RM21G4G with a 340?

[Amata Bene]

I'm new and this Road Runner is new to me. But I'm not new to Road Runners. So here is my dilemma. The VIN is RM21G4G the fender tag also qualifies that. The motor however clearly reads 340. To be more precise, it reads MH 340R the following numbers I believe are 38480155 get a little hard to read (see photos). I have had this vehicle for 18 days. Finally freed up a place in my shop for a few days to do some inspecting. I give the frame and body 8 out of 10. All rubber bushing are um... not good. A lot of original stuff under there still.
So I moved on to drive train. Not all that experienced with SB MOPARs but it didn't look like the 318's I've seen before. Indeed on closer inspection it is in fact a 340. If I'm gonna have a SB it's my opinion 340 is the better of the 3 after 68 up to 1971 ish. In the photos are numbers under the left head on the block and the machined surface at the oil pan on lower right side of block, just behind motor mount.
I am not sure how to read them to figure out year and what the motor may have come out of. Any help in deciphering would be greatly appreciated.

 

[Cranky]

On the side of the block will be a casting date and it will be 'cast' on the block and not a machined pad. Don't recall what side it will be on though. Also, the poly teen looks way different but from 67 on, they look just like any other small block. And welcome to the site!!!

 

[turbine68rt]

Most likely from a '72 Dart, Demon, or Challenger built at Hamtramck.

 

[Amata Bene]

Most likely from a '72 Dart, Demon, or Challenger built at Hamtramck.

I get the "Hamtramck" and "R" regular fuel. How do you get to 1972? I am confused on the number sequencing and which ones to use.

 

[Amata Bene]

I also thought that part of the number at the bottom would match part of the number on the top.

 

[R/T Boy 67]

H 340 is 72 340. The 2B on the lower stamping is 1972 hamtramck built car.

 

[WileERobby]

The good thing is... it's a 340 !

 

[70chall440]

Look at the raised casting numbers on the side of the block that will clearly tell you what displacement it left the factory as, not any stamped number

 

[Amata Bene]

The good thing is... it's a 340 !

The bad thing is... it's a 72. Got choices from 68 - 71 as far as better heads if I go that way. They could have at least used a 70 AR 340. Just sayin. Dose not sound or act stock. Lumpy and all over the place at idle. Touch the skinny peddle and it wakes right up! Running richer than the Bidens right now. Trying to get that sorted. Still has the little 904 and dog legged diff. All in due time. Already put out some fealers for SB 727. B body limited slips aren't to hard to find. Got nothing but time and... no money. Where's there's a will, there is a way. On the plus side the misses ain't fussin. Yet.

 

[Runcharger]

If it runs good, be happy. I've always been more into burning rubber than date codes. Sounds fairly punchy, has probably been rebuilt with good pistons and camshaft. I'd run the 904 until it calfs, they'll ET better than a 727 anyway and will live if they've been upgraded a bit.

 

[WileERobby]

The bad thing is... it's a 72. Got choices from 68 - 71 as far as better heads if I go that way. They could have at least used a 70 AR 340. Just sayin. Dose not sound or act stock. Lumpy and all over the place at idle. Touch the skinny peddle and it wakes right up! Running richer than the Bidens right now. Trying to get that sorted. Still has the little 904 and dog legged diff. All in due time. Already put out some fealers for SB 727. B body limited slips aren't to hard to find. Got nothing but time and... no money. Where's there's a will, there is a way. On the plus side the misses ain't fussin. Yet.

Guess what, that 904 ain't no slouch. Just ask my buddy who'd raced a 360 Cuda with a 904 for years. Give it a beef up when you're ready and it'll be good to go... literally.

 

[Amata Bene]

Okay, so the effects of "I got a Road Runner" liquor I drank wore off Friday evening and Saturday. Sure I went to bed with an 8 but seems that I am waking up with a solid 6 maybe 7. This girl is easy to look at and even a few strangers have whistled. But you start taking her clothes off and time has taken it's toll. Not sure yet what the hell is going on ignition wise. Tickled the carb a little bit and played with the timing and still can't get her to smooth out. You load her up and she stumbles and coughs and lurches. So I started with a compression check. #2 was the best at 150 lbs. While #3 & 5 were the lowest at 135 lbs. That puts me slap on the 10% mark. Ran all 8cyl. 3x's, very consistent. Just for giggles I let it set the 2nd and 3rd time and only lost 2lbs max for any one cyl. I know this is not a proper leak down test, just quick and dirty. I check the wires, good continuity, put a set of new plugs in her and still rough as sand paper. Yes I checked and double checked firing order. So I'm leaning towards ignition system. This is my nemesis. Like most other normal people I HATE ELECTICAL. I'm not sayin you ain't normal if you like that stuff. I'm just sayin you ain't right and god bless ya cause the rest of us need ya. Did I mention the puddle of tranny fluid (not that kind) on the floor after all the cranking and running in my shop. Yeah there's that. But hey it's still a Road Runner with good compression and that's not a bad starting place. What I need is a constant flow of money and no job. Being thirty years younger would help also but I'd settle for the free money.