Difference between J1 and J1a?

I think I have been told what the difference between the J1 and the J1a was, perhaps even on this forum, but I can't remember and can't find a string that looks relevant. Can someone tell me just how the two sub variants of PRR 2-10-4 differed?

It's NOT a matter of one having a one-piece cast frame and the other not: I think all J1a had one-piece frames AND some (but not all) J1 did, too. I found one internet source that said that the J1 were built in 1942-1944, but that all the J1a were built in 1943. This suggests that the J1a was a response to temporary (War Production Board mandated?) unavailability of some components or materials.

HELP! This is bugging me...

PRR mechanical had a single diagram, available here
http://prr.railfan.net/diagrams/PRRdiag ... z=sm&fr=ge" onclick="window.open(this.href);return false;
covering both subclasses. A note in the upper left corner says that specifications apply to both, except as noted. Dimensions are given with no note about variation, but for weights there are separate columns for J1 and J1a… with the J1a column frustratingly left blank! ***IF*** it turns out that the J1a was heavier, my first guess would be that the difference was substitution of a lower grade alloy (say, carbon steel instead of nickel steel) somewhere. But that would only be a guess.

But this
http://www.steamlocomotive.com/texas/?page=prr" onclick="window.open(this.href);return false;
web page says that the J1a weighed ten thousand pounds LESS than the J1. I'm puzzled.

And, on the third hand, the July 1989 issue of "Mainline Modeler" has an article on the PRR J1/J1a (with 1/64 scale drawings) that includes what seems to be the same PRR diagram referred to above… but with the J1a weights filled in! And here it seems the J1a is about 2,000 pounds heavier. (And, for comparison, a diagram of the C&O T1.)
…
Production tolerances in locomotive building aren't pharmaceutical grade, so there was undoubtedly some variation within each subclass. And, of course, in operation (with the stoker and the feed-water heater trying to replenish what went up the stack as smoke and steam) weights would have varied continually. But weights givn on the same PRR Mechanical department diagram should represent the same condition, so this suggests that -- at least according to the designs -- the J1a was heavier than the J1… by something under one half of one percent.

For the record:
Category --- J1 --- J1a --- T1
Wt Empty 510,140 513,630 *
Wt W'king Ord 572,640 574,730 **
On pilot tr. 63,650 64,170 61,000
1st Ax, Tr tr 64,070 64,160 66,000
2nd Ax, Tr tr 67,070 67,160 66,000
Drivers total 377,800 379,300 **
No 1 Drivers 76,800 77,270 74,500
No 2 Drivers 75,880 76,340 74,700
No 3 Drivers 72,510 72,890 75,000
No 4 Drivers 76,670 76,760 74,300
No 5 Drivers 75,940 76,040 74,100
* Not shown on T-1 diagram
* Not shown on T-1 diagram (and I'm too lazy to do the arithmetic right now)
--Note that weights are quoted to the nearest ten pounds on the PRR diagram but only to the nearest hundred on the T-1.

The weight difference is in the boiler plate and frame, model dependent

J-1's - had fabricated bar early builds and cast steel frames, all J-1a had cast steel frames I wonder if that was the difference.
J-1 6450 built 12/42 first J-1


Did find one reference -say boiler plate was the only difference - http://www.prrho.com/prr-j1-class.html" onclick="window.open(this.href);return false;
We know the frames were different so weight difference of the two types of plate make a difference?
J-1 carbon boiler plate
J1a - carbon silicon boiler plate
second reference
http://www.steamlocomotive.com/texas/?page=prr" onclick="window.open(this.href);return false; worth reading, very good detail
Based on the Chessie's 40 T-1 class engines built in the 1930s (Locobase 95). Pennsy built 127 total, modifying them slightly internally and to a significant degree on the surface, including the addition of a much larger tender. J1as used different materials in their boilers, but were otherwise identical. Some sources claim that J1a had cast-steel, as opposed to built-up, locomotive beds, but others state that both classes had locomotives rolling on both kinds of beds.
Wish I had one of each type to put on a scale!

Thanks, Rdgrailfan!
Your second link (http://www.steamlocomotives" onclick="window.open(this.href);return false;) mentions a weight difference, but doesn't explain it. (The weights it gives are well over those on the PRR diagram: I have no idea what the scale would show if we COULD put them on a scale!) The first link (http://www.prrho" onclick="window.open(this.href);return false;) says that it is the J1 that has the carbon/silicon steel and the J1a that had the straight carbon steel. This makes sense to me: I believe that carbon/silcon steel was more of a "specialty" steel, probably a bit lighter in weight for equivalent strength, and more likely to be unavailable for a time during the war. (I think the New York Central Niagaras were built with silicon steel boilers. which developed cracks and were replaced with carbon steel ones.)

(B.t.w. If you are a Reading fan, AND interested in 2-10-4 types, have you seen Richard Leonard's very neat Reading "U-1"?
http://railarchive.net/fantasysteam/rdg_2200_rcl.htm" onclick="window.open(this.href);return false; )

Now that Rdgrailfan has answered the question that was bugging me… Here's an "alternative history" question. The PRR, after the war, talked to Lima Locomotive Works about building another 25 J-1. Then they cancelled the order, building the 25 "production" Q-2 instead. (Who the gods wish to destroy, they first render insane.) I don't know how far the talks between PRR and Lima got, and I don't know whether detailed specifications were drawn up. But… Lima and Altoona didn't think EXACTLY alike, and it's not implausible that Lima would have wanted to make some small revisions in the design. What might they have been? I note that Lima DID build some postwar 2-8-4 for the Louisville and Nashville, and did make a few revisions to the design of the wartime Baldwin 2-8-4 that L&N had received. Like: using Boxpok drivers.

Did some more poking around on the steel plate weights, no direct answer but one point jumped out.
The weight of carbon silicon steel is slightly less (2/3%) than that of pure carbon steel.
Apparently the carbon steel was in very short supply up to the middle of WWII, but as war production orders were cancelled it started to became available.
I found two references to carbon silicon steel for liberty ship construction, which contributed to their flex properties when in operation. Remember the life expectancy of a liberty ship was not all that great in 42/43, this my have contributed to the decision to use a lesser (slightly) grade of steel.
Early in 1944 the US began scaling back orders for liberty ships.
Tanks used carbon steel, by the end of 1944 the end was in sight for the Europe war and orders were cut back.

I suspect the types of steel used were based on what can I get in "quantity" for the planned production on a given date, based on War demand.
Fractured logic but it points to the PRR using what they could get!

Rdgrailfan--
Thank you for following up on that! The difference in weight of the two types of steel is probably the explanation for the small difference in weight between the two subclasses. (I suspect that weight-saving was the motive for the New York Central's getting their Niagaras built, originally, with Silicon steel boilers. The New York Central didn't have 155 pound rail like the PRR, and in fact apparently had a lot of 129 pound rail on main lines, and my impression is that they were a lot more concerned than the PRR with keeping axle loads down. Certainly I don't know of any NYC steam type that was as heavy, per axle, as the J1 and Q2 of the PRR!)

The PRR wasn't the only railroad that had to make do with "second choice" steel on war-time steam locomotives. The Santa Fe's war-time 2900 class 4-8-4 was basically similar in design to the pre-war 3776 class, but much heavier because the light-weight steel used on the earlier class was not available during the war.