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Tapping a coupler screw hole in an old Roundhouse loco - what size?

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Posted by crossthedog on Saturday, June 18, 2022 8:52 PM

hon30critter
If you have already thought of these things I apologize for not giving you enough credit.

Not to worry Dave, I'm not proud. Thanks for the tips.

doctorwayne
Well, it's good to see that you've finally got a workable coupler installed, but it seems like there was a lot of unnecessary worries about how to do it.

Wayne, worry is what I do best. I usually worry myself into inaction. The success here is that I managed to do anything at all :)

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

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Posted by doctorwayne on Saturday, June 18, 2022 1:48 AM

Well, it's good to see that you've finally got a workable coupler installed, but it seems like there was a lot of unnecessary worries about how to do it.

Were it mine, a suitable file would have made enough room for a Kadee coupler in a plastic Kadee draught gear box, likely shortened somewhat (on the front) so as to be placed where the back end would not interfere with the lead truck, but still allow for the gladhand to be left in-place for easy uncoupling. 
Pretty well any size of screw, from a #2-56 to an 00-90 could have been used, as it's very easy to "bush" the mounting hole in the Kadee coupler box.

Every time we do something which we've not previously tried, we usually make a mistake (or three), but those mistakes are what teaches us to not give up.
I'm not surprised that you carried it through.

Wayne

 

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Posted by hon30critter on Saturday, June 18, 2022 12:59 AM

Hi Matt,

Congratulations on solving your coupler installation challenge!

I'm going to go out on a limb and make a suggestion regarding your locomotive project. I can see some flash and some mold lines on the locomotive. Before you add any more details, I would suggest that you clean all that up. If you have details, like the grab rails that are already installed for example, they may get in the way when you are trying to file things smooth.

I would also recommend that you take the locomotive down to the bare shell before starting to file off the flash. That will avoid getting filings into places like the motor and drive system, or the couplers, where you don't want them. It will also make the locomotive much easier to clean before painting.

Also, grind off the pointy tips on the leading truck's axles. Pointy tips look good on Harleys but not so much on locomotives.Smile, Wink & Grin

If you have already thought of these things I apologize for not giving you enough credit. I look forward to watching the locomotive progress.

Cheers!!

Dave

I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!

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Posted by crossthedog on Saturday, June 18, 2022 12:38 AM

Good news! I like to provide a wrap-up when my original issue is solved so here it is, and I hope this thread may prove useful to the next person who wants to add a front coupler to a mid-'70s Roundhouse pot-metal locomotive.

I have been successful in drilling and tapping a hole for a 00-80 screw in the center of that boss behind the pilot and I've mounted a Kadee coupler in there.


Atlantic1

atlantic2

I drilled the hole by hand with a pin vise. It was a challenge to drill the hole perfectly straight up and down to the plane of the underside of the loco, and it was equally challenging to tap the hole, but I did like you fellows said -- took it slow and used machine oil -- and I got enough threads for a grab without even having to drill all the way through the pilot.

To keep the coupler on I used one of the box lids from a Kadee ol' No. 5, cutting it down to just a narrow strip with the hole in the middle. Because the hole was bigger than the screw head, I also cut a small strip of white styrene to act as a washer to hold everything together. I used the clearance bit to make a hole in the styrene for the screw.

The only 00-80 screw I was able to get at my local hobby shop was way too long, about 7/8", but I cut it down to about 1/8" and it worked without even any filing.

It still needs some work. For starters, my LHS didn't have plastic couplers, so I used a #148. When I can get a plastic coupler I'll replace it, but since the plastic coupler doesn't have whiskers I'm not sure how that will work. The coupler has nice springiness, although it catches a little at the far edges sometimes. So it's not perfect, but for the first time in its almost half a century my old 4-4-2 is now able to pull in reverse.

