Means are provided for adjusting the relative timing of {TAIL ~ the two pistons} so as to vary the compression ratio of {HEAD ~ the engine} 
Means are provided for adjusting the relative timing of { TAIL ~ the two pistons } so as to vary the compression ratio of { HEAD ~ the engine }
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD