Torque Converters for Internally Balanced vs Externally Balanced Small Blocks

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Detroit Iron

Detroit Iron

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Background: I bought a 1975 360, which I recently built, and it came with a 904 attached to it. The guy who sold me the engine and transmission combo said they were a "matched" set.

Most of my experience with small blocks is with internally balanced 273's 318's and 340's, so I just did my thing.... rebuilt the motor and the transmission and put it in the car. I've got a vibration at speeds of about 40 mph that I'm trying to track down... I even thought it was the tailshaft busing and started a thread in this section about that. In the process of removing the tailshaft, I noticed a 1971 date code on it. Hmmmm... so NOT a matched set after all.

Further diagnosis; with the car parked, I put it in neutral and race the engine and the vibration is present. So, it wasn't the tailshaft bushing or the drive shaft at all.

So either the engine is highly out of balance which I seriously doubt, or..... the torque converter is for an internally balanced engine like a 318 since it has a 1971 date on it.

My question: Do 360's require a special torque converter? And if so I assume I'd be able to pull my inspection plate and see a weight or some sort?

I most likely won't get the inspection plate pulled until the weekend, but I wanted to get this thread started to see what my friends here have to say. Thank you all!
 
First, a question. How do you know the engine isn't "highly out of balance" if you didn't have the machine shop balance it? If it was out of balance before, guess what? It still is.

Ok onto your question. Yes! The 360 requires an externally balanced converter with weights on it, AS WELL AS an externally balance harmonic balancer with either an offset weight on it, or a large "groove" cut out of it. There are two types and I believe they are interchangeable. Although, all that said, skipping having a new build balanced is a mistake. I think you found that out.

Now, there is something different you can do with the converter. You CAN run a neutral balance converter if you use the externally balanced flex plate B&M makes that has a "bite" out of it to balance the 360. So that's another option there. But you need to inspect the converter for weights. They will be welded to the front of the converter, just behind the dust cover, if you have one. Also inspect the balancer to see if it is externally balanced. Feel free to post pictures of both and we can help determine what you have. Lastly, I would VERIFY that you do indeed HAVE a 360. A lot of people have gone through this only to discover they have a 318 or some other small block.
 
It's a 360. I built it.... .030 over new pistons bearings etc. I've never had a problem with stock motors being out of balance after a rebuild. The factory slapped them together and sent 'em and manufacturing variances were way out compared to modern piston technology. Sure, you could be right it... the motor could be out of balance, but my suspicions lean toward this converter at the moment.


Good point on the balancer, and I do have the correct harmonic balancer on the motor. I knew about that, just wasn't sure about the torque converter.
 
Just to show what you're looking for, here are the early 360 convertor weights; two weights, one on each side of the drain plug (early to mid '70s):
1757390444389.png

And here is the later 360 weight (single weight, beginning late 70s):
1757390611746.png

With a bunch of mismatched parts, you could have either one- but they are interchangeable.
A neutral/internally balanced convertor will have no weights on the face of the convertor.
To use an internally balanced convertor on a 360, just use the B&M 10239 flexplate.
 
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I had the machine shop all Mallory metal to my 360, only cost a couple of bucks. Then they were balance it internally which opened up a wider variety of torque converters, harmonic
balancers etc..
 
Detroit Iron said:
It's a 360. I built it.... .030 over new pistons bearings etc. I've never had a problem with stock motors being out of balance after a rebuild. The factory slapped them together and sent 'em and manufacturing variances were way out compared to modern piston technology. Sure, you could be right it... the motor could be out of balance, but my suspicions lean toward this converter at the moment.


Good point on the balancer, and I do have the correct harmonic balancer on the motor. I knew about that, just wasn't sure about the torque converter.
Click to expand...
You're right. They were out of balance a good bit compared to performance standards. But remember that P word. If you're building for more performance than stock, balancing should not be overlooked. Hope you get it sorted out. Keep us posted.
 
Heywodja said:
I had the machine shop all Mallory metal to my 360, only cost a couple of bucks. Then they were balance it internally which opened up a wider variety of torque converters, harmonic
balancers etc..
Click to expand...
You forgot to put the word hundred after the word couple.
 
92b said:
You forgot to put the word hundred after the word couple.
Click to expand...

It was maybe 200 more.. a relatively small price to pay for adding less complexity and access to more off the shelf parts availability for other components, like harmonic balancers and torque converter swaps.

My 408 stroker-build also went with an internally balanced rotating assembly as did the small block chebby 427 stroker for my truck.

For me it works, for others it doesn’t but that’s the beauty of freedom of choice.
 
Professor Fate said:
Just to show what you're looking for, here are the early 360 convertor weights; two weights, one on each side of the drain plug (early to mid '70s):
View attachment 1716452539
And here is the later 360 weight (single weight, beginning late 70s):
View attachment 1716452540
With a bunch of mismatched parts, you could have either one- but they are interchangeable.
A neutral/internally balanced convertor will have no weights on the face of the convertor.
To use an internally balanced convertor on a 360, just use the B&M 10239 flexplate.
Click to expand...
@Professor Fate for the win. Thank you, thank you, thank you. This really helps.
 
Did Ma EVER put 904's behind 360's?

People like BM and others used to sell converter flex plates that were weighted so you could use a neutral converter behind a 360. Some of them had dual drillings for smaller converters.

ALSO MAKE CERTAIN the 904 is not an early one with the small converter snout, which will not pilot into later engines. I forget, 68? 69? and later. 67 / earlier were for certain small snout
 
67Dart273 said:
Did Ma EVER put 904's behind 360's?

People like BM and others used to sell converter flex plates that were weighted so you could use a neutral converter behind a 360. Some of them had dual drillings for smaller converters.