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

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Posted by dstarr on Monday, May 2, 2022 5:55 PM

Lets assume you are going to use a 2-56 screw.  That size fits most coupler boxes and couplers.  For 2-56 the tap drill (drill that leaves enough metal for the tap to cut threads in, is a #50 or #51.  The clear drill (lets the 2-46 screw go clear thru, no threads, like the hole in a washer) is #41.  It may require quite a bit of filing or Dremeling to open the hole in that pilot big enough to accept a regular coupler box.  Assume that is a "hot chassis" locomotive (juice for the motor flows thru the locomotive chassis) you will want electrical insulation in your coupler installation.  Otherwise double heading with another hot chassis locomotive (the common case) you have a 50-50 chance of a short circuit if the coupler is conductive.  The common Kadee #5 is made of metal and conducts.  It may be easier to find a brass pilot that accepts a coupler box than filing that Zamac pilot out enough to accept a coupler box. 

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Posted by Overmod on Monday, May 2, 2022 2:48 AM

gmpullman
There's a certain triangular, rolled thread, among others, designed for such use. You really only need 15% or 20% thread engagement to hold a coupler in place.

It's unexpectedly hard to find an illustration that shows the 'trilobular' shape of these things, but here is a brochure with all kinds of not-clearly-illustrated variants:

https://taptite.com/assets/files/taptiteii-conti-reminc_5.pdf

I had no idea they made the things as small as 0-80... while most of the stocked variants only go down to 2/56, the "original" goes as small as 00-90!

I think there's enough material thickness that you could use a trilobe type without splitting or stripping the metal... but I'd still prefer cutting threads with a fluted tap set.  Their recommendations for clearance drilling in zinc die castings (p.12) only go down as far as 2/56 but you can extrapolate to 0-80...

Yes, I think cutting or filing the back of the V of the pilot is the easiest way to open the pilot.  You could cut, file, or saw the individual slots, but that would leave the structure looking as solid as a GSC cast pilot... many of these pilots were fabricated from used 2" or whatever boiler-tube material, simply hammered flat and drilled for attachment, which is part of the argument for using or making a brass replacement.  Zamac or other pressure die-casting alloys could make a 'scale' pilot, but it would be weak and not impact-tolerant...

Working from the back is like cutting to tune a 'ping' putter to a particular note: you don't see the 'inside' so you can take off just the material you need and the outside will still look good.

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Posted by SeeYou190 on Monday, May 2, 2022 12:32 AM

crossthedog
can a plastic coupler -- like a #26 -- be put into the narrow #262 coupler box?

It can, but the centering spring will not fit. That is why the #262 is best used with Whisker couplers... no separate centering spring.

-Kevin

Living the dream and happily modeling my STRATTON AND GILLETTE Railroad in HO scale. The SGRR is a freelanced Class A railroad as it would have appeared on Tuesday, August 3rd, 1954, in my personal fantasy world of plausible nonsense.

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Posted by crossthedog on Monday, May 2, 2022 12:01 AM

Overmod
If I'm not mistaken, a very little filing or grinding from the back will open up the 'flashed' areas between spokes of that pilot. You could remove more material and it would still be strong.

Overmod, I may have some questions for you about this. I keep rereading it and trying to figure out exactly where you mean to grind or file, and I haven't doped it out yet. I wonder if you mean underneath on either side of the vertex formed by the back of the "V" of the pilot. I may resort to chalking up my photo with arrows because I want to know what your idea was, but it's past my bedtime.

Also, I see that you noted already that Kadee's 2-56 screws might be nylon.

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

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Posted by crossthedog on Sunday, May 1, 2022 11:48 PM

@Kevin, thanks for clarifying. A further question (because while with each response I learn a little more in absolute terms, the field of unknowns seems to widen so that I actually know LESS as a percentage of what is known to be knowable -- and in fact I believe as the answers roll in I am asymtotically approaching total ignorance in this matter of adding a front coupler on a vintage "pot metal" loco): can a plastic coupler -- like a #26 -- be put into the narrow #262 coupler box?

Gratefully, as ever,

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

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Posted by SeeYou190 on Sunday, May 1, 2022 11:36 PM

SeeYou190
Delrin, just because that is what the Kadee screws are made from.

