Cylinder head date codes


Jan 16, 2020
Reaction score
Pittsburg, Tx, USA
Please forgive my ignorance, I got a pair of used 346 heads for rebuilding my 400. I need to know where is, and how to read, the date codes on these heads. I know in 73, Mother Mopar started using hardened exhaust valve seats in anticipation of upcoming unleaded fuels. I'd like to know if I can just hand lap the valves and run them as-is or if I must take them into a machine shop to have hardened seats installed.

I did take these into a local head shop (the valve spring retainers & keepers on 2 valves was missing) but "technician" on duty either couldn't or wouldn't tell me if they were or not.

Thanks in advance. Go forth & be awesome.

Head 1-1822

Head 2-2782

You tell me, when were they made?
182nd day of ‘72 and 272nd day of ‘72.
Only the 452 castings are truly induction hardened. 346 and the others from that time frame only have hardening process maybe .002-3 deep, probably through it already unless ultra low mile heads.
My personal opinion is I would not have hard seats put in unless you are going to run killer valve springs or really high mileage.
First, alot of this comes from the GM and Ford camps, both of which are known to have soft crappy iron in pass/low po engines. Chrysler did not downgrade the recipe for 2 bbl 400/ 383 / 360 and save the "good iron recipe for HP engines" they all got the same basic iron formula. So any old Chrysler heads are a higher grade than the other OEM passenger car engine.
I believe the date code reads 1-365, last digit is the year. I'm not good with numbers so don't take my words as gospel on that.
Last edited:
346 heads are "smog" heads with a flatter intake port but do not have hardened valve seats.
If you are not racing or towing with the car, a basic rebuild of the heads (new guides,valves,3 angle grind) they will last a long time even on today's gas.

To compare closely,if you heads do need a lot of work,it may be more cost effective to buy a new set of heads from 440Source or Edelbrock.
You can buy a pair and port match them yourself to the parts you have already.
Hope this helps
Echoing the above, installing hardened seats isn't needed with Mopar heads for most applications.
Thanks for the input guys. The car itself has rolled the odometer, perhaps more than once, so it's already high mileage. The original 400 heads are 2 bbl heads with the smaller valves, ports & 88cc Chambers. These 'new' heads have bigger valves & 79cc chambers, should result in better breathing & a bump in compression. Plans include a rebuild with flat top pistons, a 'Stage 2 Mopar' ground cam (based on what they used in the HP 440's), 4bbl & headers. We're hoping for 350 HP. I'm not going racing, towing or making a daily driver put of this. It's going into a heavy 73 Newport with a 400 big block, gas mileage can be expected to deliver single digit economy, so it'll mostly be a weekend cruiser. But I am anticipating moving to East Texas in a year where the speed limits are 70 or better, so I do want it to have some grunt under the Go pedal.

I'm working within a limited budget, Eddy heads or the offerings from 440Source are outside my reach. I still have to scratch up $1400 to machine the block & crank, and another grand for internals. That might be chump change for some of y'all but it's big coin for a guy working 2 full time jobs just to drag in $2k a month, net. Bills gotta come first & I have to keep the missus happy. Thanks again for the feedback.
A couple of things. The stock heads and the heads you pictured are same basic configuration valve sizes and chambers. Unless they have been modified can't see you did not picture the combustion chamber side. If you want to get the most out of it you need actual measurements not book numbers all B B Mopars are never down to theoretical design sizes. If you are having machine work done and buying pistons, this is the time to correct the mass production tolerance and make specs correct for what you want. Measure, do not assume
It only cost me $60 to have Hughes Engines install hardened seats in my 906 heads. That was in 2013 so the price might have gone up a little by now.
Long live the iron head!:D