71 RT 340 with J heads????

[MerlinsMopars]

Starting to dig in my 340 4sp challenger rt and noticed it has j heads on it. I seen somewhere that they manufactured some j heads with 2.02s in them but unsure. Block matches vin #s and i wouldn't think the rt would come with 1.88s. So I figure some of you guys might know. I am gonna put a new timing chain and intake and gonna check the cam and valve train while im digging but while im that far i might change out the heads if they arent 2.02s. Dont want to change them if i dont have to tho. What do you guys think? Is there #s that tell they are original to the car? All info is appreciated.

 

[Chryco Psycho]

Likely original , J heads mostly had 2.02 intake valves

 

[MerlinsMopars]

Wow. Never knew that. Thought they were 1.88s. Hell i just read the other day about some with 2.02s and had trouble believing that. Always learning. Thanks for your reply!

 

[340challconvert]

340 HEADS

All the X heads came with 2.02 valves. Only some of the J heads did. Other than that, they are pretty much identical.
Casting
#2531894 'X-Heads' {1968 thru 1971}
Intake Valve. 2.02".....
Exhaust Valve.....1.60".....
The 'X-Head' is generally considered the best 'Stock' Cylinder Head, as the Combustion Chamber is a little tighter than the 'J-Head'. Both 'Ports' are near identical in volume.

1968 thru 1970 ~ 340..... 10.5 - 1 Compression-Ratio
1971 ~ 340..................... 10.3 - 1 Compression-Ratio

According to Chrysler, the later {mid-1970 and later #2531894 'X-Head'} Cylinder Head Castings had approximately +2.0 CC's larger Combustion Chambers than the earlier Castings.

The J-head was phased in sometime in 70 and for the 70-71 model years it too had the big 2.02 valves until 72 at which point the 1.88 valve was introduced.

Finally, there is a minor difference in the combustion chamber volume between the two. That pretty much it in a nutshell?

2531894 'X-Head'--mid-1967 thru late-1970 Casting

2531894 'X-Head'.......... late-1970 and 1971 Casting

#3418915 'J-Head'.......... 1971 Casting {w/2.02" Intake Valve}

#3418915 'J-Head'.......... 1972 {340 Engine} or 1971/1972 {360 Engine}

This info should help!

 

[MerlinsMopars]

Yea man. Good solid info. Is there date casting on em? If so where would that be?

 

[340challconvert]

date is under valve cover and is based on 10,000 day calendar

Use
Chrysler's 10,000-Day Calendar

 

[Chryco Psycho]

The TA heads with the modified pushrod location away from the intake ports also used the Later J castings FYI

 

[MerlinsMopars]

Ah ha! Like tranny's. Thanks man!

 

[Edward Hackett]

Starting to dig in my 340 4sp challenger rt and noticed it has j heads on it. I seen somewhere that they manufactured some j heads with 2.02s in them but unsure. Block matches vin #s and i wouldn't think the rt would come with 1.88s. So I figure some of you guys might know. I am gonna put a new timing chain and intake and gonna check the cam and valve train while im digging but while im that far i might change out the heads if they arent 2.02s. Dont want to change them if i dont have to tho. What do you guys think? Is there #s that tell they are original to the car? All info is appreciated.

 

[Edward Hackett]

My AAR 340+6 came with 'J' heads, 2.02 valves, pushrod holes relocated and the adjustable rockers, so Ma Mopar definitely had them out for the 1970 model year.
I've read that the 'J' heads were originally cast for the upcoming 360 engine, and it was a toss up which the '70 340s came with.

 

[i_taz]

The J heads on my '72 340 are 1.88...

 

[MerlinsMopars]

Tore the motor down last week and there were 2.02s in there.

 

[Racer Dave]

My T/A heads are J castings with 2.02s, but I have some later J heads that are 1.88. I don't know if this a reliable ID or not, but the 2.02 head looks like this:

And the 1.88 has an extended pad below the exhaust port ("smog" heads). These also have hardened exhaust seats because unleaded gas was becoming law.

 

[MerlinsMopars]

Thats good to know. I learned so much recently with this car as almost all the other mopars ive had over the years werent original and i would always just build what i wanted. Im sure its like a cold but i regret so badly how much good stuff i got ridd of or junked over the early years. Once parts became scarce I became i hoarder. Lol. These are what came off and in the machine shop now. Def 2.02s

 

[MerlinsMopars]

Dave, do you have any recommendations for machining? I asked them to cut the guides so I could run a larger cam later on if i want to but would pocket porting make them any better? I opened up a set of 1.88 j heads that didnt have those lower bolts and they put 2.02s in em and pocket ported them also but i would think the casting would still be the same, just smaller valves. Figured id ask while the shop is refreshing them.
Thx,
Scott

 

[fasjac]

You’re not alone, if I’d kept the cars and parts since the day, I wouldn’t have to be working anymore. Lol would a could a should of.
Very nice!

 

[Chryco Psycho]

There is a lot of potential in porting I tend to use a wing shape around the guide on both the intake & exhaust side as well as cut away the lip behind the valve seat , I always work on the roof but leave the port floor alone

 

[aussiemark]

My T/A heads are J castings with 2.02s, but I have some later J heads that are 1.88. I don't know if this a reliable ID or not, but the 2.02 head looks like this:
View attachment 68112

And the 1.88 has an extended pad below the exhaust port ("smog" heads). These also have hardened exhaust seats because unleaded gas was becoming law.

View attachment 68113

They are U heads.

 

[Racer Dave]

Dave, do you have any recommendations for machining? I asked them to cut the guides so I could run a larger cam later on if i want to but would pocket porting make them any better? I opened up a set of 1.88 j heads that didnt have those lower bolts and they put 2.02s in em and pocket ported them also but i would think the casting would still be the same, just smaller valves. Figured id ask while the shop is refreshing them.
Thx,
Scott

You'll want to machine the guides anyway to use the "positive stop" type Teflon seals.

If you are thinking MUCH larger cam later on, the stock springs are rather small diameter and don't allow a lot of lift before coil bind, so opening up the spring seat diameter and plunging them slightly deeper will help on both fronts and give you better spring choices. You do have to be careful doing this though because you don't want the seat to break into the intake runners or water jackets, as there's not a whole lot of material there. I have one set fully ported (bowls and runners) with seats cut to fit a spring O.D. of 1.480" and 1.800" installed height with stock length valves. That will allow up to about .600" lift. But save that for later......

Pocket porting with a nice 3-angle valve job is worthwhile IMHO. Also what Chryco said about the port floors is good advice. The short-turn radius is really tricky on Mopar small block heads best left to pros with both a flow bench and some empirical knowledge. A wrong move in this area can totally kill the head's flow.

 

[EV2&Q5]

Since you stated you have a 340 Challenger R/T then you can only have a 1971 Challenger with a Thermoquad. You would have J/U heads and they would have to be 2.02 intake valves. Some say the 71 340s with the 800cfm TQuads were the best.