Whoops! Scratch that too.

They are actually made from Nylon.

-Kevin

Living the dream and happily modeling my STRATTON AND GILLETTE Railroad in HO scale. The SGRR is a freelanced Class A railroad as it would have appeared on Tuesday, August 3rd, 1954, in my personal fantasy world of plausible nonsense.

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Posted by SeeYou190 on Sunday, May 1, 2022 11:21 PM

crossthedog
I may try the #8 first. You said it supposedly would work here, but you didn't mention whether or not it's nonconductive, whereas you specified that about the 20 series.

The Kadee #8 is conductive. The possibility of a short is remote, and I am overly cautious about these things. However, if you have ever had to deal with an intermittent flying short, you might become overly cautious along with me.

crossthedog
Do you think a #242 might actually go into that spot? It would be great if I could avoid filing out all that metal. And secondly, why Delrin, specifically?

You would need to remove a lot of material to make it fit.

Delrin, just because that is what the Kadee screws are made from.

crossthedog
This looks frighteningly difficult. Why replace it, and why brass?

I did not realize the pilot already had an opening for the coupler. Scrap this comment. No pilot replacement is warranted.

crossthedog
No one has yet mentioned the #262 coupler box.

The #262 coupler box only works properly with Whisker couplers. It is narrower, and it is my favorite for freight cars. It also does not allow as much side-to-side coupler swing as the #242 coupler box.

-Kevin

Living the dream and happily modeling my STRATTON AND GILLETTE Railroad in HO scale. The SGRR is a freelanced Class A railroad as it would have appeared on Tuesday, August 3rd, 1954, in my personal fantasy world of plausible nonsense.

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Posted by cowman on Sunday, May 1, 2022 6:50 PM

Any locos that I wanted a front coupler for I went to the Kadee listing to get the coupler number, then to the page where  it shows what is  needed for installaton in that particular loco.  Mine were all for plastic locos so things ma be different.

Good luck,

Richard

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Posted by TwinZephyr on Sunday, May 1, 2022 6:33 PM

The instructions for this locomotive (which can be found at hoseeker.net) call for a 0-80 x 1/8" screw and a flat square cover plate when installing the front coupler.

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Posted by gmpullman on Sunday, May 1, 2022 5:25 PM

crossthedog
And I guess what I'm calling the 'stump' or 'chimney' is officially called a 'boss'?

Boss (engineering)

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In engineering, a boss is a protruding feature on a work piece. A common use for a boss is to locate one object within a pocket or hole of another object. For instance, some motors use a precisely machined boss on the front face to locate it on the mating part. Like a process on a bone, bosses on castings can provide attachment points or bearing surfaces.

The term 'boss' when used in engineering can also relate to a finishing edge around (usually) a circular opening that allows the opening to locate onto, or within another opening thus locating or joining two items together with a view to the location or joining being temporary or semi-permanent. A common everyday example of a boss is the housing of the rotation spindle in a washing machine drum, or on a cylinder lawn mower at the end of the cutting blade cylinder which may house a bearing set to allow the cylinder to rotate through one plane, but held firm in another plane.

A boss may also refer to a mounting feature that will receive a screw or thread-forming screw.

The word boss comes from the Middle French word embocer, which means protuberance.

Cheers, Ed

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Posted by crossthedog on Sunday, May 1, 2022 5:17 PM

I'm still paying attention and taking notes. No one has yet mentioned the #262 coupler box. My understanding is that that one is narrower than normal. Do we think that would be an option? after removing the stump of course.

And I guess what I'm calling the 'stump' or 'chimney' is officially called a 'boss'?

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

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Posted by gmpullman on Sunday, May 1, 2022 3:58 PM

Overmod
Ed, didn't they used to be called bottoming taps?

I guess that's up to local custom? Supper or Dinner? (Beans on the railroad).

