Tweaking the Big Boy Centipede Truck
Fine tuning for stability.

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The big boy's tender is very top heavy and needs the rear truck tracking and geometry to be as near perfect as it can be to keep the tender from wobbling down the track.  The tender will be prone to tipping and clunking if the rear wheels are not aligned correctly Lionel's tooling for the truck frame was not the best and variances were the result. 
Here is a fix that with a simple jig will allow you to get the truck re-aligned correctly.

The rear truck frame needs to be completely removed from the tender. Start with the speaker removal

Then remove the sound boards.

The frame screws are removed next. Don't loose the insulators. There may be shim washers between the truck frame and insulators. Be sure to remove those shims, we will not re-use them.

The alignment jig is simply a piece of 1/8 or 3/16 aluminum drilled to match the truck frame mounting holes. The jig needs to be stiff, true and flat.

Assemble the truck frame on the test jig with all of the wheels in place. Be sure to swap the rear two wheelsets so a blind set is rearmost.
You want the blind wheelset in the rear as the frame is weaker there and may distort over time.  Also the truck tracks better in curves.
While you have it apart Locktite the coupler plate mounting screws

You will need a piece of test track that is straight and square mounted to a board for clearance testing.
Check the clearances between the blind drivers and track to be sure the blind wheels are not bearing any weight.   

File the blind wheel's journals if needed so that the wheels clear the track.
The reason for only the  flanged drives to bear  the weight is to keep the flanges from riding up the sides of the rails and causing a clunk when they fall back down on the rails.

Now re-assemble the truck with only the flanged drivers

Place the assembly on the test track and gently apply pressure diagonally on the plate and try to feel any play when rocking the plate. Do both the green and the yellow directions. You will probably notice that one direction will be noticeably looser than the other.

If you notice a difference in the diagonal play then you need to measure all of the flanged journals. 

You only need to notice the difference in their measurements and the goal is to adjust the frame so all are within +/- .001 or as close as you can get them.
I write the measurements in pencil on the jig by each journal for reference.

Then the critical job. VERY GENTLY press the frame at the  largest measurements in SMALL steps in a vise. This will reshape the bend at the top of the frame where the screw attaches. Be sure to re-tighten the screws after every squeezing.
Squeeze once and measure. Repeat if needed. Then do the other high ones until you walk all to within +/_.001 . of each other.  Then re-test the diagonal rocking and it should be minimal now. Re-assemble tender when happy with the results.