I was working on my L13A recently and needed to judge the position of the pistons. I'd read about the trick of putting a thin ratchet handle or extension through the spark plug hole to observe the top and bottom of the piston's travel as you turn the engine over by hand. Now, I didn't feel completely comfortable with the idea of putting a rigid metal piece into the engine like that, so I found a suitable length of wooden dowel. This worked initially, but at some point the dowel got grabbed between the piston and the spark plug hole and started to bend. I tried to get it loose but it was already trapped. Turning the engine backwards was tempting but I knew that could cause engine damage. A piece of the dowel eventually broke off completely and got lodged in the combustion chamber. Sinking feeling. After trying a few strategies I was eventually able to extract it using a vacuum cleaner with a thin extension hose poked through the spark plug hole (I took out the rear spark plug to help with airflow). Whew.

So, be very careful if you try something like this on your engine! I wonder if the technique is a safer proposition on engines where there is a single spark plug central and perpendicular to each piston. I can't help thinking that the angle of the spark plug holes on the L13A and friends would make it a far more dangerous proposition. There are markings on the crank pulley and on the camshaft sprockets (if you have the valve cover off) that can be used instead, and you should be able to see the top of the piston through the spark plug hole with suitable lighting.