Will my 12" clutch fit on the stock flywheel?

[eborges19]

Hi everyone,

First of all, sorry if this has been answered elsewhere, I couldn't find a thread that really answered my question. I bought a clutch for my truck (350sbc with a SM465 behind it) and I am trying to figure out if I need a larger flywheel or not. The block is a 3970010, meaning it was made between 1969-1979 which was before GM starting manufacturing 1-piece rear mains.

From there I can deduce that I have a flywheel for a 2-piece rear main (168 tooth), but I don't know If the stock flywheel is a large enough diameter for a 12" DIA clutch. I've done some research that suggests I would need a 14" flywheel, but would I run into issues with the bell housing or starter? Any info you guys can give me would be super helpful, as this will be my first time changing out the clutch.

This is the clutch kit I bought:
SACHS K190902HD

If I need to buy a flywheel, I was thinking about purchasing this one off of Jegs:
https://www.jegs.com/i/JEGS/555/601205/10002/-1


Thanks!

 

[jjester6000]

The problem would,t be the diameter, but the bolt pattern for the pressure plate. 168 is the largest they come.

I remember when I upgraded from a 10 in clutch to an 11 in my truck, and I ran into a problem installing the pressure plate on my old flywheel (I did actually have a correct flywheel, but it was for a 1pc rear main seal engine). I ended up running out and picking up a new one that supported both bolt patterns.

168 is the largest they come, so you shouldn't have any clearance issues, just make sure that your flywheel matches up with the pressure plate.

If it is original, it probably only has 1 bolt pattern drilled into it unlike most aftermarkets.

 

[AuroraGirl]

The problem would,t be the diameter, but the bolt pattern for the pressure plate. 168 is the largest they come.

I remember when I upgraded from a 10 in clutch to an 11 in my truck, and I ran into a problem installing the pressure plate on my old flywheel (I did actually have a correct flywheel, but it was for a 1pc rear main seal engine). I ended up running out and picking up a new one that supported both bolt patterns.

168 is the largest they come, so you shouldn't have any clearance issues, just make sure that your flywheel matches up with the pressure plate.

If it is original, it probably only has 1 bolt pattern drilled into it unlike most aftermarkets.

You seem knowledgeable about clutches and stuff.
I have old... uh.. assemblies on shelves. How to tell if they are good. The flexplate is what I assume has the metal blades that are on the inner part? The flywheel is usually on the engine i do know that. I seem to have two or 3 of these assemblies i speak of.

 

[eborges19]

The problem would,t be the diameter, but the bolt pattern for the pressure plate. 168 is the largest they come.

I remember when I upgraded from a 10 in clutch to an 11 in my truck, and I ran into a problem installing the pressure plate on my old flywheel (I did actually have a correct flywheel, but it was for a 1pc rear main seal engine). I ended up running out and picking up a new one that supported both bolt patterns.

168 is the largest they come, so you shouldn't have any clearance issues, just make sure that your flywheel matches up with the pressure plate.

If it is original, it probably only has 1 bolt pattern drilled into it unlike most aftermarkets.

Thanks for your reply, that helps a lot. I found this flywheel that I think will work, and its made by the same manufacturer as the clutch kit I purchased.

Think this'll work?
SACHS NFW1007

 

[pduffyd]

hi Eborges19

I put a 12" clutch on my sm465 behind my 454.
Ive got the 168 tooth flywheel, I know its different on a BB to an SB, but I had to change my bellhouse to a 460486 to accomodate the 12" clutch. I used to have a 292 L6, in the truck ( long gone), which had the stock 11" clutch in it.
Here is a photo of the new bellhoise notice the big bulge on the lower bellhouse for the 12" pressure plate.

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I used to have the cast iron one but could not find the tin lower cover anywhere so I had to do this. So I dont know if the stock cast iron bellhouse is ok with the 12" clutch. Just something else to check. Good luck.

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[fast68chevy]

i have tin covers for those iron housings... and bolt pattern on flywheel is what matters. there are many diff flywheels/clutch diameters they dont interchange. because only one bolt pattern in them, for a given clutch diameter... for example on the big grain trucks, the clutch is so big on them the bellhousings are too big for the sbc motors the factory actually made a special steel adapter ring that bolts to the motor bellhousing so the extra large bellhousing will bolt to the motor. bottom line is you need specific flywheel for specific clutch diameter size. and the truck iron housings were all made to accomodate 14" flywheel. so yes they will take 12" clutch.
but this only applies to light duty 1 ton and under.. not to 1.5 ton and up.

 

[pduffyd]

whow, didnt know that! That would be a BIG clutch.
Yeah I couldnt find the part number for the tin, but I came across this bell for $100, so I had to get it.
It would be nice to have the cast iron back on though.

This is my set up, the measurements maybe useful to you.

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[shiftpro]

You seem knowledgeable about clutches and stuff.
I have old... uh.. assemblies on shelves. How to tell if they are good. The flexplate is what I assume has the metal blades that are on the inner part? The flywheel is usually on the engine i do know that. I seem to have two or 3 of these assemblies i speak of.

I hope someone will chime in with a better answer... here's what I can offer...

Those fingers or 'blades' as you described are the spring that forces the pressure plate into the flywheel. That's the pat that gets worn out. When worn out you can see the fingers sitting at different angles in respect to each other. Someone that knows for sure can take a 2 second look and determine if it's good or done. There must be more to it than that, but this is all I have.