Hot Rods Question on Flexplates

Knowledge of flexplates is limited - Car model A coupe 1970's Small block Chev 350 CI non performance with 350 t/h Transmission.
I'm looking in a Summit Racing catalog and this is what I read ??

Chev V-8 1955-1985 ( Internal Balance 153 tooth)

Chev V-8 1955-1985( Internal Balance 168 tooth)

Chev V-8 1986- 1994 External Balance 168 tooth

Need explanation of what internal and external means in regard to flex plates and what number of teeth to go with which would fit my application .

Also have read the reviews on people that have purchase their flexplates and some complain that the holes on the flexplates are not drilled properly and are oval shaped where the torque convertor attaches ??

Gene

 

well the external balance is out because of the years and it uses a special bolt pattern on the hub and hub hole diameter is smaller ( 3") so it will not interchange ,
so you have to know the starter that was used to decide which flywheel was used ,
if the bolt holes are inline its 153 ,
if its staggered its 168 .
if all three holes are drilled and tapped then its either /or

. the 168 is usually used in trucks and also they use a larger dampner on the front and the starter should use a brace at the rear and its recommended for high rpms its made for torque ( flywheel effect )

. most of the light duty motors used the 153 flywheel

 

And the complaint on the oval holes is from people that know not what they are doing.. Almost all SBC replacement flexplates have an oval hole in the torque converter pattern. Look at a factory unit. One hole in the pattern is oval. This allows for factory tolerances in the pattern. After mkt flexplates have two bolt patterns. Thus two oval holes..

 

and if you're really freaked out by the oval holes, three good flat washers tig tacked to the flex plate solve the problem.

 

your not supposed to use the bolts as driving stud, but clamp the converter tight to the flexplate with a wide head bolt or a bolt with a built in flange . (http://static.summitracing.com/global/images/prod/xlarge/mor-38765_w_xl.jpg) , and never use flat or lock washers either they can throw the balance off ( spring washers specially if one breaks ) blue locking compound is prefered , a couple of .oz off far out on the fly wheel can cause problems ( think what a couple of oz does to a car wheel and its the same thing . )

 

when I did this for a 348/700R4 and used the washers, they were within .002 of center, .001 thickness, and when run on the balancer didn't affect anything. It wasn't easy, but it can be done following good engineering practices.