Welcome to moparts

Moparts Tech Archive

Small Block

SB Heads Q's

J heads! X heads! O heads

Moparts Member
Posts: 104
From: Mission Viejo
Registered: Sep 2001

I'm still kind of new to the small block relm here. I've heard of J heads and X heads, but what are O heads? They're on my '70 Dart 340. The original owner told me that everything was left pretty much original when the motor was gone through, so I think that they are original. Any quick eductaion I could get?


Moparts Member
Posts: 1378
From: San Marcos, CA
Registered: Jan 2001
posted 04-14-2002 04:34 AM

to add to the confusion, there is also a U head too!


Moparts Member
Posts: 1158
From: Old town,fl,usa
Registered: Oct 2001
posted 04-14-2002 05:25 AM

O heads that i have seen are a 915A casting from '73 with smog tubes and 1.88 valves,U's are quite common the U will be a 915C casting,the J is a 915D casting,you can see the j and u w/out removing anything,it is stamped on either side of the center exhaust port,as is the X,the o is machined on a pad in the same area but is usually covered in grease and needs to be scraped off,whereas the x,j & u stick out like a sore thunb.sorry to disappoint but the o heads i have in stock are 73 non performance smog heads from a 73 challenger 360 w/1.88 valves,all the others are 2.02 heads that i have anyhow.


Moparts Member
Posts: 1130
From: Portland Oregon
Registered: Apr 2000
posted 04-14-2002 07:36 AM

My 70 340 Duster I had had both on it, a o and a u, it had the original as far as I know 70 heads and they had to be taken off to be rebuilt. They were both big valve 202 heads. the o and u are probably just a casting flaw in the U's that are really 0 heads and they used the same castings for a while and just put whatever valves the heads needed for the year and app they were destined for.


Moparts Member
Posts: 4118
From: Valley City, ND. USA
Registered: Oct 2001
posted 04-14-2002 08:22 AM

And from what I have been told in the past and read in Mopar mags the letter designation really don't mean squat.

To add to my above post these same people that told me about the letter designation figures the the letter is the nothing more than the foundry letter designation where the head was cast. ???


Moparts Member
Posts: 1262
From: NW Arkansas
Registered: Sep 2000
posted 04-14-2002 11:39 AM

My "O" heads are 915B castings with 2.02"/1.60" valves. There are no smog tubes. The "O" are machined in the heads. The letters are as prominent as any "X", "J" or "U".


Moparts Member
Posts: 914
From: Ft. Mohave, Arizona
Registered: May 2001
posted 04-14-2002 12:37 PM

Lets not forget the "Z" heads. My dads 360 has these on the engine when we got it. Not sure of the casting numbers though.


Moparts Member
Posts: 480
From: Polson, MT
Registered: Jan 2002
posted 04-14-2002 01:07 PM

I've had "O" and "J" heads that were smoggers, and others that were the good 915 castings.


New Member
Posts: 64
From: Copenhagen Denmark
Registered: Mar 2001
posted 04-14-2002 01:17 PM

Just to add to the confusion, right now there are a set of T/A heads on e-bay with excellent pictures, one 915-B the other one 915-C respectively U and J.

(believe it or not I have learned a lot from E-bay pics)

They seem to be machined correctly (T/A) so how this comes U-J

I am puzzled, I mean if they are legit T/As they are made with in a very short time frame in the spring of 70, I think I have seen even 915-A.

Can any one from any coast enlighten me?



Moparts Member
Posts: 3658
From: Orange, CA. USA
Registered: Dec 2000
posted 04-14-2002 01:19 PM

It basically boils down to the casting dates on the heads and the valve size. X's were used from '68-70. In '70 the U's and the O's came out. In '71 up J's, U's, O's and Z's came out in various valve sizes depending upon engine type.
Just to mess things up again there was a 598 casting that was an X head


Moparts Member
Posts: 2491
From: Maryland
Registered: Nov 2001
posted 04-14-2002 02:35 PM

I have a pair of 340 TA heads. Ones an O and one is a U head. I also have a pair of o heads that are equivelant to X heads but dont know if the 2.02's are original.


