Screw sizes: speaker mount in Piko BR 194

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notofthiscenturyTim

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Does anyone know the size of the four screws needed to fit a speaker in a Piko BR 194 on the mounting provided on the base of the loco?

The speaker is a Visaton FRS 7 which is the same as the one sold as part of the official Piko sound conversion kit. I know that screws are included in that kit but I haven't been able to find out what size they are. They look like thread forming screws designed for thermoplastics.
 
I would think they are self tappers as you say, but sorry, I don't know the size.

I'd be tempted to try what I have starting with the smallest until I had a size that 'bites'....
 
Gizzy said:
I would think they are self tappers as you say, but sorry, I don't know the size.

I'd be tempted to try what I have starting with the smallest until I had a size that 'bites'....
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Thanks. To clarify, the screws on the Piko product shot look like this. I'm pretty sure they are self-tapping ones of the thread-forming type which displace rather than remove the material they're being screwed into. I think this is what makes them good for plastic and why they don't have the pointy end found on other types of self-tapping screws designed for wood or metal.
I've been finding it really hard to get a set containing these types of screws in different sizes.
 

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Greg Elmassian said:
I am of the opinion that the pointy end is much better.

I would not worry too much, as long as they are sized correctly. Think about it, the ones without pointy ends need to be sized more closely, since they do not have the point to lead the screw in.
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Thanks Greg.
My concern with the pointy ended ones is that they have a larger thread profile angle which is designed for wood and metal and which creates more internal radial stresses during the removal and deformation of plastic which can lead to cracking. It may well not happen with the Piko's plastic but I'd rather not chance it.
I'm going to buy the set of Piko screws and at least if none of them fit I'll have a better idea of what size is likely to.
 
With the flat end, you need to be sure that the minor diameter of the screw is smaller than the hole or it will help expand the hole too much... it's the difference between putting a tapered rod into a hole, or possibly forcing a rod into a hole too small...

The larger profile angle should reduce the amount of thread in contact and be nicer to the hole, right? like the difference between a coarse screw and a fine pitch screw?

you do mean "larger thread profile angle" as the "coarseness" of the thread pitch right?

Or are you possibly talking about the thread contour itself?
 
Greg Elmassian said:
With the flat end, you need to be sure that the minor diameter of the screw is smaller than the hole or it will help expand the hole too much... it's the difference between putting a tapered rod into a hole, or possibly forcing a rod into a hole too small...

The larger profile angle should reduce the amount of thread in contact and be nicer to the hole, right? like the difference between a coarse screw and a fine pitch screw?

you do mean "larger thread profile angle" as the "coarseness" of the thread pitch right?

Or are you possibly talking about the thread contour itself?
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Thanks Greg. The ones with the flat end need a pilot hole of the correct size so that only the threads engage with the plastic. It looks like the Br194 has pilot holes pre-moulded.

I think diagrams might help with the thread questions. Here is a diagram showing the different thread profile angle aka flank angle of fasteners designed for plastic (30-48° depending on the properties of the plastic) and for wood/metal(60°). The coloured diagrams on the right show the material that the fastener is being screwed into. CM is the thread diameter and is larger for fasteners for plastic because plastic is softer. I've also attached a diagram showing the features of a thread forming screw which looks like the sort used by Piko. Credit for both stanleyengineeredfastening.com
 

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Very cool, yes that was the point I was making, you need the right match of hole diameter and screw if you use the "flat ended" screws, that is why I normally recommend the pointy ones, it seems that there are slightly different ones for the 2 major manufacturers I use.

That was what I was trying to say with; "With the flat end, you need to be sure that the minor diameter of the screw is smaller than the hole or it will help expand the hole too much..."

So flank angle is part of the thread profile, and not the pitch. Nice to learn another term that is industry recognized.

I always try to keep the original screws close by, stuck to an old computer hard drive magnet, so I don't mix them up with others.

Thank you! Greg
 
Just to close out this thread. Many thanks to all who contributed especially @WillemD - Dankjewel!
I used the screws from the Piko spares set earlier today and they worked perfectly.
The sound is spectacular. Piko did a great job providing a built-in grille and set of mounting points for the Visaton FRS 7 speaker. Positively neighbour-disturbing. I may need to reprogramme those volume CVs.
I'm also very happy with the Drive-XL DCC decoder and SUSI SX6 sound decoder from MXion/ML-Train. Total cost to digitise the Croc, including the Visaton speaker and Piko screws set, was exactly £106. Using official Piko parts was going to be approx £280 as far as I could see.
So I and my three-year-old "engineer" are very happy bunnies - "Daddy, I want to make the Crocodile whistle" etc.
 
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I like the FRS7... the fuel tanks at the rear of the pictures have FRS 7's

assembly_line.jpg
 
Greg Elmassian said:
I like the FRS7... the fuel tanks at the rear of the pictures have FRS 7's

assembly_line.jpg
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Yes, it sounds great to me. Thank goodness there's space in the loco carcass to fit all 65mm of it.
Makes me inclined to go for more Piko standard gauge electric and diesel locos in the future as there's so much more space to fit a full-range speaker compared to LGB narrow gauge steam models, beautiful though they are.
 
Greg Elmassian said:
I like the FRS7... the fuel tanks at the rear of the pictures have FRS 7's

assembly_line.jpg
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Agreed. I put one in my Piko railbus, where of course it fitted perfectly since the mounting is designed for one.. Growls nicely.
 
notofthiscenturyTim said:
Yes, it sounds great to me. Thank goodness there's space in the loco carcass to fit all 65mm of it.
Makes me inclined to go for more Piko standard gauge electric and diesel locos in the future as there's so much more space to fit a full-range speaker compared to LGB narrow gauge steam models, beautiful though they are.
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PIKO steam locos also have room and fittings for beefy speakers, at least Visaton 50mm, even the BR 80 0-6-0T.
 
idlemarvel said:
PIKO steam locos also have room and fittings for beefy speakers, at least Visaton 50mm, even the BR 80 0-6-0T.
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That's good to know. Didn't realise you could fit a 50mm speaker into the BR 80.

I am being a little unfair to LGB as the two I have are much older than the Piko designs with either no provision for sound or sound as something of an afterthought, like the Stainz speaker arrangement in the cab roof.
 
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