a friend of mine has a 68 F250 with a 390 and a t18, and a few weeks ago was pulling some high RPMs on the freeway when the #1 piston decomposed and sent the top of the rod through the cylinder wall.
we found a used 390 that looked halfway decent, it had been rebuilt, bored .030 and rods and mains both .010 undersize, with a tag on the front that showed it was done by a real shop. at the time of purchase, my friend heard the engine run, but that was all he really knew.
we don't know how long ago this was, but we do know that there was minimal cylinder wear after this rebuild, though the rings were shot, the bearings were showing a lot of copper, and the cam gear/sprocket had several teeth missing their nylon coating.
the block was quite dirty, though the heads, valve covers, and intake were nice and clean, and all the valves sealed surprisingly good.
we decided to go ahead and freshen up the bottom end, and install new valve stem seals because the old ones were in small pieces (at least those i could find were).
thats where things started to get interesting. right after i dropped the first exhaust valve on the ground and wiped it off, i decided to check the valve seat with a magnet, and found it magnetic. i know what that means, and started piecing together a story. it seems to me that the unhardened valve seats (for leaded gas) caused the owner a problem, so he had a valve job done not too many miles ago. it would appear that this was an occasional-use rig, as the seals had time to decompose while the valves still sealed decently.
so i took the heads to my machinist and asked for hardened valve seats (for unleaded gas), and the valve grind that has to go with it. well he found the valve guides worn and needing replacing, being too slack even for the oversize valves the head already contained. with the price tag attached, we decided that we needed to check the heads for cracks before dropping 5-600 into them, and thats where things are starting to turn ugly on us. one head had cracks coming off every spark plug hole, and a couple others as well. the second head only had a single crack off one spark plug hole.

i have yet to pull the heads off the original engine, but i suspect at least one of them to be condemned by the decomposed piston, not to mention that they're believed to not match each other.

this leaves us with 2 options: buy new (maybe rebuilt) heads from a parts store - but i've heard stories about various factory build heads having horrible valve setup. or we start with another pair of heads from the local junkyard, again with no guarantee.

what would y'all advise doing from here? we've already rebuilt the bottom end, so scrapping it for a 460 is not part of the plan

 

find some used heads , off craigs list or pick ur part .
there was i guy on ebay selling rebuilt heads for a good price.

 

D2 heads are the ones with hardened seats, but these are all so old now that you just never know....but there were millions served, so start looking I guess. Unless you want to spend "a little" extra:

Edelbrock 60069 - Edelbrock Performer RPM Cylinder Heads - Overview - SummitRacing.com

 

i was under the impression that ALL modern heads have the hardened seats, and that ALL hardened seats do not respond to a magnet. is this correct?
today we pulled the heads off the original engine, and one was damned by the decomposed piston (dented up plus a missing exhaust valve), but the good head showed magnetic valve seats, valves that still sealed decently (a drop of water leaked past in 10 seconds on the worst ones), and my friend had put 80k miles on it himself, with unknown history before that. almost makes me wonder if my knowledge of valve seats is wrong ??
for the "d2" heads, what years did those come in, or were they aftermarket? i don't really want to spend all day pulling heads off at the junkyard if i don't have to

 

Not sure why hardened seats wouldn't respond to a magnet? Stainless, very high grades, won't pull a magnet, but hardened isn't stainless... ?

 

"D2" is short for "D2TE", the first part of the casting number for heads engineered in 1972. (D = 1970s, 2 = 1972) Since that was the dawn of unleaded gas, those had the hardened seats.

"Modern" heads have hardened seats. The trouble is, the most "modern" FE carries back to that 1972 engineering change.

IIRC, the early heads were induction hardened, so that should hold a magnet. That hardening does not survive a valve job, thus the need for replacement seats in some heads.

More blather about heads:

The FORDification.com Forums • View topic - dumb question about 390 GT heads

Heads are constantly being discussed in this forum--you could spend all day reading thread after thread.

 

Josh, it sounds to me you are doing a basic rebuild, not exactly a performance build. With only software (so to speak) replacement parts on this build, dont worry about utilizing the heads from auto zone, etc. Use them. For a machine shop to rebuild those "old head "right", here in Virginia Beach anyway, will cost about as much as a rebuilt motor from a large chain part store. PS Recon is JUNK and ATK is not much better!!!!!!!!!! I've installed and ran both brands. PSS who the F... bores one cylinder out to .060 and leaves the rest at .030? Recon did! I owned it.

 

thats good to know!!
today found me at the junkyard, where i pulled a pair of heads that look too good to be true. perfectly clean inside, the outside only had a thin layer of gunk on it, even the head bolts looked brand new. the intake was already missing, and i only broke a couple exhaust bolts getting everything apart. i plan to have mike do a magnaflux, and if they pass, do any necessary valve work. i pulled them off a 74, but didn't look close for casting numbers.
as for price tags, the first heads i brought in needed valves, seats, and guides. the surface checked out, and the springs were good. he quoted me about 550 for everything. then i asked him to magnaflux the heads and we found all the cracks.

and yes, we are just doing a basic rebuild, getting the truck back on the road, but keeping the 4bbl intake and headers that the truck already had in it, as thats easier than changing everything else back to stock.