Hot Rod Forum banner
1 - 9 of 9 Posts

·
Banned
Joined
·
159 Posts
Discussion Starter · #1 ·
I have a 87 350,building a 377 stroker and I am confused on the internal external balance cranks.It seams there is always 2 sides to the story every where I read looking for a straight answer to guide me.From what I read on my net. searches the only externally balanced sbc is the 400..... The book I bought thats a interchange manual for sbc's says all 350's are internally balanced till 86 when they introduced the one piece RMS where they made them externally balanced,which requires the use of matching 1986 or later flywheels/flexplates.....the stroker crank i bought is a one piece RMS externally balanced, when I got the motor it didnt have the flexplate/flywheel but did have the harmonic balancer which I noticed it had balancing holes drilled on the inside of it.......and when I pulled the crank out of the motor it also had balancing holes drilled in its counterweights,but the stroker crank that I got had no holes drilled in it......in short,if I use the new stroker crank allong with the old pushrods,harmonic balancer and a flexplate(automatic) from a 86-earlier 1 piece RMS 350 will it all balance out? Just doesnt sound like it would to me.....The 350 I am using is a stock 87 and was almost complete except for the missing flexplate........
thanks.....
GM :thumbup:
 

·
WFO
Joined
·
5,030 Posts
Exactly what are you proposing to build?

Most of the time when someone says "377", it's based on a 350 crank in a 400 block (destroked). This will be internally balanced.

Or a 350 block w/a 3.75" (size of a 400 SBC) crank. Usually will be externally balanced, but can be internally balanced, as well.
 

·
Registered
Joined
·
973 Posts
in short,if I use the new stroker crank allong with the old pushrods,harmonic balancer and a flexplate(automatic) from a 86-earlier 1 piece RMS 350 will it all balance out?
Ayuh,...
The 1986,+ Earlier motors are 2 piece rear main seal crankshafts with a 3, 1/4" bolt pattern....
The 1987,+ Later motors are 1 piece rear main seal crankshafts with a 3" bolt pattern...
The Flywheels/ Flexplates are Not interchangable...

You'd better do somemore Learnin',... Your questions aren't making a whole lota Sense....
As noted, your question asks about a 377,+ your discription is about a 383,....

Btw,....Pushrods have Nothing to do with Any of this....
 

·
Banned
Joined
·
159 Posts
Discussion Starter · #5 · (Edited)
bondo said:
Ayuh,...
You'd better do somemore Learnin',... Your questions aren't making a whole lota Sense....
As noted, your question asks about a 377,+ your discription is about a 383,....

Btw,....Pushrods have Nothing to do with Any of this....
Ayuh.....thanks for pointing out my short comings......feel better about yourself?
Yes what I meant to say was connecting rods,not pushrods.....my bad,but I am building a 377(350 block,stock 5.7"connecting rods",3.75 crank and 4.00 bore)now if it were bored .030 over yes that would make it a 383 but I am not so theres ya some learnen's Bondo......

bondo said:
The 1986,+ Earlier motors are 2 piece rear main seal crankshafts with a 3, 1/4" bolt pattern....
The 1987,+ Later motors are 1 piece rear main seal crankshafts with a 3" bolt pattern...
In the realy real world that I live in its
The "85" and earlier(before) that are 2 piece
and heres the real kicker
The "86" and later(after) are the ones with the onepiece
maybe you should do some "googling" before you post
GM :thumbup:
 

·
WFO
Joined
·
5,030 Posts
greenmoonshine said:
Ayuh.....thanks for pointing out my short comings......feel better about yourself?
Yes what I meant to say was connecting rods,not pushrods.....my bad,but I am building a 377(350 block,stock 5.7"connecting rods",3.75 crank and 4.00 bore)now if it were bored .030 over yes that would make it a 383 but I am not so theres ya some learnen's Bondo......


In the realy real world that I live in its
The "85" and earlier(before) that are 2 piece
and heres the real kicker
The "86" and later(after) are the ones with the onepiece
maybe you should do some "googling" before you post
GM :thumbup:
So, if you bought an external balance, 1-piece rear main seal crankshaft w/a 3.75" stroke, this will require a "400" damper and flexplate- ones that are counterweighted for a 3.75" stroke crank.

IIRC, this is not the same thing as a 1-pc. RMS 4.3L V6/305/350 crank's flexplate (the 4.3L V6305/350 1-pc. RMS flexplate has a different counterweight value). The '86-up damper is neutral balanced, so it will not work w/the 3.75" external balance crank, either.

The balancing holes drilled in the original 350 balancer are just to neutral-balance it, the crank was drilled to internally balance the crank, and is seen on all SBC's, as a matter of routine.

bondo said:
The 1986,+ Earlier motors are 2 piece rear main seal crankshafts with a 3, 1/4" bolt pattern
The '85 and earlier crank flange bolt pattern is 3.58" diameter.
 

·
Banned
Joined
·
159 Posts
Discussion Starter · #8 · (Edited)
cobalt327 said:
the crank was drilled to internally balance the crank, and is seen on all SBC's, as a matter of routine.
Then why is the 86+ listed as externally balanced if the counterweights are drill from factory to make it internally balanced?This is the part that confused me...
Now, what you said about the longer stroke throwing off the factory balancer and flexplate makes perfect sence and I never even factored that into my equation.....can the original balancer off the 350(neutral balanced) be drilled and fitted with weights to match the 3.75 stroke for balancing? or will I have to just buy one for a 400 like you stated? Would I have been better off just buying one that was internally balanced?
Just found this in case others need this info http://www.hotrodders.com/forum/internal-vs-external-balance-32238.html
Thanks
GM
 

·
WFO
Joined
·
5,030 Posts
greenmoonshine said:
Then why is the 86+ listed as externally balanced if the counterweights are drill from factory to make it internally balanced?This is the part that confused me...
The counterbalance added to the 305-type, 1-pc. RMS flexplate is only there to account for the (now) missing counterweight that was present on the 2-pc. RMS cranks. If you look below, you'll see the difference- and why the counterweight is needed.

This doesn't mean that the crank isn't drilled to attain balance- it is still drilled to balance it just like any crank- internal or externally balanced, is.

can the original balancer off the 350(neutral balanced) be drilled and fitted with weights to match the 3.75 stroke for balancing?
Not an OEM damper. There are aftermarket neutral balance dampers that can have counterbalance weight added to make them compatable w/an externally balanced engine.

AFA whether an internally or externally balanced set-up is better, internally balanced engines can be more durable in high RPM apps, but cost more to balance, if Mallory or "heavy" metal is needed. IB can make parts easier to obtain- sometimes a consideration when flywheels are involved.

For most apps, external balance is fine.
 

Attachments

1 - 9 of 9 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top