J (915) vs. Magnum heads

skrews said

Figure on completely rebuilding the Js if they at the point of needing a valve job. Valves, guides, springs, locks, retainers, valve job, milling to equal out the compression loss, ect. Not to mention needing the LA valve covers, rockers, shafts/hold downs, head bolts, intake, ect.
Even once this is all done, you still need a fair amount of port work to get the Js to out flow the Magnums. Just swapping the Magnums for the Js in near stock form would result in a performance loss.

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Unless he is using stock mag springs, which in that case he would need new springs if putting a real cam in anyways, he could swap them over.

Head bolts, what I think its only a couple that are shorter?

According to everyone on this site, minus you, the valve covers interchange.

.020 would even them up, thats about $65-90 for the pair.

Other than rocker gear in which the j's shaft mount is better, and chamber...they arent much diff.

If you do your homework you will find that the mag has a few more cfm in stock form in the .200-.300 lift range, otherwise the j heads flow more and are better suited for a higher lift camshaft.

In the performance world where every part is pretty much messed with to make more power, I dont see enough return investment in messing with Magnum heads.
I do think that if the op is cruising a stock/next to stock mill that the magnum head is fine.

Unless they are ported, its almost apples to apples but with one of the apples having crack indusive worms.