Captive nuts.

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BurnitBlue

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I have failed to source 10mm captive nuts in stainless steel. I am moving my engine about 2 inches aft to give better clearance between Prop and P-bracket.

Fed with looking and I have decided to make my own. Oh the gloom of it. When I started thinking of how to make such a seemingly simple thing I realised that it was far from simple and I cannot think of a single way to make one. And I need eight so a simple method is needed.

BTW I did manage to source some 10mm galvanised captive nuts from Biltema but they were too flimsy to trust with the weight of an engine.

Any ideas please. Thanks
 
BurnitBlue said:
I have failed to source 10mm captive nuts in stainless steel. I am moving my engine about 2 inches aft to give better clearance between Prop and P-bracket.

Fed with looking and I have decided to make my own. Oh the gloom of it. When I started thinking of how to make such a seemingly simple thing I realised that it was far from simple and I cannot think of a single way to make one. And I need eight so a simple method is needed.

BTW I did manage to source some 10mm galvanised captive nuts from Biltema but they were too flimsy to trust with the weight of an engine.

Any ideas please. Thanks
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Cant do much without knowing what the captive nuts will fit into...
Would they be like J-nuts or U-nuts or cage nuts? Do they fit in a hole or clipped to the edge of a beam... and all that m'larky

mebby a sketch of what you think and/or have will help.
 
Lon nan Gruagach said:
Cant do much without knowing what the captive nuts will fit into...
Would they be like J-nuts or U-nuts or cage nuts? Do they fit in a hole or clipped to the edge of a beam... and all that m'larky

mebby a sketch of what you think and/or have will help.
Click to expand...

Ditto.
Hence, Praxinoscope's suggestion of rivnuts might work, or be quite inapplicable.
 
BurnitBlue said:
I have failed to source 10mm captive nuts in stainless steel. I am moving my engine about 2 inches aft to give better clearance between Prop and P-bracket.
Click to expand...

Why do you want to increase the distance between the prop and P bracket by 2" as this is not a good idea. The normal clearance is 15mm but may be increased (such as when fitting a rope cutter) but suggested maximum is 1.5 times diameter, otherwise you may experience excessive cutless wear.

Even if the distance is too little, moving the engine is not the right way to do it. Either fit a spacer between the flanges aft of the gearbox, or have a longer shaft made.

What are you trying to achieve by moving the prop aft?
 
BurnitBlue said:
I have failed to source 10mm captive nuts in stainless steel. I am moving my engine about 2 inches aft to give better clearance between Prop and P-bracket.

Fed with looking and I have decided to make my own. Oh the gloom of it. When I started thinking of how to make such a seemingly simple thing I realised that it was far from simple and I cannot think of a single way to make one. And I need eight so a simple method is needed.

BTW I did manage to source some 10mm galvanised captive nuts from Biltema but they were too flimsy to trust with the weight of an engine.

Any ideas please. Thanks
Click to expand...

I'm afraid that I do not understand what "captive nut" means when used as a term in isolation? There are many ways of creating a captive nut but without a lot more information about the circumstances it's difficult to give meaningful advice. :confused:

Richard
 
The captive nut will be underneath a partially GRP sheathed strip of plywood bolted on top of the original engine beds using the original captive nuts. This was to raise the height of the engine bed to fit the new engine a Yanmar 3YM30. I did this a few years ago. The original captive nuts embedded in the engine bed are OK for the original engine but completely useless for the Yanmar. For a year I have used the Yanmar feet bolts (10mm) upside down with the head under the spacer and normal nut on top.

I want to use the normal system for the engine bolts that point down. They can be removed completely when I need to check engine alignment. Right now when I remove the nuts, the studs are sticking up and making small movements of the engine almost impossible.

I need to move the engine back at least an inch to clear the P-bracket in forward gear. Cone type gearbox which engages by moving the shaft.

Hope that makes it clearer what I mean. Thanks for the replies
 
Perhaps an easy way to describe a captive nut (my name for that type of nut) is a nut that is embedded below the surface and captive so that it will not turn when the bolt is tightened. Nor will it fall out when the bolt is removed.

Sorry, maybe there is a proper name for that type of nut. .
 
