408 Stroker Engine Vibration

[Roadhunter]

I have an annoying engine vibration in my 408 stroker and am looking for suggestions. This has an internally balanced Scat forged rotating assembly which was installed by the builder(not me) and was sent out again to a different shop and rebalanced. The notes he made was that it still had the vibration "can I live with it?". I asked him about it and if i should try a new fluid balance and he said everything was balanced together. It is annoying to be driving at cruising speed and get that rhythmic vibration through the steering wheel. Is the a risk of doing some major damage if I keep running it this way? I have put less than 50 miles on it since I have owned it and has less than 800 from being rebuilt.
Thanks

 

[TT5.9mag]

If it’s out of balance it will hurt itself eventually. If your machinist asked you “if you can live with the vibration” get a new machinist. You need to figure out what is causing the imbalance, flywheel, flexplate, converter, harmonic balancer, or the rotating assembly, and FIX it.

 

[jimjimjimmy]

A vibration of any kind is not good .

 

[Super-Cuda]

I have an annoying engine vibration in my 408 stroker and am looking for suggestions. This has an internally balanced Scat forged rotating assembly which was installed by the builder(not me) and was sent out again to a different shop and rebalanced. The notes he made was that it still had the vibration "can I live with it?". I asked him about it and if i should try a new fluid balance and he said everything was balanced together. It is annoying to be driving at cruising speed and get that rhythmic vibration through the steering wheel. Is the a risk of doing some major damage if I keep running it this way? I have put less than 50 miles on it since I have owned it and has less than 800 from being rebuilt.
Thanks

Hey Roadhunter.

A few things..

Do you feel a vibration at idle?

Did you get a report of the balancing (rod weights, piston weights, etc..)

No way you should get an engine vibration if the balancing was done properly.

What about the harmonic balancer? Did you use the correct unit for your combination? (internal or external balance)

 

[Mopar Sam]

First off, are you sure its an engine vibration.
Does it do it in neutral with the engine rpms at the speed you feel it on the road?
Rhythmic vibrations are usually drive shaft , pinion angle etc. related.

 

[George Jets]

Back to basics:

The Chrysler LA 360 is an externally balanced engine, requiring a weighted flexplate (such as the B&M #10239 or Summit SUM-G124SFI) to compensate for its cast crankshaft. In contrast, a 408 stroker derived from an LA 360 block typically uses an internally balanced crankshaft, which requires a neutral balance flexplate (such as a stock 318 unit) and a neutral balance torque converter.

Using the wrong combination causes severe vibration; specifically, an internally balanced 408 engine paired with an externally balanced 360 flexplate will result in significant driveline shake. If building a 408 stroker, you must verify the crankshaft balance specification with the manufacturer, as some aftermarket kits may be internally balanced while others retain external balance characteristics.

Re-verify everything...


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[George Jets]

What transmission did you originally have mated up to your 360 external balanced engine?

727 with balance weights welded on the torque converter with a neutral balance flex plate?

If so you need to remove the torque converter (welded on) balance weights and use a neutral balance flex plate to go with your neutral balance 408 engine.

Not using one of these in any combination.

This is not designed to offset your weighted 727 torque converter for your 408 application.

Have to check all possibilities, lots of variables where things could go wrong.


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[Roadhunter]

I will get some specs and post it. It was not done for me but I have the build data. This guy is a well known builder around my area and not his first time. It does it with the trans in neutral.

 

[George Jets]

Showing some more threads out there that the 408 stroker engines do need the weighted flex plate for the 727 neutral trans. (Suggesting it's an external balance 408 build)

___________________

My 408 stroker motor calls for a "weighted 360 pre magnum flex plate"

???


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[George Jets]

Even Blueprint Engines was talking about needing the weighted flex plate with their 408 builds in 2025, mating up to a neutral balance 727 trans.

__________

Time to do some more research to find out what the 408 build needs.


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[Mopar Sam]

I will get some specs and post it. It was not done for me but I have the build data. This guy is a well known builder around my area and not his first time. It does it with the trans in neutral.

Ok, doing it in neutral tells us it is a problem with the rotating assembly. When you get a chance, can you lay eyes on the flex plate and the front damper? You would be better off with a helper to rotate the crank a bit at a time to see if the flex plate is external or neutral. Pictures would be great. Also, builder's and machinist's notes will help.

Was any explanation given as to why the machinist could not balance it?

No way would a crank/rotating leave my place like that. Even if you had to save for mallory(tungsten) slugs.

 

[Roadhunter]

Here is what I have. Let me know your thoughts.

 

[Roadhunter]

Here is what I have. Let me know your thoughts?

 

[Roadhunter]

Sorry for the double post it didn't look like I hit reply.

 

[NC Engine Builder]

B&M converters have been not great in my experience

 

[George Jets]

Neutral Balance Flex Plate
#10230

Flex Plate #10230 for a 727 on the B&M Flex Plate sheet

Provided that is the one that is actually in the car.

______________

Here is an external balance one for a 360/904
#10239

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[George Jets]

Front Damper looks to appear to be a neutral balance also, unless it has a Half Moon carved out of the back side.


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[George Jets]

New B&M Torque Converter

Good diagnosis by NC Engine Buider on chasing this vibration problem down.

Possibly bad torque converter.


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[George Jets]

Probably be a good idea to pull the stick on the 727 transmission and see if the fluid is starting to discolor?

As if that new torque converter is beginning to fail, could be taking out the front trans pump and bushing.

Could go as far as taking out the whole trans if you keep driving it with that vibration.

If things look suspicious, next step drop the trans pan and start looking for shavings.


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[autoxcuda]

I have an internally balenced Scat stroker kit.

I mated it right to the old 727 trans that was behind my original 68 340. No issues

 

[Roadhunter]

Probably be a good idea to pull the stick on the 727 transmission and see if the fluid is starting to discolor?

As if that new torque converter is beginning to fail, could be taking out the front trans pump and bushing.

Could go as far as taking out the whole trans if you keep driving it with that vibration.

If things look suspicious, next step drop the trans pan and start looking for shavings.


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Thanks, I will try that.

 

[MOPAROFFICIAL]

First off, are you sure its an engine vibration.
Does it do it in neutral with the engine rpms at the speed you feel it on the road?
Rhythmic vibrations are usually drive shaft , pinion angle etc. related.

Exactly. The only part is where the "machinist" asked if he could still live with it knowing that it still was out of balance, or else why would he say that... I don't get it.. but yeah vibration in the steering wheel could be wheel balance could be the engine header laying against the steering box... you never know

 

[dr dodge]

I have an internally balenced Scat stroker kit.

I mated it right to the old 727 trans that was behind my original 68 340. No issues

balanced old school flex plate?

 

[autoxcuda]

balanced old school flex plate?

Just an original stock flex plate. No balance marks in it.

Truly internal balanced motors shouldn’t need anything special

 

[George Jets]

As things are being run down here, I would think it would be a good idea to have @BluePrint Engines to chine in here (or call them) for their recommendation on the proper Balance Flex Plate for a 408 stroker build.

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