The question was: Can a J4 drive built for a 400 ton hull allow a 300 ton hull to make a five parsec jump?

CT LBB 2 Building Ships pages 20-21: "1. Custom hulls with mass displacements other than the hull sizes shown on the drive potential table are treated as the next larger size. The maximum hull size possible in these rules is 5000 tons."

In CT LBB 2 a J4 drive sized for a 400 ton hull will not allow a custom 300 ton hull to make a five parsec jump.

A CT LBB 2 600 ton hull with a J4 drive can use drive types M, N, or P. Using either a 600 ton J4 type M or type N drive appears to be able to allow 400 ton hull to make J6. The M type J4 drive requires 65 tons of hull space and the type N use 70. The two jump drives exceeds the 400 tons available Engineering space of 50 tons by 15 and 20 tons. A 400 ton hull J6 drive requires 0.1 x 400 x 6 = 40 x 6 = 240 tons reducing the Main Compartment's 350 tons to 110 tons and stealing 15 tons leaves 95 tons. Installing the smallest Power Plant 4 is Type H and requires 25 tons of space 95 - 25 = 70 tons of unused space. The power plant requires 60 tons of fuel which leaves 70 - 60 = 10 tons of hull space. A 400 ton hull has a 20 ton bridge and requires a Model/4 computer of 4 tons 10 - 24 = -14.

The use of a J4 drive built for a 600 ton hull on a 400 ton hull will not jump 6 parsecs there is not enough room to fit the required components.

My point is that, using CT's LBB set (1-3), where standard drives are referenced as to J capability according to displacement, the *exact* same drive can be put into different size hulls & produce differing J-max. The idea that, somehow, those drives' J-max is somehow permanently set & cannot be altered is an idea that I believe cannot be true. If I were to salvage one of those J drives from a larger ship & put it into a smaller one, the rules clearly indicate that the drives J-max could be increased depending, of course, on the actual Dtons of the new hull. Hence that long-ago GM's assertion that a J-drives J-max is permanently fixed is clearly incorrect. The statement "Once a J(n) always a J(n)" just cannot be true. (Wasn't that the original question?)