This is the start of hints and tips on narowing the gauge on the Bachmann 4-4-0 locomotives sold as the "Jupiter" and as UPRR #119. The first part of the text is from Ted Brandon. Other bits are from Garth Hamilton and Roy Plummer. These are from posts made to the Nn3 email list (Nn3@egroups.com)<P>
-Rick-
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From Ted:

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I may be able to be of some help about converting a Bachmann 4-4-0. First of all, there are two basic versions of this loco: the old one, with a rather heavy driveshaft from tender to loco ( with hexagonal cavities in both ends to engage the hex nuts in both tender motor and loco shaft). The new version, out recently, has an improved transmission system which, while similar in basic design, uses a very slim shaft with a simple universal joint at each end. In addition, the new version has much better details and painting and also runs better out of the box.
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First recommendation: If you have an old version, throw it out.
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Second recommendation: If you have a new one, run it on standard gauge track and forget the conversion.
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If all else fails, begin by looking at Page 18 of the new, 5th Edition, Nn3 Manual, available from NTRAK Publishing for $12.00 postpaid. This is the original article on such conversions, and several have been done with mixed success. I personally found the methods to be less than ideal for several reasons: 1. The old type drive shaft would seriously limit the ability of the loco to go around the tight curves common on narrow gauge railroads in general, and on Nn3 layouts in particular ; 2. The front truck would not traverse enough for those curves; 3. While narrowing the bottom plate on the loco is not too difficult, carving out the plastic superstructure wheel wells to narrow the drivers is a real challenge.
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Now, coming to the tender: Here, the tender IS the motor and the motor IS the tender; there is no separating or replacing the motor. Concerning the trucks, you have two choices: replace with MT trucks (which creates other problems) or modify the existing trucks. If you replace the trucks with MT's, the tender body will sit higher, causing misalignment of the drive shaft between tender and loco. Visually that is not too bad, but, unless you carve out the hex-holes (on the old version), it will run badly.
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You will also have to find ways to pick up and transmit to the brass strips under the tender body, the essential for operation: electricity. If you decide to modify the original trucks, try the method in the Manual. But, rather than do this, I have been toying with a new idea: To cut away the plastic truck bolster between the two metal wheel retainers, then glue the retainers (with their wheels) back-to-back, with a plastic strip inbetween to obtain the correct gauging and assure insulation. You have to drill a new hole to mount the trucks, of course. I have not tried this yet. If you do it, let me know how it works. 
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Good Luck!<BR>
Ted Brandon<BR>
TEDBRANDON@aol.com<BR>
Rio Verde & Western RR (Nn3)
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From Garth:
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Have not done a pilot change on this engine, but I have mounted Nn3 couplers on the tender. I first removed the brass strip covering and two small screws and placed them in my special small parts drawer and then removed the coupler parts and put them in my junk box. The pocket is to deep so I placed two styrene shims .030 thick into the pocket glued them in place. Then I did a height check and filed the top of the shim to reduce the height slightly and then drilled and tapped for 00-90 screw to hold the coupler in place. Last part of the job is to touch up the edges of the styrene where it shows. On the coupler pocket, Yes I opened the back of the pocket up to accomodate the coupler box. 

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The tender trucks have two springs that hold the tender above the truck and make contact between the brass plates and the raised portion on the metal portion that hold the wheels. The wheels should turn freely on the metal axle stubs and  a very small amount of greese will keepthem free moving. All the grees you need can be picked up on the head of a dress makers pin. You may find the wheels are dirty and jammed on the axle stubs, be careful how you pull the wheels off as the white metal casting is not over robust when pulled  in tangent directions. 
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Taking the engine apart. the plate come off easily and with the engine on its back nothing else will fall out. Before taking the plate off suggest you remove the side rods with a small philips screw driver. After removing the plate just gently pull each driver out and set it aside. Then the brass bearing that hold the double worm shaft can bew pushed out the rear of the engine through the cab. The main crank and the steam chest have to be slid backwards to come off the two flat arms that hold them. There is usually glue on the inside where the arms meet the steam chest and I used a needle that had the point bent 90 degrees to the shaft and used that to drag the glue out of the joint. I have also used the hookd blade of the exacto knife to this and I gently work at it until it is free. 
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To narrow the steam chest you are going to have to cut the bars that hold the truck in place. I cut them one at a time at opposite side so them when separated I had the steam chest from one side with one bar of the slot that hold the pilot truck. To refit after narrowing the engine just nip a small piece off of the two arms sticking out and test fit one side at a time. The last thing you do is to cut tha bar that holds the pilot truck in place. The rear brace on the steam chest slide is just pushed into a hole in the casting and should come out easily.
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From Roy Plummer: 
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The weight increase only applies to the new type tender or if you remotor the old with a smaller motor ( Mashima 9x16). First pull out zamac weight from front of tender, remove trucks and brass contact plates from base of tender, pull off plastic drive dog from motor shaft. With suitable pliers pull motor out by drive shaft, (now is a good time to lubricate motor). Mark the motor with a spot of paint on top to retain correct rotation. In the back of the tender shell you will see a recess about 4mm square across the full width.  Make up a piece of lead to fit with a hole in the center to clear the motor tail shaft. On the rear of the motor there is a rebate on both sides; fill these with lead so that the rear is flush. At the front make two 2mm thick lead plates to fit the top and bottom of the plastic front brush housing. The lower plate needs to be reduced in width to clear fingers on contact plates. Stick edge of plates to zamac weight with super glue so that they will slide over motor on assembly. This tiny weight increase will make a great difference to electrical contact and ride. 
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I reduced the tender wheel treads by filing down on a peg in the work bench, reducing truck frames to match. On the locos, old and new, remove the cab zamac weight and discard, cab windows can now be fitted, make up a cab floor in 2mm thick lead, then a D shaped piece to match up with boiler, resembling rear of boiler and backhead in cab. Glue to new floor. You can also add an enginer (head and shoulders only) to the new floor. On locos with the old drive shaft you may have to file a notch to clear - glue the new floor into the cab with Goo or similar. To happily traverse Marklin smallest radius curves, you have to elongate the slot that retains the pilot truck and remove a little material from inner front face of the cylinders to clear wheels. I hope this all makes sense, please let me have your opinion. One remaining problem, wheel spin on Marklin points, I think the flange bottoms out at the frog lifting the traction tyre clear of the rail head.