It was the heat treat process itself that cracked the heads, I bought a brand new pair from mopar not re-man and they were cracked, I got my money back and have been running cracked magnum heads ever since.

The only un-cracked ones I found were on 5.2s, someone said they were actually made in a different factory so may have been treated a little better.

Some things may make them worse during use like

1. EGR slowing down the burn speed so the mix is still burning as the valve is opened instead of burning faster in the chamber where it should be before the valve opens.

2. Overheating because of many things common to these engine including thermostat housing leak, timing cover leak, radiator leak and water pump leak. Even so I have never seen coolant use as a result of cracked magnum heads, it always turns out to be something else.

3. Another possible difference in the 5.2 and 5.9 is the 5.2 has more cylinder pressure than a 5.9 because of a higher compresison ratio and a better cam profile, that makes the mix burn in the combustion chamber instead of on its way out the ex valve.

A better reason to me to run a head like an EQ with hard ex seat is because the factory heads get severly receded ex seats after 150,000 or so miles. I would run the EQ as a first choice in a truck over the eddy because it's iron and flows better. If it's a hot rod and not a truck then sure go with the eddy.

I probably went through 200 heads to find an uncracked pair on my last hot rod build, the only reason I bothered is because I was putting in bigger valves.