Fox .15 Glow Plug Question

Ok. So just another little question regarding the Fox .15x. This time it is about the glow plug. The engine came to me with a short glow plug installed. However, tonight I looked and found it had I believe only one other short glow plug. So for testing purposes I tried a couple I had on hand. I tried a standard long, standard long with idle bar, and a medium such as an OS #8. I used only one gasket per plug. When installed, they all allowed the propeller to turn, and there was no contact with the piston that I could feel. Anyways with that said, what is the recommended glow plug for that engine, or can I use what I have on hand that works without contact. I am not oppowed to buying a new plug, but would like to use up what I have. As well, I am not sure they sell short plugs locally, but will call tomorrow to see. The medium length plugs like the OS #8 are readily available. Thank you again in advance for any help in this matter.