I have a PT-40,with a OS LA-40 using a no.6 glow plug.
Can I use a no.8 in the engine. Will no.8 wreck the motor ? and
do I have to use a higher % nitro fuel ?

A #8 will work just fine. It won't hurt your engine at all -- it is the most popular plug for medium - large 2-c glow engines. Any fuel from 0% - 15% nitro will work well.

thanks for your imfo.
Winter is coming and I'm going to recover my
PT-40 and want to put a different engine on it.
Any sugustions for a good replacement for the
OS LA 40 ?

Jolly here are some good almost direct replacements ( as memory serves):

Os 46 AX and 50SX, Magnum 46 and 53 XLS, and the TT 46 Pro.

Some other goodies:

Irvine 46 and 53, Enya 50 SS, Super tiger GS 45 and 51, and the MVVS 49.

Mark Shuman

PS: The #8 will be just fine.

OS 52 Four Stroke.....direct bolt in, same bolt pattern.