Installing On30 Low frame trucks (29904) to underframes (29906.)

[rob1910]

I tried to install these low frame trucks (29904) to underframes (29906) using machine screws included with both items.
(These underframes come 3 to a package, are black and also available in a rust color.  The trucks come a pair to a pack.)
Included with #29906 are bolster adapter washers (otherwise the 4'1" will not seat correctly,) but the included machine screws had no nuts and the underframe bolster holes are too big for the approximate 2mm screws provided.  I called Bachmann Tech Services and he pulled out the tech sheet and suggested the answer was to use a self tapping screw threading upwards through the truck and into the underframe bolster housing. (In lieu of a nut  permanently imbedded in the plastic underframe bolster floor.)
I am hoping someone out there has come across the same issue and has an answer for me and others in the same boat.
Is a self tapping screw the way to go and if so, what diameter and length is required?
Just setting the trucks with the bolster extension, looks good with proper coupler height and the truck rides smoothly on the underframe. (This test run was on 18" radii track.)
Is a self tapping screw the way to go and if so, what diameter and length is required to secure trucks (29904) to underframe (29906?)
While on subject, my intention is to shorten some of these 25'6" length underframes and shorten them to 18' for scratch building flat and box cars.  To my knowledge Bachmann does not offer one and other manufacturers frames are pretty expensive.  Any info/advise appreciated.

[dutchbuilder]

The standard boxcar frames are made for the standard boxcar archbar trucks.
The screws that come with those frames go through the frames into the floor section of the boxcar.
I myself used M 2.5  screws.
You can tap a good thread into the frame.
If you don't have M2.5 , M 3 also works fine.

Ton