early GM (60s, 70s, 80s) Flywheel and Clutch Info:
The following information is based on GM production combinations, and is by no means complete and thorough. You’ll need to confirm what you actually have in your car, as things change over the years,
especially with aftermarket parts, block machining, etc.
2 Flywheel sizes:
- 153 tooth, 12.75” diameter. Used on 350ci and smaller engines.
- 168 tooth, 14” diameter. Used on SBC and BBC, 400ci and larger
2 Flywheel weights:
- Internal Balance; Engine is balanced internally, and the flywheel neutrally balanced. Used on SBC 350ci and less, 427 BBC and less
- External Balance; Engine is balanced externally, and the flywheel has a counter balance weight. Used on SBC 400ci, and BBC 454 and up.
- It is possible and common to change an externally balance engine to be an internally balanced engine.
The GM engine blocks are machined to accept 2 starter bolt patterns:
- Straight across pattern; always for SBC, and always for the 153 tooth flywheel
- Diagonal pattern; always for SBC 400ci, and all BBC blocks, and for the 168 tooth flywheel.
- Both patterns; 350ci and less SBC often have both patterns, and can accomodate either flywheel size.
There are 3 clutch sizes:
- 10.4”; the most common (aka the “10 1/2” clutch)
- 12”; for truck applications
The 10.4” and 11” clutch can be used on either flywheel, and your choice depends on the clutch pressure plate bolt pattern. Generally, the 11” clutch will only fit on a 168 tooth
flywheel, however, aftermarket companies do sell a pressure plate for the 153 tooth flywheel and and 11” disc.
The 153 tooth flywheels generally have a 10.4” clutch.
The 168 tooth flywheel can be ordered to accept either a 10.4” or 11” clutch.
Similarly, you can use a 12” disc with an aftermarket pressure plate on a 168 tooth flywheel. McLeod sells such a package.
2 flywheel types based on rear main seal:
The early GM blocks had a 2 piece rear main seal, and the later models had a 1 piece rear main seal. They flywheels are different for each, and more info is coming on this topic.
Which flywheel diameter is right for me?
Well, that’s mostly dictated by which block you have, and which starter bolt pattern(s) you have. However, it is possible to have the block machined and tapped for the opposite bolt pattern, and/or to purchase
an aftermarket mini-starter that will allow either flywheel.
Which clutch size is right for me?
Boy, there’s lots of opinions on this question. Here’s mine. Unless you’re running really high HP/Torque, and just want every last ounce of gripping power, get the
10.4” clutch. Why? The 10.4” is plenty of clutch for most applications, especially street car applications. Your radial tires will spin long before a 10.4” clutch slips.
Also, the 10.4” clutch has less rotating mass, which will result in a better shifting transmission (because the addtional centrifical force of the larger disc will keep the input shaft spinning, and the input
shaft must slow down in order for a smooth shift). Further, bear in mind that clutch technology has improved dramatically over the years. Even 30 years ago, the 10.4” clutches were used behind the
Hemi engines. If Mopar felt that a 10.4” clutch was good enough for them back then, it stands to reason that today’s much improved clutches will work even better.
If you’re running slicks, and a high HP engine, then by all means, the additional gripping power is a good thing.
If you don’t agree, that’s ok. It’s just my opinion, I’m not a clutch engineer/designer. :-)
I always recommend that you ask around, ask the
experts, and take what you feel are the best recommendations that suit your needs. It’s always good to do your homework, and get the varied opinions so that you make knowledgeable decisions.