Moparts Tech Archive

Manual & Clutches

Clutch tech info, what I've learned so far

E body, Small Block Clutch Fork
10.75" overall
2:1 ratio
- 3.25" from fulcrum to T/O bearing contact point
- 6.5" from fulcrum to center of pushrod hole

Borg & Beck Pressure Plate
Approx 6:1 ratio on the fingers

Clutch Disc
Typical street/stip - .340" uncompressed, .310-315" compressed

PP/Fork Travel

JUST tested new PP and IT WORKS PERFECTLY! I took my old PP apart and could see no obvious failures?

Measurements I just made....

Takes .650" of travel to compress the fingers enuff to "just" release the clutch disc (unload the marcel AND obviously overcome some of the PP spring preload).

Takes another .510" to open up a .050" gap (thats obviously like a 10:1 PP finger ratio which looks about right - since I opened my old PP I will try to get a rough measurement to confirm this ratio - so with a 3100lb clutch, thats 310lbs at the T/O and 155lbs at the clutch fork pushrod side) now I'll have to measure all the other ratios)

Thats what, 1.160" of PP finger tip (at the T/O bearing end) travel to disnegage the clutch. PLUS anuther .250" recommended T/O bearing to finger clearance. 1.410" total. WOW. Way off my guesstimated above.

THEN with a 2:1 fork ratio that 2.8" of fork tip travel! GREAT INFO TO HAVE WHEN UNDER THE CAR TRYING TO SEE IF YOU ARE CLOSE.

I also bored a 2-1/4" hole in the very bottom for the bllhousing so I can check the clutch gap when everything is bolted together. McLeod RECOMMENDS drilling the hole. Wonder why they don't put a home in thier safety bellhousings?

OH and with a .050" gap at the clutch disc the fingers are a good .250" away fromn the clutch disc tophat springs! The old PP had to have some bent fingers or something.

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OK so from my previous post I see that I need 1.16" of travel at the fork tips. I measured the ratio of the fork tip to pivot is (3.25") and from pivot to location on fork where it clears the bellhouwing opening (2.75) = 1.18:1

SO

If I need 1.16 at the fork tip divided by 1.18 then I need .98" of travel at the spot wher the fork just clears the bellhousing. With ONE .125" spacer under the pivot I have .75" of clearence with the fork engaged in the T/O and the T/O resting on the PP fingers. NOT ENOUGH.

Gonna go try 2 hardened washer (like I had when I was just taking a shot in the dark during my original ass'y). I need at LEAST another .25" of clearence where the fork clears the bellhousing. Kinda doubt (2) .125" hardened washer will do it

OK (2) hardened washer under the pivot give me .97" of clearence - .22" more by adding (1) additional .125" hardened washer under the pivot.25" total. Thats still not enuff cause the rubber boot still has to fit between the fork and that opening. The boot is pretty thick around the opening; maybe .187"

SO if adding the second hardened washer gave me an additional .22" of clearence. A third, .375" total should give me the RCH that I need minimum PLUS another .200" to clear the rubber fork boot.

ONE LAST THING. blackcrowe, in another post mentioned his fork pushrod being on a heavy angle instead of coming straingt into the fork. I checked my Z bar by holding in place - SAME PROBLEM. Will require a pretty heavy angle to actuate the fork - maybe 15+ degrees THEN I NOTICED the tab on the Z bar (an old used one I cleaned up and powdercoated) that pushes the fork pushrod IS BENT over about 1/4 to maybe 1/2" AWAY from the fork!!!! CAN YOU SEE THAT!

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This is from McLepd's website.... "When using a scattersheild and motor plate, it is recommended that you make use of the adjustable ball stud or intermediate T.O. Bearing. To ensure proper adjustment for release on most high performance or drag cars, a hole should be drilled in the bellhousing under the clutch assembly large enough to permit entry of a feeler gauge."

THEY make "scattershields" too. You'd think they would put a hole in them.

Stopped by Mancini's today. FORGOT MY CAMERA. Art graciously cut open both the SB & BB clutch fork/pivot boxes. Told him I was doing research for Moparts.com

- FORKS ARE IDENTICAL
- BB pivot bracket is like 1/16" taller. Mounting and L shape is identical to SB.

I doubt ANY problems would be created if you installed the wrong fork - least anythig that couldn't be adusted out. According to my notes above, a .06" diff at the pivot would be approx a .11" dif at the opening (more clearance) and a .22" diff at the pushrod hole (moves the end of the fork closer to frt of moder).

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