Street Twin Saga and Tutorial

Drivetrain Intro  Street Twin page 2, Girling slave cylinder  T56 rebuild  T56 to TH400 conversion

Fall '00 update:  At my request, George from McLeod put kevlar pucks on one side of each of the two disks, so it's now a Dual Friction Street Twin.  The takeup is buttery smooth, with absolutely zero chatter, but I don't have enough miles on it to test the absolute holding power.  I asked for the upgrade after spinning the regular version on a run that immediately followed a pro car that did a half-track burnout.  Steeeenky!  The 2 dual friction disks should be good for at least 900hp, which is not in my plans.  Yet.

If my alpha-tester story doesn't interest you, scroll to the bottom of this page for a series of pix that show how the Street Twin goes together.

I ordered my McLeod Street Twin 7/24/97 from Dennis Smith of Internet Racers Supply.   If you're familiar with LT1 f-bodies with the M6 transmission, you probably know that it uses a pull-type pressure plate patented by Valeo.  As of this writing, March, 2000, it is still the case that all clutches for our cars use a Valeo pressure plate.

McLeod's first stab at the Street Twin, however, came with a "conventional " 3-finger push-type clutch and a hydraulic ram to operate the throwout bearing.  Unlike the stock setup, which uses a slave cylinder located outside the bellhousing to push on a clutch fork, this arrangement used a doughnut-shaped hydraulic ram that fit tightly around the tranny input shaft collar.  Two hydraulic lines passing through the hole vacated by the clutch fork provided pressure and bleeder connections to the ram.

For you hard core clutch techies, the pressure plate was a Borg and Beck hat with Long style fingers and 9 springs for a total spring force of 2800#.  As sexy as this setup was, it had one big drawback.  It didn't fit.  Even after I ground away a bunch of aluminum from the front of the tranny so the ram could be moved another 200 thou away from the flywheel, there just wasn't enough inches between the flywheel and the tranny for all that hardware.  Turns out McLeod had only sold a couple of these things, and had *never* installed one on a 4th-gen, so they offered to exchange mine for their Valeo-based rev 2.0 at no cost.

OK, so now I have serial number 0001 (or thereabouts) Street Twin rev 2.0.  The good news is ... it fits!  The bad news is ... it takes both arms pushing as hard as I can, braced against the seat back, to engage reverse.  Getting first only requires a max effort push from one arm.  Once moving, the higher gears weren't so bad, but this clearly was intolerable.  The clutch was not completely disengaging.  After a couple weeks of exchanging faxes and phone calls, I told Red and George that I'd be paying them a visit at McLeod World Headquarters in Placentia, CA (not far from Disneyland).

To their credit, McLeod had shop foreman George spend most of an 8 hour day trying to get that $#@! clutch to disengage.  Eventually, we settled on replacing the stock 1" diameter clutch slave cylinder with a 3/4" bore cylinder.  This cured the disengagement problem, but caused a new one.  Just as the clutch disengaged,  the clutch fork would hit the spinning pressure plate.  Made quite a racket!  It was getting late on a Friday afternoon, so we kludged up a pedal stop using a bolt with the head ground down.  Nothing is quite as anxiety-provoking as heading out into Friday afternoon LA rush hour traffic with a clutch you *do not trust*.

But the clutch did not misbehave, as long as I didn't push the pedal really hard.   A couple days later it occurred to me that shaving 0.1" or so from the clutch fork pivot would allow it to fully disengage the clutch disk without hitting the pressure plate.  After talking it over with George, I gave it a shot.  It worked.  Finally!  Full disengagement, no fork/pressure plate contact.

Since those first experiments, McLeod has changed the slave cylinder to 7/8", and recommends that the fork pivot be cut 0.050".  But I'm sticking with a 3/4" bore cylinder.  It decreases the pedal travel required for disengagement, which makes shifting at the strip just a bit quicker.

The pic below is Red posing with the car just outside McLeod's shop.

I've noticed that some f-olks confuse the "twin" in Street Twin with the "dual" in Dual Friction.  Centerforce (and probably others) use Dual Friction to describe a single clutch disk that uses two different friction materials to face the two sides of the disk.  One side is generally a very aggressive, high coefficient of friction material, often in the form of pucks, while the other side is a more conventional, organic-based semi-metallic material.  The aggressive pucks improve the clutch's holding power (supposedly), while the conventionally-lined other side allows it to be reasonably streetable (a "streetable" clutch, IMHO, is one that can be slipped to start the car moving without grabbing or chatter).

The Street Twin uses *two disks* separated by a floater plate which is tied to the flywheel.   The sequence of pix below show the stackup of a Street Twin.

There are only three main factors that determine how much power a clutch can transmit without slipping; pressure plate spring force, friction material surface area, and the coefficient of friction of the material used to line the disk.  Since LT1 clutches are stuck with the Valeo pressure plate, which uses a diaphragm spring, there isn't much the designer can do to increase the mediocre spring force.   Centerforce uses small weights hooked to the diaphragm spring fingers that add their centrifugal force (inertia for you physics majors) to the spring force, but given the very shallow angle they are working through, it's hard to believe they have much effect.

Most LT1 clutch makers get their holding power gains through semi-metallic, ceramic, kevlar, or some other aerospace friction material.  Think of it as replacing your stock brake pads with Z-pads from Performance Friction, or a race pad made of an even more exotic material.  Just as the Z-pad improves stopping power, the aggressive clutch disks improve holding power.  But they are still limited by the diaphragm spring force and surface area of a single disk.

This is where the Street Twin comes in.  With two disks, it has twice the surface area of any other LT1 clutch, which gives it twice the holding power with equivalent disks.  Back to the brake analogy, think of replacing your stock 11 inch rotors with 14 inch rotors.  Even with stock pad material, a 14 inch rotor is going to stop you *much* faster than the stocker.  The holding power is so good that McLeod can use a fairly mild friction material, which makes engagement buttery smooth, as opposed to the grabbiness of some of the race disks.  It's been a pain getting here, but now that it works, I wouldn't swap it for any other clutch.

Onward to the Girling Slave Cylinder mod