Internal/external balancing

-
Captainkirk

Captainkirk

Old School Mopar Warrior
Joined
Apr 24, 2006
Messages
3,371
Reaction score
1,509
Location
Northern IL
Does anyone know with certainty the approximate date the 340 motors switched from internal to external balance, with corresponding changes in timing mark indicators, dampers and radiators/water pumps?
 
Last edited:
thought all 340's were internal just cast/forged crank with proper damper ???

water pump,damper.timing cover. radiator 70 and after
 
It can't be internally balanced with the addition of a balancer weight. I'd say when they went to cast with the weird balancer.
 
Sometime in 1972 for cast crank 340's which were external balance thru 1973 when they were discontinued. The switch to lower driver side water pump and radiator was for the 1970 model year.
 
dano said:
Sometime in 1972 for cast crank 340's which were external balance thru 1973 when they were discontinued. The switch to lower driver side water pump and radiator was for the 1970 model year.
Click to expand...
So, what about timing cover and relocation of timing mark on harmonic balancer?
 
jazak5 said:
thought all 340's were internal just cast/forged crank with proper damper ???

water pump,damper.timing cover. radiator 70 and after
Click to expand...

Cast crank 340's are external and their own deal. 318 and 360 external balance dampers and flywheels do not work with them.
 
Captainkirk said:
Does anyone with with certainty the approximate date the 340 motors switched from internal to external balance, with corresponding changes in timing mark indicators, dampers and radiators/water pumps?
Click to expand...


Pretty sure it switched after 1971. My 1972 340 duster was lower compression, externally balanced, and electronic ignition.
 
My 71 340 was forged, neutral balance. Unfortunately, the original crank "may not be" anymore
 
pishta said:
It can't be internally balanced with the addition of a balancer weight. I'd say when they went to cast with the weird balancer.
Click to expand...


You can make an externally balanced crank internally balanced by the addition of Mallory metal. Cranks are so cheap I’m not sure why a cast crank is ever used.
 
Being certain about something approximate?
 
I have a cast crank 340 listed now. Someone said the change was like February of ‘72 was the change to cast crank, low compression pistons, and the balancer and weighted converter.

the timing cover change was after 68 from what I’ve seen. Went from drivers side to passengers side, and coincided with the release of the 318 and 340.
Cast crank was late ‘72 model year change. I also have a ‘72 forged crank engine...
 
gregcon said:
Being certain about something approximate?
Click to expand...
Yeah, like within a quarter year of production or so. Not like "between 1970 and 1973"

Reason I'm asking...building a '72 340 that came with a cast, externally balanced crank. I'm replacing it with a forged, internally balanced '69 340 crank. I need to buy a new damper that will fit the forged crank and need to know which timing cover, water pump and radiator to use.
 
Captainkirk said:
Yeah, like within a quarter year of production or so. Not like "between 1970 and 1973"

Reason I'm asking...building a '72 340 that came with a cast, externally balanced crank. I'm replacing it with a forged, internally balanced '69 340 crank. I need to buy a new damper that will fit the forged crank and need to know which timing cover, water pump and radiator to use.
Click to expand...


70 and later small blocks all get the timing cover and water pump that have the outlet on the passenger side. 69 and earlier have the outlet on the driver side. So the timing cover, water pump and such should match the year of the car.
 
yellow rose said:
70 and later small blocks all get the timing cover and water pump that have the outlet on the passenger side. 69 and earlier have the outlet on the driver side. So the timing cover, water pump and such should match the year of the car.
Click to expand...
Thank you! So, I would buy an aftermarket damper marked as "fits 70-72 with forged crank"?
 
when they switched to cast crank on the 318 they stayed internal balance. 318 and 340 have same main size, same stroke. why didn't they just use the 318 cast crank in the later 340s? any advantage to external balanced engines vs internal? I cannot think of any.
 
The 340 pistoons and rods are more heavier.
 
yeah and.....
they are equally heavier. so should still be OK as internally ballanced. They ran 340s internal balance from 68 til (at least) 71 without issue. so why the change (and confusion) with switch to external?
 
volaredon said:
yeah and.....
they are equally heavier. so should still be OK as internally ballanced. They ran 340s internal balance from 68 til (at least) 71 without issue. so why the change (and confusion) with switch to external?
Click to expand...
If I'm not mistaken, one of the reasons for the switch was because cast cranks were cheaper to produce and required less machining. You balanced the crank assembly using the weighted damper. Another reason was the engines were de-rated and had a lower CR. Or so I'm told.
 
so again... the cast 318 crank would have gone in and worked fine and retained internal balance.
 
When a crank is balances, its balanced to its rods and pistons as a unit. You put a neutral balance 318 crank in a 340 with its heavier pistons and rods and its not going to work. The 340 rods and pistons were heavier than than the 318's even though the 340 cranks were drilled to help a little. Its not like a 4 banger 180 crank where if the piston pack and rods weigh the same its balanced (as they counterbalance each other). a V8 has 2 banks at 90 and they are both above the crank centerline so they need crank counterweights to match the bobweight of the piston pack and rods. The cast cranks were lighter than the forged (less dense) so they had to add weight to the balancer to "Detroit" balance it as a unit while using the same rods and pistons. Its pretty easy to spin the assembly in the motor and then only balance the damper to the spinning motor to get a 'good enough' balance for a street motor. Going back to my first comment in Post #3, it cannot be "internally balanced" if it in fact needs the offset weight of the special "cast crank" damper that looks like an egg! As YR stated, anything can be made to be internally balanced with tungsten, but its like $50 a slug and sometimes you need a few! Far cheaper to just get the correct balancer and be a little over so your shop can take metal out of the balancer perimeter to get you to zero. All 318s were internally balanced, and all their balancers were neutral also. 318 and forged 340 balancers were compatible as they were both neutral.
 
I don't know if it's a fact, but rumor has it the cast crank blocks were painted blue, not orange like the forged crank motors. But after 50 years, that might not matter one whit.
 
Captainkirk said:
Does anyone know with certainty the approximate date the 340 motors switched from internal to external balance, with corresponding changes in timing mark indicators, dampers and radiators/water pumps?
Click to expand...
Near as I can tell, it's around 04/01/72 when they changed over from forged to cast cranks. If you're going to swap in a forged crank in place of a cast crank, I'd rebalance the engine, get a '72-up 318 harmonic balancer (so you don't have to modify your crank pulley), remove the weights on your torque converter (or get a 318 flywheel if your car has a manual trans), and reuse your existing timing cover and water pump.
 
You must log in or register to reply here.
-

Back
Top