I don't know how big of a "stash" of hardware the OP has on hand but when I'm sometimes confronted with looking for fastener or other hardware needs I dig into the many bins of "too good to toss out" hardware I keep around:

 IMG_7186_fix by Edmund, on Flickr

I've never tossed any kind of appliance or electronic gear without first extracting and harvesting all the little screws, cogs, wire harnesses and such.

In the case of the blind hole in pot-metal I could probably find a "washer-head tapping screw" that would fit the bill wothout any tapping at all.

There's a certain triangular, rolled thread, among others, designed for such use. You really only need 15% or 20% thread engagement to hold a coupler in place.

Related to keeping a handy assortment of screws on hand, I recently picked up this assortment (there are others out there, too) of "laptop hardware screws".

M2-M3 Screw Assortment

An advantage with these particular screws is that some of them have a nice, wide and flat head on them that are ideal for the clearances sometimes needed for three axle trucks or close-fitting draft gear parts.

I don't know why a 0-80 tap is called for? Those are used for the smaller, side mount holes on the Kadee draft gear. The center hole is made for a 2-56 screw.

 [edit] OK, I see that if the existing boss is kept then a smaller tap would be needed for the inner diameter hole. Embarrassed

 

Good Luck, Ed

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Posted by Overmod on Sunday, May 1, 2022 3:29 PM

Ed, didn't they used to be called bottoming taps?

Here's a sample set of three (taper, plug, bottom) and they call it 'bottom' there:

https://drillsandcutters.com/0-80-hss-machine-and-fraction-hand-tap-set-tap-america/?gclid=Cj0KCQjw37iTBhCWARIsACBt1Iyb_4HjiX7anoJMCwtmpEy5gLmw_rzyuMKClwcRnDS2ddXenX45NrMaAqbfEALw_wcB

To be honest, I'll bet the hole is long enough, and the screw length to hold the thing potentially short enough, that a typical taper tap alone might still give him enough full thread to hold.

He could also 'cheat' by chucking the screw in a Dremel and tapering the tip a bit...Whistling

 

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Posted by Overmod on Sunday, May 1, 2022 3:21 PM

If I'm not mistaken, a very little filing or grinding from the back will open up the 'flashed' areas between spokes of that pilot.  You could remove more material and it would still be strong.

I wonder if, for the number of times you'd actually hook something to that front coupler, you could cut some strategic slips of Kapton tape and arrange them in the pilot recess and perhaps around the 'stump'.  A thin piece of something like styrene would give you the custom 'washer' to substitute for the missing broad-headed or washered screw when you use the 0-80.

The plastic screw (may be nylon rather than Delrin) is proposed because it is dielectric (insulating)

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Posted by gmpullman on Sunday, May 1, 2022 3:16 PM

I hate to throw another curveball in here but keep in mind that if you plan to tap a blind hole you'll need a "bottom tap".

There are primarily three types of tap, taper, plug and bottom. Often these are sold as sets. In order to cut threads "nearly" to the bottom of that hole you'll need a bottom tap.

Alternately, you could drill the hole all the way through then use the taper tap to pass all the way through. A taper tap cuts the threads progressively where as a plug tap starts cutting right away, thus may break easier in a blind hole. That's where the plug tap comes in by cutting the "lead-in" threads.

crossthedog
but as several of you have since pointed out, I cannot use that coupler because it's conductive.

Kadee makes an extensive line of non-metallic couplers:

https://www.kadee.com/shop-by-category/couplers/ho-scale-couplers/26-ho-scale-20-series-plastic-couplers-with-gearboxes-long-2564-centerset-shank

 

Good luck, Ed

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Posted by crossthedog on Sunday, May 1, 2022 11:24 AM

Why does everything I want to do seem to end up as a kitchen remodel? ("I'll just pull out the old dishwasher and install the new one in the same spot... it should hook right in!")

Thanks Overmod for quickly answering my original question. That was actually the size I was eyeing to purchase. Now, on to all the fretsome issues my plan has exposed...

doctorwayne
Well, unless I'm not seeing everything clearly, it looks to me as if there's part of the pilot casting that's blocking the opening through which the coupler's shank should extend. If that were my locomotive, the first operation would be to open that area using a cut-off disc in a motor tool, then use the same disc to remove that stubby stump where you plan to place the screw.

