Screw thread spec for Bachmann 4-4-0 bottom plate?

[Maletrain]

A club member has an old Bachmann 4-4-0 with the screw that holds the bottom plate under the drivers missing.  So the bottom plate drops into the track and the loco spins its wheels without moving.

Does anybody know the spec for the threads on that screw?  The version has "Bachmann" and "China" molded on the bottom plate, and is the "American" type.

http://spookshow.net/loco/bach440a.html

[Lemosteam]

Bachmann Parts store for $3.11 + shipping:

https://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=70_79&products_id=5334

[Maletrain]

Yes, we had found the Bachmann screw set, but its $3 for the set and $9 for the shipping, plus there is the statement in Spookshow's review that some of the screws were changed between the versions.

That is why I was hoping somebody knows the actual thread specs for that screw in that version.  I may very well have a suitable replacement in a drawer.

I may just bring the member's loco home with me and carefully see what fits.

Calling Bachmann parts didn't help, because they have no dimensional info on their stock.

[Lemosteam]

@Maletrain I can look at my old white box model.  I have a metric thread gage. and I can measure the length.  I'll get back to you.

[Ed Kapuscinski]

I'd bet it's an M2 or M3. The big question would be length, but you might be able to figure that out with some creative measuring.

[kiwi_al]

Diameter 1.62mm
length 7 mm
So you need a M1.6 or an imperial equivalent.

Actually, you probably could just use an M2

[Maletrain]

That diameter seems like an M1.6 screw.  I would not want to try to force an M2 into an M1.6 threaded hole, because, besides the shaft diameter difference, the threads have different pitches (0.35 vs 0.40 mm).

I can measure the depth of the hole without much trouble.  I have M1.6 x 3mm screw, so I could check fit of the threads with that. 

But, another possibility for that diameter is a 0-80 Imperial screw, which has a slightly finer thread pitch (~0.32 mm).  I have more lengths of those, so I'll see what fits best.

[Maletrain]

@kiwi_al , I am assuming that the screw you measured looks like the one in Lemosteam's picture that is just a silver panhead machine screw that is sitting among several brass-looking machine screws with squared edge truss type heads?  And that the 7 mm length does not include the head?

[kiwi_al]

It's the silver panhead and you are correct, the 7mm length is the shaft only not including the head

Edit: see pictures:

[peteski]

Yes, it is a plain Phillips pan head screw with 7mm long threaded part. Just like the  one in the Bachmann screw assortment photo or in kiwi_al's photo.

[Maletrain]

To finish off with the answer:

The screw that holds the bottom plate for the Bachmann 4-4-0 version that says "Bachmann" and "China" and has metal gears is a M1.7 machine screw that is something more than 5 mm long, but less than 7 mm long.  If the screw is too long, it goes too far into the threaded hole in the brass bearing for the worms, so it contacts the worm shaft and binds them.  I just kept filing off a bit more length until they tightened up without binding the worms.

I repaired 2 of them, and they turned out to be the same screw spec in both locos. But, they had both been messed with before I was asked to fix them, and one had the round part with the threaded hole turned slightly, so misaligned and not open far enough for the proper screw diameter, while the other had a different screw glued in place.

The brass shoulder screw used to attach the tender to the loco is also M1.7.

While I was in there, I cleaned the worms and gears of a lot of black gunk, which made them run much more smoothly at low speeds.

Another problem I found was that one of the tender wheels was bent under and its truck frame was dragging on the rail, making to run sporadically.  I bent that back to horizontal, and it solved that problem.

But, in reboxing the loco, I noted that it is hard to get the tender to sit straight enough to drop down into the conforming clear plastic cradle - and that pushing it into the cradle with force looks like it would bend the axle that I found to be bent.  The problem is that the wheels are just slid onto the ends of fixed half axles that are connected to the truck frames on only one side of the frame for each wheel.  So, pushing hard on the wheel can pretty easily twist the frame so that the wheel is pushed up into the truck frame and the frame can contact the rail.  But, I initially had not noticed that because I was focused on the loco bottom plate dragging on the rail, too, due to the missing screw there.

Thank you to all that helped, and especially to Mark for the disassembly pictures in his Spookshow review.