I'm looking to get a TCI flexplate since I hear they are pretty straight and good quality, unlike my wobbly pioneer one. However, on summit and elsewhere there are two different part numbers for my engine; 399173 which they list as "external" balance and 399174 which they list as internal balance. Now from various threads I've read that all the 87-up SBC engines were internal balance with the 400 being the only exception. I think that summit might be listing them incorrectly though and actually mean that the external balanced one is the one with the weight, which is what I need. Does anyone have some clarification on which one I'm supposed to use? Maybe someone who has bought one of the two can chime in?

Link to summit search results:
http://www.summitracing.com/search/p...49306%2B400252

 

What year engine? If it's the '91
You need a flexplate for 1 pc. rear main seal, Internal Balance

 

Only old-style 2-pc RMS SBC's are TRULY internally balanced.
Technically, Summit is correct in their description of the 173 part number. Despite the automotive community's common referral to all non-400 SBC's as internally balanced, the fact that the 1-pc RMS requires a weight on the flexplate/flywheel makes it just as EXTERNALLY balanced as the 400 SBC's.
Their 174 part number would be for an aftermarket SBC rotating assembly that is in fact internally balanced with no need for a weight on the flexplate. Don't go by the pic in that add; it shows a weight on the flexplate.

 

Should be the same style engine as the original (it's a rebuilt). OK, I suppose I'll go with the 399173 since I've also heard of people using that one on our stock engines. Thanks for the clarification!