The casting does NOT in fact block the opening, although the way I shot the first photo does suggest that. Here's a front view.

The opening is narrow, however, and won't fit a #5 box, as you all probably already know. It's about 7/32" wide.

hon30critter
Before doing any of that, I would check to see that a coupler will actually fit over the stub without being either too tight or too sloppy.

I stuck a #148 whisker coupler (sans box) in there and clipped off its hose, just to see how far over the pilot it would extend, and it reaches far enough that the pilot would not interfere with coupling to a normal freight car, but as several of you have since pointed out, I cannot use that coupler because it's conductive.

SeeYou190
That stump will fit the hole in an "NMRA" horn hook coupler, and that coupler will spring-lever-center against the side of the recess. It will be difficult to make that stump work with a Kadee coupler, but suppsedly it can be done with a #8 coupler.

I may try the #8 first. You said it supposedly would work here, but you didn't mention whether or not it's nonconductive, whereas you specified that about the 20 series.

SeeYou190
The locomotive should have included a small screw with a very wide head for this coupler.

Long gone, if ever it was. Oddly, the box does still contain a thin, very wide metal washer, which would itself cover the entire draft box area if I had a screw for the hole. I don't know if this washer was original to the box. Circa 1979 this loco chuffed full steam and pilot-first off a dead end track and plummeted into the abyss, where the pilot truck shattered. You can see in this photo that in repairing the engine, my dad replaced the original wheels with freight truck wheels with the pointed ends. So the washers may have come from that misadventure. 

SeeYou190
Get rid of that pilot and replace it with a brass pilot.

This looks frighteningly difficult. Why replace it, and why brass?

SeeYou190
Install a Kadee 20 series coupler* in a #242 snap together coupler box. Use a Kadee black plastic 2-56 delrin screw.

A couple questions here: Others above have surmised that the space will have to be enlarged to fit any kind of box, and it looks that way to me. Do you think a #242 might actually go into that spot? It would be great if I could avoid filing out all that metal. And secondly, why Delrin, specifically?

Thank you all very much. As usual I have vexing choices to make now.

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

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Posted by SeeYou190 on Sunday, May 1, 2022 8:39 AM

That stump will fit the hole in an "NMRA" horn hook coupler, and that coupler will spring-lever-center against the side of the recess. It will be difficult to make that stump work with a Kadee coupler, but suppsedly it can be done with a #8 coupler.

The locomotive should have included a small screw with a very wide head for this coupler.

What I would do... take any advice at your own risk.

1) Get rid of that pilot and replace it with a brass pilot. Preferably one that already has a coupler opening in it. If you get a pilot with more vertical bars, or even steps, the coupler can be shorter and look better.

2) File off the stump completely.

3) Drill through and tap for 2-56.

4) Install a Kadee 20 series coupler* in a #242 snap together coupler box. Use a Kadee black plastic 2-56 delrin screw. Once coupler height has been adjusted, trim the screw flush with the top of the pilot deck and touch up. No one will notice it.

* The locomotive has a metal frame, and 20 series coupler are non-conductive. This will 100% eliminate any possibility of introducing a difficult to find intermittent short circuit.

Maybe this will help.

-Kevin

Living the dream and happily modeling my STRATTON AND GILLETTE Railroad in HO scale. The SGRR is a freelanced Class A railroad as it would have appeared on Tuesday, August 3rd, 1954, in my personal fantasy world of plausible nonsense.

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Posted by hon30critter on Sunday, May 1, 2022 1:26 AM

Hi Matt,

I just bought a 0-80 tap and drill set from Amazon. It was about $20.00 Cdn. There were much cheaper taps available but the reviews were not very favourable.

If you haven't done a lot of tapping, perhaps I could offer you a few pointers:

First, I wouldn't assume that the hole is the correct size even if it was intended for a 0-80 thread. If you buy a matched tap and drill set the drill will ensure that the hole is the right size for the tap to work properly. If the hole is even a bit too small, the odds on breaking the tap are much greater.

