yes, you need a compressor. the spring is nearly twice as long uncompressed and very strong. the locks are held in by the spring trying to uncompress.

i suppose if you had a compressor but difdn't have air, you could change the seals. i'd worry about dropping a valve unless you turned the motor by hand to tdc for each pair of pistons. engine tdc for 1 and 6, 120 degrees for 2 and 5, another 120 for 3 and 4. good luck, chuck.