I use a couple of sockets and a pair of shaft collars for a seal driver to install the seals without folding the seal casings. Best seals to use are the ones which are rubber outside diameter not steel, they are more forgiving to drive in and there is less chance of leaks with rubber on aluminium rather than steel on aluminium. Seal spec is SC 15x21x4.
Only install a spi seal on a cleaned and inspected casing, this is a one shot deal for the seal so if you have more machining to do or have not fully cleaned the carter then do not install the seal.
Check there is slight lead in for seal on the carter half
Place 10mm socket into vise
Place two 1/2 collars or wood block to level and support carter
Place carter onto 10mm socket
Put little 2 stroke on socket
Place spi seal with garters facing out onto socket
Slide to seal seat
Place 14mm socket onto 10mm socket
Drive seal into seat with light taps of hammer
CHECK SIZES FIRST.
This works with my 10mm socket as a pilot but check first if socket is a good fit into the crankshaft orifice on the carter half. You may need to use a 11mm or a 3/8 socket depending on the socket. Also check first if your 14mm socket driver is a good fit to the pilot socket you selected, it should be a good fit not overly tight but not rattly loose.
Make sure there are no nicks or burrs on the side of the pilot socket - any imperfections will rip the spi seal and render it useless.