ALSO MAKE CERTAIN the 904 is not an early one with the small converter snout, which will not pilot into later engines. I forget, 68? 69? and later. 67 / earlier were for certain small snout
Click to expand...
Indeed they did put 904s behind 360s. I believe Cordobas and stuff like that. This is a 1971 transmission, so I think the converter snout is correct for the 360. I think you're right though.... '67 was the last year for the smaller snout.
 
Last edited:
Heywodja said:
It was maybe 200 more.. a relatively small price to pay for adding less complexity and access to more off the shelf parts availability for other components, like harmonic balancers and torque converter swaps.

My 408 stroker-build also went with an internally balanced rotating assembly as did the small block chebby 427 stroker for my truck.

For me it works, for others it doesn’t but that’s the beauty of freedom of choice.
Click to expand...
Yup. Considering machine work for an entire build can get into the thousands, a few hundred more for a proper balance is chicken scratch.
 
Whelp, I see weights. You guys are gonna fight me on this, but I'm still not totally convinced the problem is with the engine. Hear me out.

This engine and transmission sat under a tarp for an unknown amount of time. I bought the combo and built both units. As I stated before the only change to the rotating assembly I made is I replaced the cast pistons with new cast pistons. Yes, the problem could have existed before I built the engine, but I'm ignoring that possibility for now. Haha...

My thoughts are that the torque converter could still be the problem. I don't fully understand how torque converters work, so maybe I'm wrong.

At any rate here is my plan moving forward. I'm going to replace the torque converter since it's an unknown, and the cheapest first step. If that doesn't solve the problem, I will pull the engine and have the entire rotating assembly balanced.

I'm looking for input on what converter to buy. Places like Summit and Jegs sell B&M and Boss Hog in the low stall\stock-ish replacement range. There are not many choices for 904s, and even less choices for 904s with a 360. I heard another member here mention Circle D Specialties in Texas makes application specific torque converters, and I did submit a price inquiry with them, but I'm guessing their stuff will be in the $1k price range which is out of the question for me.

I just wanna go 80 mph on the freeway. Gheez!

IMG_2708.JPEG


IMG_2709.JPEG
 
Detroit Iron said:
Whelp, I see weights. You guys are gonna fight me on this, but I'm still not totally convinced the problem is with the engine. Hear me out.

This engine and transmission sat under a tarp for an unknown amount of time. I bought the combo and built both units. As I stated before the only change to the rotating assembly I made is I replaced the cast pistons with new cast pistons. Yes, the problem could have existed before I built the engine, but I'm ignoring that possibility for now. Haha...

My thoughts are that the torque converter could still be the problem. I don't fully understand how torque converters work, so maybe I'm wrong.

At any rate here is my plan moving forward. I'm going to replace the torque converter since it's an unknown, and the cheapest first step. If that doesn't solve the problem, I will pull the engine and have the entire rotating assembly balanced.

I'm looking for input on what converter to buy. Places like Summit and Jegs sell B&M and Boss Hog in the low stall\stock-ish replacement range. There are not many choices for 904s, and even less choices for 904s with a 360. I heard another member here mention Circle D Specialties in Texas makes application specific torque converters, and I did submit a price inquiry with them, but I'm guessing their stuff will be in the $1k price range which is out of the question for me.

I just wanna go 80 mph on the freeway. Gheez!

View attachment 1716453513

View attachment 1716453514
Click to expand...
Have you checked the balancer?
 
Detroit Iron said:
I'm looking for input on what converter to buy. Places like Summit and Jegs sell B&M and Boss Hog in the low stall\stock-ish replacement range. There are not many choices for 904s, and even less choices for 904s with a 360. I heard another member here mention Circle D Specialties in Texas makes application specific torque converters, and I did submit a price inquiry with them, but I'm guessing their stuff will be in the $1k price range which is out of the question for me.
Click to expand...

Circle D is going to be VERY expensive..

Google is your friend

Dodge A904 Transmissions
 
Detroit Iron said:
Yes sir. Had Damper Dudes build the damper that came with the engine. Stock 360 damper.
Click to expand...
And it has been CONFIRMED to be an external balancer? You got a picture? If you think I'm hammerin on it, I am because I'm just trying to help.
 
RustyRatRod said:
And it has been CONFIRMED to be an external balancer? You got a picture? If you think I'm hammerin on it, I am because I'm just trying to help.
Click to expand...
Hey man we come here for help and input, so I appreciate you. Indeed I will respond with a balancer photo as soon as I'm done wasting my time at work I will get one posted. Haha
 
Detroit Iron said:
Thank you. I ordered one on your recommendation from Florida Torque Converter. Looks like good positive feedback about this company online. Boss Hog Converters.... not so much.
Click to expand...

I hope it works out for you; I watched them build out a 904 lockup for my transmission, it was amazing to see how little there really is inside of those...
 
Detroit Iron said:
Further diagnosis; with the car parked, I put it in neutral and race the engine and the vibration is present. So, it wasn't the tailshaft bushing or the drive shaft at all.
Click to expand...
Do you have any estimate what rpm range the vibration is at? My experience is that it will be most noticeable above idle and below 3000 rpm.

Has it been determined that engine is running on all 8 and in a good state of tune? I have worked with engines that have the balance wrong and often times it is mistaken for an engine miss. I suppose the opposite could be true.
 

92b said:
Do you have any estimate what rpm range the vibration is at? My experience is that it will be most noticeable above idle and below 3000 rpm.

Has it been determined that engine is running on all 8 and in a good state of tune? I have worked with engines that have the balance wrong and often times it is mistaken for an engine miss. I suppose the opposite could be true.
Click to expand...
This is very true. I will further diagnose this and post results here. The engine idles nicely and sounds good, but I agree that I need to check for mis-fires etc. I will do that tomorrow Sat 9/13.
 
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