Second, as Overmod suggested, you need to use a lubricating oil. I use 85 weight Nano oil but any standard 3 in 1 oil or reasonably thick engine oil will do. Note that most cutting lubricants are designed for machine use where speeds will be higher and the lubricant has to act as a flushing agent as well. When you are tapping by hand, speeds are much slower.

Third, also as Overmod suggested, you need to take your time. The proper procedure is to rotate the tap 1/4 turn forward and then reverse it 1/2 turn. Reversing the tap cuts off the displaced material and helps to clear the tap threads. When you go forward again, only rotate the tap 3/4 turn and then back it off 1/2 turn again. In other words, you are only cutting the threads 1/4 turn at a time. You should also remove the tap completely every two or three full turns and clean the cutting debris from the tap threads. Before putting the tap back in, add some more oil.

Before doing any of that, I would check to see that a coupler will actually fit over the stub without being either too tight or too sloppy. If it does fit, you would be wise to check the coupler height if that is possible at your stage of assembly. Ultimately, it might be easier to simply remove the stub, file the opening to fit a Kadee coupler box, and just use the 2/56 tap that you already have.

You might also need a long shank coupler to clear the pilot.

Have fun,

Dave

I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!

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Posted by doctorwayne on Sunday, May 1, 2022 1:09 AM

Well, unless I'm not seeing everything clearly, it looks to me as if there's part of the pilot casting that's blocking the opening through which the coupler's shank should extend.
If that were my locomotive, the first operation would be to open that area using a cut-off disc in a motor tool, then use the same disc to remove that stubby stump where you plan to place the screw.

Left in place, that stump would not allow installation of a Kadee draught gear box.  If after those operations, there's not enough clearance for a draught gear box, I'd suggest using a needle file to enlarge the height (or depth) of the opening.
For a cast metal locomotive, the metal Kadee coupler needs to be in a plastic box to prevent electrical shorts.

Once the opening for the draught gear box is large enough and at the proper height, you can temporarily insert it to enable you to mark for the location of the screw which will be used to secure the coupler box in-place.

Use a #50 drill bit to make the hole for the screw, then use your 2-56 tap to create the threads.  The screw should be long enough to pass through both the draught gear box and the thickness of the pilot deck.  Once the coupler is in place, use a suitable saw (or the cut-off disc) to remove the portion of the screw which is sticking above the pilot deck, then file-down any remaining excess.

I should also mention that you may need to use a specific Kadee coupler...perhaps one with an under- or over-set head, and/or possibly a coupler with a longer shank to prevent interference with the pilot when the loco is coupled to another loco or car.

Wayne

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Posted by Overmod on Saturday, April 30, 2022 10:58 PM

I suspect this would take a 0-80, which used to be a commonly-encountered size in model railroading.  I think you can cut 80tpi in that soft casting, with correct lubricant, but be careful and go slow -- a hardened 0-80 tap may break VERY easily and you'd need to resort to watchmaker tricks  to get the broken tip out...

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Tapping a coupler screw hole in an old Roundhouse loco - what size?
Posted by crossthedog on Saturday, April 30, 2022 10:18 PM

Hi all,

My old Roundhouse Atlantic has never had a front coupler. There's a very small stump with a hole in it in the middle of the coupler box area, but there's never been a coupler box installed nor a screw in the stump. My smallest tap and drill set is a 2-56 / 51. Here is the tap juxtaposed to the hole.

It looks too large, the tap is almost as wide as the stump or chimney that the screw would go into. I have almost zero knowledge of tap sizes, whether some are common (except the 2-56), etc. Do any of you have any experience with this? What size would you suppose is a good size to try here? I'll have to buy another tap and drill, or maybe a set that would include useful sizes. Or is there a simple way to figure out how big the existing hole is (there are no threads in it) and buy a tap that matches that drill size? I just need a little guidance.

Thanks,

-Matt

Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.

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