3″ – 4″ scale

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3″ – 4″ scale

This topic has 11 replies, 6 voices, and was last updated 10 June 2024 at 09:01 by UncouthJ.

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9 June 2024 at 17:19 #735090
UncouthJ
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@uncouthj
<p style=”text-align: left;”>Hey folks…</p>
I’m sure there were some very detailed answers to this on the old forum, and not going to get into the loss of content upset here…

However… If I have plans for a 3″ traction model, specifically a Suffolk dredging tractor, can I just add a 1/12″ to each dimension to make a 4″ scale or is it more complex?

Ta muchly

J
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9 June 2024 at 17:53 #735102
norm norton
Participant
@normnorton75434
You will have to re-draw it. Not too hard. Most of the dimensions can be multiplied by 4/3 to give you a 4″ scale model, but some of the dimensions on the 3″ drawings will have been set to specify standard material like 1/8″ plate or a 1/2″ shaft. Now you are going to have problems if you try and apply 4/3 to everything.

Threads cannot be upped by 4/3. You will have to choose a suitable replacement ME (32 or 40tpi) or BA thread. That’s why it is a re-draw and redesign.

Can you not find a 4″ engine that you would like to build?

Norm
9 June 2024 at 18:09 #735106
JasonB
Moderator
@jasonb
As Norman says 4/3 is the ratio you need, the four (4 x 3″) takes it up to full size and the three (12″ / 3″) brings it down to 4″ Then adjust to sensible sizes.

The boiler will also need new calculations as that can’t be done by the simple 4/3 method
9 June 2024 at 18:10 #735108
UncouthJ
Participant
@uncouthj
Yeah I get certain things such as you mention will be ‘bodge its’. I’m after function over form. Just want something to pootle round the rally fields and I adore the Suffolk.

ATB

J
9 June 2024 at 18:18 #735110
norm norton
Participant
@normnorton75434
Yes the boiler is a nuisance because an existing design can be approved under the UK scheme but if you rescale it exactly to 4/3, or any other scale you think of, it is a new design. Scuppered is the term.

Norm
9 June 2024 at 18:37 #735114
UncouthJ
Participant
@uncouthj
Ah ok that’s definitely an important consideration. I’m 90% sure I have the drawings for the boiler itself in 4″ in one of my boiler books, so that might not be too much of an issue.

Didn’t know boilers were graded on an existing or new template. Thought they just had to pass a hydro test.
9 June 2024 at 18:48 #735116
bernard towers
Participant
@bernardtowers37738
Boiler testing info avail from FMES, it would make a good read for you and also help take away any nasty surprises.
9 June 2024 at 19:08 #735118
UncouthJ
Participant
@uncouthj
Can’t seem to like your post in the new format so have a couple of these instead 👍👍 thanks Bernard
9 June 2024 at 21:05 #735127
Grindstone Cowboy
Participant
@grindstonecowboy
Just for the record, the ability to ‘Like’ a post never existed in the old format either 🙂

Rob
9 June 2024 at 22:10 #735131
UncouthJ
Participant
@uncouthj
<p style=”text-align: left;”>Oh did it not? Confusing my forums…</p>
10 June 2024 at 08:51 #735154
duncan webster 1
Participant
@duncanwebster1
On 9 June 2024 at 18:18 norm norton Said:

Yes the boiler is a nuisance because an existing design can be approved under the UK scheme but if you rescale it exactly to 4/3, or any other scale you think of, it is a new design. Scuppered is the term.

Norm

Just get a copy of the Australian code and do a few sums. Most of it is in look up tables, so that doesn’t need sums.
10 June 2024 at 09:01 #735156
UncouthJ
Participant
@uncouthj
Good tip 👍
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