Thread: Clutch size question

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Well the weather seems to finally have broken and I plan to start assembling the parts I have collected over the winter. I have one naggin dlillemma. I picked up a bellhousing that the owner claims is good for a 10.5" or 11" clutch. My 454 wrecker motor came with a 12" disc. I plan on using a 15530202 bell from an 85 pickup truck. What are my options?

1) Can I use my 12" clutch set up in my bell? Simply picking up a fresh clutch / press-plate and have my flywheel turned...

2) Go with a 11" clutch / press-plate set up...and possibly need to purchase a smaller flywheel to accomodate the smaller press-plate.

I do not know if an 11" pressure plate will bolt onto a flywheel from a 12" setup. I do not have any old parts around for mock up, and my motor is currently bolted onto a stand which also makes it impossible to mock up.

Anyone ever been in this scenario? Any input would greatly be appreciated. Or does anyone have an old 10.5" or 11" pressure plate laying around that they could measure the bolt holes for me?
thx Joe

Originally Posted by DennyW

454 with moderate power, I would go with the 12" clutch setup.

I agree, do you know if a 12" pressure plate is that much bigger than an 10.5 or 11"...? I am not sure it will fit inside my bell, and since my engine is on a stand and I my hoist is lent out, I can't mock it up...

I guess I will just have to see if my 12" setup will squeeze inside my new bellhousing. wish me luck.

Originally Posted by Gusaroo

I agree, do you know if a 12" pressure plate is that much bigger than an 10.5 or 11"...? I am not sure it will fit inside my bell, and since my engine is on a stand and I my hoist is lent out, I can't mock it up...

Thanks for the reminder on why I don't loan out tools or shop equipment!!!!!

i like my engine feet, they work great for doing mock ups.... thats what always annoys me with stands, you always end up going oh crud, i wanted to check something with the flywheel but i have to do it hanging on a stupid chain.... time to put on the steel toes

Heres another question:
Are sbc pressure plates and BBC pressure plates interchangeable?

Or in other words, does the SBC flywheel and the BBC flywheel have the similar bolt patterns for mounting the pressure plate?




BTW, my 12 inch clutch setup DID fit inside my new bellhousing, even though I was told it was for A 10.5 clutch set up yehaa!!

Gus....try to stay with me on this one...I am about to start another fire storm in this thread....... There are many flywheels (salvage yards if you wish) that accomodate multiple pressure plate sizes. 12" is good. I run 11" because I look for total contact area. If the 12" is only 12 on the outside diameter and yet 11 in the inside, you have no more surface area than a little 9.5" or a decent 10". Think about how a Lenco setup uses 6-6.5 inch contact area. We need surface area way more than diameter sizes to stick the torque to the ground. Can you apply a 10 or 11 or 12 inch disc in the bell? yes the fly wheel doesn't grow, just mating area (machined surface) and the right ones will allow you to put in what you wish. Remember, Surface (Contact) area is what you want. Too many suppliers cut corners to save a nickel and you have a huge disc and no "meat" for application.

One thing I have not seen mentioned yet is that the 454 is an externally balanced engine. What this means IF you end up swapping flywheels is that you will have to have one that is correctly balanced for a 454.

It's been along time since I did a 12" to 11" clutch swap, but it seems like the Flywheel would accept both. It seems like the flywheel needed to be changed when going from a 10.4 to 11" or 12"

Okay, I tried to reply earlier and lost power at the computer....what a master piece too. Now I will condense to answer Denny's question. So many flywheels have been offerd for the mulitiple bolt hole applications for varied size clutches. The flywheel does not grow; only the surface area and offerings of bolt patterns to allow your choice in clutch. My original post in this thread was to enlighten the fact to those who wish to read, was the fact that a 12" clutch can suffer mating and apply surfaces because of the manufacturer. I can run an 11" clutch with more grip than some 12 inchers. Solely because the outer diameter may be 12 but the inner diameter is a mere 10.5. Where I can use a 9" I.D. to apply more frictional surface maintanig the O.D. of the 12 I was trying to achieve. Sometimes a smaller clutch can have more total area than a large one. My point was merely to explain that a 12 is not always a 12 because it will be worthless if the builder does not see that the unit is as weak as a 9, a 10, an 11, etc. Look and measure. Bigger is not always better. If you can score a good unit with large frictional area, run the darn thing and don't look back.

Previous post was done under duress caused by being exhausted. I know it is not as clear as I had hoped it would be.

With the new clutch technology with the Soft-Lok's and two sets of springs with different rates and some with two sets of fingers, clutch size is no longer the determining factor... As Nitro said, the contact area is what counts, not the od of the pressure plate. I use a 10.5" McCleod diaphragm that hooks harder and lasts longer then any 11.5 or 12 inch Borg & Beck style 3 armed overweight clutch/flywheel setup that ever came out of the factory.... When I step up the power (over 500hp), it'll probably go down to a 6.5" double disc.....

For the 454, having the external balance flywheel is going to be the main concern. If the flywheel isn't drilled for a 10.5 or 11 inch clutch, it's a fairly simple operation for a machine shop.....

I'll jump in with a little math to add to the good warrior's explanation.

If you have a clutch with a 12" friction area O.D. and a 10.5" I.D. (the friction material being 1 1/2" wide), then you multiply .7854 times 12 times 12 and find 113 square inches. Now multiply .7854 times 10.5 times 10.5 and find 86 square inches. Now, subtract 86 from 113 and find the actual lining area is 27 square inches.

If you have a clutch with a 11" friction area O.D. and a 9" I.D., (the friction material being 2" wide), then you multiply .7854 times 11 times 11 and find 95 square inches. Now multiply .7854 times 9 times 9 and find 63 square inches. Now, subtract 63 from 95 and find the actual lining area is 32 square inches.

Perhaps, but should we talk about the paddle clutches and the lack of friction area??? The material used on the clutch and the clamping pressure of the pressure plate also comes into the equation.........

Well I am aware of the balanced flywheel, and my 454 is out of a truck, so I am hoping to stick with the 12" setup I had (mostly just to save me the expense of purchasing a new flywheel). As I started shopping thru the catalogs and online for a 12" clutch and Press Plate, I am noticing that most don't specify whether it is BB or SB. Which leads me to think they are interchangeable, hence my question.


As far as the diameter of the clutch vs actual surface area grabbing, follow me on this thought. If the clutch surface area is futher from the center of the flywheel, less force or surface is needed, correct?

Guess I don't follow that. Only thing I see the larger size doing is adding to the weight of the clutch and flywheel and increasing the weight of the rotating mass..... Heavier weight would tend to hold the revs higher on shifts, a lighter mass on the end of the crank will allow the engine to accelrate and decelerate faster. Look how much quicker a car with an aluminum flywheel and light clutch will RPM.... I don't see where the clamping force would be effected by centrifugal force as it is different forces moving in different directions....