When I was fixing air/craft, a captive nut was one secured by a cage, or some such.
Sounds like you have to move the mounts to a new position. So, need to find fresh points to put the holding bolts in?
Just doing this on an old boat. So opening up the GRP bearers and filling with epoxy, then will tap the threads for the bearer bolts into the epoxy. Covered in the West Epoxy online book.

As usual, a slow typer, so replying to #7...
 
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I think I'm getting the picture.

First question is if you currently use bolts upside down with the head at the bottom, why can't you use the bolts the right way up with the head at the top and the nut at the bottom?

If you really want captive nuts in GRP-sheathed plywood you would either have to remove the plywood and securely glass-in the nuts below or use something like Tappex nuts, epoxied into the plywood from above. These will provide a secure fitting without having to remove the plywood but it will depend upon how thick the glassed-in plywood is.

Richard
 
BurnitBlue said:
The captive nut will be underneath a partially GRP sheathed strip of plywood bolted on top of the original engine beds using the original captive nuts. This was to raise the height of the engine bed to fit the new engine a Yanmar 3YM30. I did this a few years ago. The original captive nuts embedded in the engine bed are OK for the original engine but completely useless for the Yanmar. For a year I have used the Yanmar feet bolts (10mm) upside down with the head under the spacer and normal nut on top.

I want to use the normal system for the engine bolts that point down. They can be removed completely when I need to check engine alignment. Right now when I remove the nuts, the studs are sticking up and making small movements of the engine almost impossible.

I need to move the engine back at least an inch to clear the P-bracket in forward gear. Cone type gearbox which engages by moving the shaft.

Hope that makes it clearer what I mean. Thanks for the replies
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Now I understand. The engine needs to be located so that there is betweem 15-20mm of clearance aft of the p bracket.

If the original beds are wood encased in GRP with your additional ply spacer then suggest you use M10 stainless lag bolts which are like coach screws but without the square under the hex head. Drill a pilot hole (which from memory is 8mm) and screw in. This works well and a lot less hassle than trying to fit captive nuts or insert tapped plates. It will attach the engine mounts very firmly to the original beds.

An alternative is to leave the engine where it is and buy a ready machined spacer of the appropriate thickness from www.ropestripper.com If you are otherwise happy with the way the engine is mounted now this is far and away the easiest solution.
 
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Bingo. Two solutions from YBW. Brilliant. A machined spacer complete with the exact flanges suggested by Tranona solves my immediate problem so I can get afloat as soon as the Ionian heat drops a bit. At £87 for a 25mm spacer it is a bargain. During the winter I can use the suggestion from more than one poster to make a strip of metal tapped for 10mm. One for each engine foot. I did think of a strip earlier but I balked at the geometry and precision need to make a strip for one side (two engine feet on the same strip (four precise holes). Sounds obvious to make one strip for each engine foot, especially as I can use one of the feet as a template to mark out the holes.

Thanks again.
 
Might I also suggest you plug the existing holes and once you are satisfied with the location of the engine use the lag screws i suggested. This is exactly what I did when I changed engines where the mounts were in slightly different locations - one set overlapping the old ones. Used M10*50mm. Remember now the pilot holes were 10mm for the first part where the screw is unthreaded and 8mm for the screw part. Much simpler than trying to insert threaded plates. As a permanent fix you won't then need the spacer as you set up the shaft exactly where you want it and then drill.

When replacing engines the only fixed datum point is the flange on the shaft and you fit the engine to that, making any changes necessary to the beds.
 
John the kiwi said:
That seems like a sensible approach. Whatever you do, you should remember that for safety the engine mounts should hold the engine in position with the boat inverted.
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Yes,,you should have seen our faces when we discovered the both the aft engine mounts on the old engine had bolts and no nuts. It’s keep kind of har to get regular nuts entered onto the bolts inside the engine bearers.
 
Sounds like you have a solution (a spacer woyld have been my prefered way) but if you stull want captive nuts im stainless to go into plywood try these: https://m.ebay.co.uk/itm/M5-M6-M8-M...m4084.l1313.TR11.TRC1.A0.H0.XStainless+t.TRS0

If you do want to move the engine, Personally I'd make a steel plate tapped with the new bolt holes with a counter sunk bolt to go into the original mounts rather than using grp/plywood. Inside the boat a plate of the thickness you are talking (must be at least 10mm if you ate using plywood) could just be mild steel. Even unpainted it would take years to corrode significantly and with the right paint would last the life of the engine - the engine block will be iron anyway.
 
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