1973 340 Balance question

[ejazz]

Hello everyone, looking for some technical help!

My buddy and I are putting together a pretty much stock 1973 340 small block and I am not sure on the whole balancing issue. There is so much info between the 340/360s when it comes to internal vs external and so on. Here is what I have:

Block # 2780930-340-S
Crank # 3462387
Balancer # 3751160
Rods # 2899495 (6) & #3751015 (2) New bushings installed

I realize two of the rods are from a pressed pin 360 but at some time they were drilled and bushed for a floating pin. The crank was also cleaned up (.010).

My question is this: What flex plate and what torque converter do I need? I am running an 904 behind it. Do I need a weighted and balanced flex plate, weighted converter, do I knock weights off, etc! I did not get a flex plate with the engine and I will be buying a rebuild kit for the 904 with a new convertor. What do I order? Thanks!

 

[Adam]

If it is an external balanced engine you have two options. You can use a standard flexplate with a weighted converter; or unweighted converter and balanced flex plate.. Like this one:B&M Flexplates 10235

You do not need both. Ideally, everything should have been balanced as it was being built.

 

[Steve340]

I think I read somewhere the cast or later 340 cranks were externally balanced. Therefore the flex plate and crank damper will be counterweighted.
A torque converter should be neutral balance so you can forget about that. It is not a factor for engine balance.
If it was it would need to phased to the crank and would only be able to go on on one place like a crank damper.
However a badly balanced torque converter can cause problems of its own.
The rods and pistons can be a fair few grams different and you will not notice a shake or vibration.
The good news is if you get the counterweight or non counterweight issue wrong you will know real quick.
The engine will vibrate its ass off.
Adam was right you would have been better to have got all the bits together and made sure they were compatible before assembly.

 

[Adam]

A 360 uses a balance weight on the converter. Not really a problem because one hole on the flex plate is slightly offset and they will only bolt up one way.

 

[Steve340]

Ok something I did not know. Yuck no wonder there is so much confusion.
He may be better off going for the counterweighted flex plate and the neutral torque converter?
The flex plate would be easier to alter if he fires the motor and it vibrates.

 

[Adam]

Yes, and then he can use any converter he likes.

 

[ejazz]

Thanks guys. I got a hold of the fellas at TCI Transmissions and they set me straight. I need a weighted flex plate and a neutral torque convertor. You guys were right on! Thanks again!

 

[moparleo]

Remember even though you can use the same parts from different engines, even if they were both the same ,engines are balanced individually not as a bulk production part. Take a 360 rod and piston from one engine and another rod and piston from another engine and they will probably not weigh the same. The best thing to do is to always do it right the first time with cost secondary. If you build with cost primary, you will spend more at the end.

 

[brasil]

I am wondering...because if the two 360 rods... isn't it better to go with 8 equal rods ?

Greetings Juerge.

 

[valkman]

I am wondering...because if the two 360 rods... isn't it better to go with 8 equal rods ?

Greetings Juerge.

If all the rods are weight matched you shouldn't have a problem.