The efficiency of a piston-in-sleve design is reduced, drastically, as you introduce free space in to the combustion chamber.

Keep in mind that converting heat to motion can be easily expressed a math problem. You have a specific amount of heat that is output by the nuclear reaction. The units that are used to measure heat can be directly converted to the units that measure motion energy and electromagnetic energy.

So, let's assume that you have built a massive piston engine that has a target thermal efficiency of other piston engines in common use. A highly efficient diesel ship engine (some of the most efficient made) will reach 50% thermal efficiency. Most car engines are around 35%.

Because of the intense heat of the fusion reaction, you must make some design compromises. Namely, the massive distances required to protect your materials. Your materials must be far enough away that the heat reaching them is below their thermal design limit. This means that all the heat energy between the temperature of the actual thermonuclear reaction and your material design limits must be dissipated somehow. In your design, you propose to use an air-gap, which means the heat will dissipate in to the air, which will be discharged during the exhaust cycle. There's a big loss in efficiency.

You'll also have the problem of heat soak. As you repetitively detonate thermonuclear bombs in your combustion chamber, some of the energy will transfer in to the materials that make up your engine. You'll have to carry this wasted heat energy away some how. In cars, this is a water cooling jacket and radiator. In power plants, this heat energy is normally used to boil water in what is called a "combined cycle" generator. You could apply the same principle here. But let's look at the piston-in-sleve side of things, because that's your primary design idea. The combined cycle side of the equation is irrelevant, because thermonuclear reactions generate heat that is easily converted to steam without the intermediary piston engine mechanicals.

Inefficiency scales with size, so it doesn't matter that we're generating insane amounts of energy, we're still facing losses here due to design compromises. The losses due to the massive air gaps required cannot be overstated. The compression ratio in a car engine has a lot to do with its efficiency. A car engine that loses even 20% of it's compression can fail to sustain operation. That is, the energy created by the fuel-air explosion isn't sufficient to overcome the parasitic forces present in the engine.

So let's make a wild guess and say that your engine is one-tenth (optimistic) as efficient as an optimally efficient diesel engine; around 5% thermal efficiency. Ouch.

So what is the thermal efficiency of a thermonuclear power plant that simply uses the heat energy from the reaction to generate steam? Current designs are limited to around 30% because of materials limits. In a typical nuclear reactor, the rods are actually inside the pressure vessel, which must be actuated to regulate the reaction, as well as accessible for servicing.

Keep in mind that I have a very limited understanding of these principles, but much of it is hands-on. I used to be a small-engine mechanic. I worked for a retired Army power plant engineer, who taught me more than I'll ever need to know that a gigantic piston-in-sleve design is a monumental waste of time.