The Pescara patent protection required an outward compressing and modifications in gas exchange.

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Pescara patent: 
Means for driving the propelling system for aircraft

1967 End of SIGMA 

After 1962 retirement of the Pescara chief engineer Robert Huber, the decline of free-piston technology began. There were several reasons:

• The slow-speed diesel engines meanwhile equalized the efficiency of free-piston gas turbines.
  
  
• The Pescara GS34 engine - an improved prototype with a technical level of 1939 - was not developed further. The engine was a victim of its own success, a succeeding model was never developed.
  
  
• Various successors (Stabine, Stelzer, Jarret, copycats) had conceptual or technical problems and damaged the reputation of the free-piston engines very much.

1969 Stabine SA, Le Creusot, Prototype

The pomised power output of some 15000 KW was never achieved. More than 150 Mio French Francs were invested, then the company went bancrupt.

This was a technical suicide for the free piston technology, that was developed by Robert Huber of "Bureau Technique Pescara" with 35 years of experience (1927 - 1962).

In relation to power, the moving mass of the Stabine engine was more than twice as heavy compared with the GS34. Accordingly, the piston acceleration was too low and thus the thermal efficiency was not achieved.