I think it makes a difference on type of use. My mags only had 294 hours on them since new from Lycoming, but new was 2005. There obviously was times of little use and much downtime. Both of my mags needed a complete overhaul on inspection - 725.00 in parts plus labor. Might as well just buy a new mag. I don’t think you can plan on a Slick mag making it to the TBO of your engine. A Bendix mag, maybe, but Slick mags are just not robust enough to last that long. I fly my airplane frequently and I plan on having a replacement plan set up when 500, or fewer, hours rolls around. I have one SureFly right now, and may go with a second one if this first one is reliable.

When we took my mags apart, it was surprising to see the amount of wear that happened in such a short time, and again, a lot of that has to do with lack of use. I purchased a mag from a friend that had an almost new one and had the 500 hour inspection performed on that one, but I don’t expect it to last forever.

Note: When you buy a new mag, they will charge you $250 more if you don’t have a ‘core’ to send to them. I asked my accessories mechanic what they do with those cores. He said they just throw them away because they want to get it out of the system. Goes to show you how long they think they will last.

Incidentally, if you need to buy a new mag, and you don’t have a core to send them, let me know. A core will save you $250. I’ll sell you a core for $100........ um, plus shipping......

Not worth the risk
 

I would fail the block assembly due to the possible FOD contamination on the cracked timming hole, it is not worth messing with. If it breaks off it could damage the gears or it could just find a corner of the mag and never move....but its not worth the gamble in my opinion. Kelly aerospace sells distributor block assemblies for most slick mags now at a lesser cost than the slick counter part. They are available thru aircraft spruce. In the past when doing a 500hr inspection I usually just assume I will be needing to replace the points and distributor block assembly. The most common dist block assembly part number will be ES3822 (not a guarantee as I don't know the model of your mag.)

Mitch AP/IA