340 8 bolt crank info wanted

-
kenelder

kenelder

70 340 4spd 6 pack
Joined
Aug 14, 2018
Messages
106
Reaction score
79
Location
Largo, Florida
I am trying to get more info on the 340 crank pictured below. It has an 8 bolt hub and the snout shows 37686 stamped on it. I’ve been told it is a Mopar Performance stroker crank but I cannot find any info on the stamped number. If any of you could help with info on this crank I’d sure appreciate it.

Thanks!

IMG_2975.jpeg
 
I have a book from 1995 that lists 8 bolt forged cranks, but those numbers don't coincide with the book #'s. They list 9 different strokes for an 8 bolt crank. Destroked at 3.06 to stroked at 3.79 and of course a 3/38 stroke.
 
Thanks guys, must be a job # like the first reply says.

Does anyone know if the 8 bolt stroker cranks were all internal balance? Since they were for 340 I’d guess they were all internal.
 
If it's a forged crankshaft it should be internally balanced. The 72 on up 340's had cast externally balanced crankshafts. 65'
 
If you have the heads off you can measure the stroke. That will give you some more info. Did it come with a balancer? You can tell if it was a externally balanced from that. Likely if it is an aftermarket steel crank it will be internally balanced
 
Thanks, but heads are on it and it's in the car. Balancer was internal balance, so was flywheel. But it has a vibration that feels like it's out of balance, trying to figure it out.
 
kenelder said:
Thanks, but heads are on it and it's in the car. Balancer was internal balance, so was flywheel. But it has a vibration that feels like it's out of balance, trying to figure it out.
Click to expand...
Can you describe the balance issue. Have you pin pointed it to the engine?
 
I have pin pointed it to the rotating parts- flywheel, clutch, crank, engine internals. It's not the trans or driveline, does it in neutral sitting in the driveway. Ruled out ignition, misfiring, pulleys, external accessories, etc. I replaced the harmonic damper with a Fuidampr hoping that would fix it, but no difference at all so it's not torsional vibration.

I didn't build the engine. It's supposed to be about 382 cubic inches with 4.064 bore and the stroker crank and if that's accurate then the stroke must be 3.74". Don't know what rods are in it, but it has J&E pistons. I was told it was balanced when built. I've got pics of it during the build and it looks like a quality job so I have to think it's balanced properly. Tons of power but I can't use it since it has this vibration that shouldn't be there.

Next step is to remove trans and pressure plate/clutch and run it and see if that fixes it. It's a new McLeod billet flywheel, new clutch, new pressure plate for internal balance engines. I did not have them balanced, probably a mistake but I've never had to do that on any 340 before. McLeod recommends balancing them (wish I had known that) but I really doubt they are off very far. This vibration is bad enough that when you get on it it makes you want to back off before anything comes apart. Starts above 1500 rpm and never goes away, doesn't reach a peak either. Feels like something out of balance.
 
If the pan was off you might find the last 3 digits stamped on the front the front counterweight. See picture in this add for a 3.51 stroke MP crank with the stamping 710.

[FOR SALE] - Mopar Performance crankshaft

By the way Mopar Performance made. 3.71” stroke 8 bolt flange forged crank who’s last 3 digits that should be stamped on the front counter weight should be 714. This strike would be close to your number calculated.

IMG_1152.jpeg
 
Also if your cylinder bore is 4.070” (uses pistons that are 4.064” plus 0.006” piston to bore clearance), you would get 382 ci with a 3.74” crank.
 
Thank you very much Rocket!!! I'll bet it is the 714 crank, that would make sense. And yes you are right it's likely 4.070 bore. I really appreciate the information, thank you all very much.

Now to fix that vibration
 
kenelder said:
I have pin pointed it to the rotating parts- flywheel, clutch, crank, engine internals. It's not the trans or driveline, does it in neutral sitting in the driveway. Ruled out ignition, misfiring, pulleys, external accessories, etc. I replaced the harmonic damper with a Fuidampr hoping that would fix it, but no difference at all so it's not torsional vibration.

I didn't build the engine. It's supposed to be about 382 cubic inches with 4.064 bore and the stroker crank and if that's accurate then the stroke must be 3.74". Don't know what rods are in it, but it has J&E pistons. I was told it was balanced when built. I've got pics of it during the build and it looks like a quality job so I have to think it's balanced properly. Tons of power but I can't use it since it has this vibration that shouldn't be there.

Next step is to remove trans and pressure plate/clutch and run it and see if that fixes it. It's a new McLeod billet flywheel, new clutch, new pressure plate for internal balance engines. I did not have them balanced, probably a mistake but I've never had to do that on any 340 before. McLeod recommends balancing them (wish I had known that) but I really doubt they are off very far. This vibration is bad enough that when you get on it it makes you want to back off before anything comes apart. Starts above 1500 rpm and never goes away, doesn't reach a peak either. Feels like something out of balance.
Click to expand...
Sounds like you are on the right track. I did just balance my 340 flywheel and mcleod pressure plate as a unit and was surprised at how much weight it took to correct it. If the flywheel and clutch doesn't change the vibration, it is possible they may have messed up the calculation for the engine balance. Hopefully you can solve it.
 
Thanks for the support! Fingers crossed it’s the pressure plate, really hate to take such a good running engine apart.
 
If you pull a spark plug, can you get a bamboo skewer down the hole straight enough to measure the up/down travel of the piston? Might give you a ballpark idea of stroke but it sounds like the stroke isn't the issue and if the clutch parts check out, you might be getting up close and personal with the crank. Drop the pan first to see if anything is loose or wonky on the crank weights?
 
Thanks Gzig5! I'm not so interested in the exact stroke but I wanted to know if this crank is for internal balanced motors. Since it is an 8 bolt crank for a 340 and I was told it was an old Mopar Performance crank I believe it was for internal balanced engines. Just wanted to know as much as I could before I start taking things apart! If it isn't the pressure plate then I guess I'll be getting a much closer look at the crank.....
 
The new Mopar Performance cranks I have seen did not have any of the crank throws drilled for balance. So similar to forged 318 or forged 273 crank but unlike the 340’s forged cranks which had two of the rod throws drilled through at the factory. When your motor was built, if for example they used the stock damper and flywheel and did not rebalance the assembly this could cause it to be unbalanced. If it was assumed the new MP crank had the same weight and weight distribution of a 340 crank that could cause it to be out of balance.

Many variables to consider.
 
I would guess that any (non-hemi) Mopar crank with eight bolt flywheel would be a race-quality piece, steel or billet, and internally balanced.
Here's hoping your clutch/flywheel/pp are way out.
 
You must log in or register to reply here.
-

Back
Top