Moparts Member
Posts: 221
Registered: Dec 2001
posted 04-14-2002 02:37 PM

funny the ones on my 71 340 are the Os iv heard its a casting mistake they are Js instead and they are big valve ones


Jesse Lackman
Moparts Member
Posts: 428
From: Center, ND 58530
Registered: Jan 2000
posted 04-14-2002 05:37 PM

And they all have the crappy exhaust port floor. To get good exhaust ports (in cast iron) one has to find the 308, MP high swirl castings, Magnum, Magnum R/T, or W-2.


Moparts Member
Posts: 363
Registered: Jun 2000
posted 04-14-2002 11:30 PM

I believe that all 68-70 340 engines left the factory with the x head castings.Does your dart have a production date of 4/70 or later?


Moparts Member
Posts: 119
From: ill.
Registered: Oct 2001
posted 04-15-2002 12:27 AM

Here's what I have to be an original 70 T/A 340 heads.

Rt. head was an O. #3418915B 12109 I am guessing is the cast date. Has 360 on a runner. Also AAWJ.
Lt. head was is a U. #3418915A 02090 cast date?
Also 360 on a runner. AAWJ too.

Both these heads have big valves.
And cast iron adjustable offset intake rockers.

Quick measurement is 71-72 cc.


Breather Question for cast valve covers s/b

Moparts Member
Posts: 388
From: newhall.ca
Registered: Sep 2000
posted 05-13-2002 12:53 PM

With all this talk latly about the breathers i have finally realized why i always smell smoke when i'm at an idle. and possibly why oil leaks all over my engine. neways, how do i go about putting a pcv valve on my black wrinkle valve covers. i have already punched out one of the cast in holes and i need to know what pcv(part #) to use and what rubber grommet to use. Basically i need to know what parts to use with #'s.


Moparts Member
Posts: 366
From: Sacramento, CA
Registered: Nov 1999
posted 05-13-2002 02:00 PM

I know your looking for #'s, but I can't help you there. What I did was went to the local speed shop and looked in the bulk chrome isle. I got a breather for one side, and a cap for the other that was made for a PCV valve, which came with a grommet in it. The PCV valve for spec'd for my car fit the cap. This setup didn't require punching any holes in the valve covers. Since the covers aren't stock, you my be stuck with trial and error method, but Autozone should have grommets and PCV valves aplenty to match up while your at the store.


Moparts Member
Posts: 440
From: ARLINGTON WA 98223
Registered: Oct 2001
posted 05-13-2002 02:11 PM

The MOPAR PERF part nunber is P5249642 works great on my SB abd BB valve covers


360 build up w/308 heads

New Member
Posts: 58
From: San Antonio TX
Registered: Jan 2002
posted 01-18-2003 11:52 PM

I am looking to build a street performance 360 with 308 heads stock size valves and mild port work. What are the cc size on these heads? My trans is a 727 with 2200- 2400 stall speed. my rear gear is 3.55. What do you recommend for pistons, comp ratio (pump gas),and cam shaft I am thinking maybe a solid cam. How much horsepower can I expect with this. All of this is going into a daily driven A-body. I really appreciate your real world experiance.


Moparts Member
Posts: 430
From: Alton. Illinois
Registered: Nov 2002
posted 01-19-2003 01:16 AM

I think they are approx 72cc, but u may want to have them CCd just so u will know........here is my 360 combo, not done yet ........ still gathering parts and asking ??????? and need the trans assembled
72 360 .030 over KB190 pistons, J heads with 2.02 and 1.60 SS valves, bowl work and blended in, 10.1 CR and an comp cams Xtreame energy cam XE274H www.compcams.com with 2200-2500 stall 355 gears will go to 3.91s later and a 904 tranny with low gear set, and a dual plane intake with a 750 carb and headers,
from the specs of that cam, it should be a decent one, it is a dual pattern cam.....and it going into my 96 Dakota daily driver


Moparts Member
Posts: 306
From: Huntington, New York, USA
Registered: Jul 2002
posted 01-19-2003 07:03 AM

I've a few cams in my 360 combos. FIrst I ran 10.7 comp which helps, perf rpm cam, althouh a chevy grind, makes great power. Maybe too much duration for everyday. I had to switch to my "spare" motor because of a block problem on my 360, but I have a 340 now, 9.83 comp, with a Hughes 2430AL.I have e brocks, but your 308s are decent. The 2430 is pretty good, works great with my a/c, and still have good power.



Back to Tech Index
Tech Index