You can use either set of marks. The marks on the forward and aft surfaces of the flywheel indicate exactly the same thing. If you put #1 cylinder at TDC and line up the front TDC mark with the spot on the starter you will find the back side TDC mark lined up with the crankcase seam. Some baffle installations make it hard to use one set or the other.
Don B
RV 9. Rebuild in Progress
When you buy the pin, be sure to buy two.
Maybe so, but in that case the guy that put the mag on had the aft marks lined up with the front starter mark. With Mike Hilgers help I used the spark plug block tool to find the hard stop on #1 cylinder on both sides and determined for certain where my TDC physically exists, it agrees with the front marks on the flywheel. The buzz box says the mag is nowhere close. It's easy enough to retime them, it just hacks me off that it wasn't done correctly during assembly. Makes me wonder what other shortcuts the guy took during the rebuild. I already found one other problem, it had the wrong vacuum pump drive gear put on the engine, the yellow tag had the correct part number but it was most definitely the wrong part installed, it was the one for an O-540 instead of my O-360.
Just to be sure, the 4373 is an impulse mag, so you need to pull the prop well past TDC until the impulse releases(with the p lead grounded and mixture ICO) , then back it up past TDC far enough to take the back lash out of the gear train to get an accurate indication coming forward again at the desired 20-25 deg. advance. If the impulse has not released, the buzz box does indicate way off.