" P-bracket!
 
superheat6k said:
My boat was built in 1990, so this would suggest my shafts are metric 38mm, but the engines are American Cummins, imperial, with Hurth gearboxes. The simple measurement with a vernier is spot on 1.5".

The R&D coupling has bolts with a fine thread and a somewhat odd 16mm AF nuts (or 5/8" AF).

I am replacing the Cutless bearings and fitting PSS seals to replace the original stuffing boxes.

So do I use 38mm (1.496") or 1.500" bearings and seals. Or do I get the micrometer out and measure the shaft properly ?
Click to expand...

Depends not only on when the boat was built, but also on where.
The American engines point to possible US origin. You don't mention the make.
My previous boat was UK- built in the early 80's and was a very mixed bag, having been re-engined on the early 2000s with a Beta, but still retained its 1" dia shaft, which required a 1" anode. Strangely, all the s/s woodscrews used were of aeronautical origin, requiring a special trefoil screwdriver head.
 
I got a cutlass bearing for the from Johnson Duramax on the US of A. They don't have them on the shelf so I had to wait a month before they sent it (didn't worry me)
If you go to this site you can download the pdf specifications for Imperial as well as metric

http://www.duramaxmarine.com/bearingLine2.htm

Duramax Johnson Cutless Bearings – Your Complete Guide ...
https://citimarinestore.com › citiguide › johnson-cutless-bearings-your-co...
Another indicator that it is time to change your cutlass bearing is if you hear a rumbling or vibration that increases over time. The sound of a bad bearing is kind of like a drumming – if you hear this, it is past time to change your bearing.

Cutlass Bearings - when to replace? - Yachting and Boating World
www.ybw.com › Forum › Forum › Motor Boat Forum
Oct 5, 2016 - Re: Cutlass Bearings - when to replace? They need replacing if there is more than a couple of mm sideways movement but 470 hours seems low. I'd expect around 1000 hours in clean water (Med).
 
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I removed my R&D coupling yesterday so can help with the bolt size, they're 7/16" diameter, probably unf but didn't check the pitch. The shaft on our boat is 40mm dia. I suspect the R&D use they're own bolt sizes, i didn't measure the Volvo coupling bolt holes accurately so I don't know if an m12 bolt will fit when i change back to a fixed coupling arrangement.

Edit... if I were to machine a shaft I'd use precision ground bar to get the straightness, don't know if this is common (or good) practice but if your mrasuring 38mm and not 38.1mm id go for 38mm
 
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I've never bought metric bar, so I don't know what the standard sizes are. If you were designing something wouldn't you go for 40mm, a round number?

I'm guessing it's a 1.5" shaft. The 7/16" bolts might confirm you're working in inches, or might not!
 
AndrewB said:
For a cutless bearing, you need to get the overall diameter precise as well. Given the age of your boat, I'd expect the measurements to be imperial. From bitter experience I know that a 45mm cutless bearing will not fit in a 1¾" P-bracket!
Click to expand...

+1
Current boat during survey had imperial 1 3/4 bearings in a 45mm stern tube and was found to be spinning.

If no mic is availible a tape measure can be used as a pi tape if narrow enough
 
I cleaned the shaft today in readiness for next weekend's task to remove them. It was easier tho clean them with the props still on as this gave me something to rotate them with.

A lovely brand new 10 tonne hydraulic gear puller made short work of getting the props off BTW.

With a clean shaft and close up careful nip of the vernier I would definitely say 1.500" rather than 1.496". Looking at cutless bearings 1.5" is a common size whereas 38mm is less so. I won't order these until the old ones are removed. Looks like the upper cutless is also a full length (my Turbo 36 had half length uppers).

I did manage to free of 7 of the 8 cutless securing grub screws.

As the shafts are coming out for the new PSS seals it seems prudent to replace the cutless bearings which are quite for anyway, at the same time.
 
superheat6k said:
My boat was built in 1990, so this would suggest my shafts are metric 38mm, but the engines are American Cummins, imperial, with Hurth gearboxes. The simple measurement with a vernier is spot on 1.5".

The R&D coupling has bolts with a fine thread and a somewhat odd 16mm AF nuts (or 5/8" AF).

I am replacing the Cutless bearings and fitting PSS seals to replace the original stuffing boxes.

So do I use 38mm (1.496") or 1.500" bearings and seals. Or do I get the micrometer out and measure the shaft properly ?
Click to expand...

Wouldn't be influenced by the R&D bolts. If the shaft measure 38mm ish or 1.5" it's more likely to be imperial, 38mm shafts would be very unusual. A digital vernier should give you 38.1mm, if you have an "analogue" vernier i'd break the mic out.
 
Trev

Either measure with a mic or pop a shaft in the boot of the car and run it round to Paul at Hamble Props, he'll tell you.

He'll have cutlass bearings in stock too, or Aquafax.
 
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