There must be a short tube that somehow found its way into your kit (or a long engine block.)

The two tubes are:
BT-20AE 1.5"
BT-20M 2.25"
Total 3.75" when stacked.

Two engine blocks at .125" each = .25"
Two empty casings at 1.75" each = 3.50"
Total 3.75" when stacked.

I opened a kit to check the parts and they stack correctly. The overhang of the sustainer engine is .375" into the booster.

__________________
Carl McLawhorn
NAR#4717 L2
semroc.com