Desert Nomad

Wow, that's great information Lar's. Wish I would have known about this procedure prior to replacing my clutch last year. I had a flat finger clutch originally installed. Replaced it with a LUK original bent finger type part last year. Travel a bit better but not great. I also have an adjustable ball but like Lar's states above, even that won't give you optimum travel if disengagement clearance is not right. All my rods and 'Z' bar is correct for a '67'.

GTOguy

Thanks for the great post, Lars. I've talked with you in the past over on the GTO forum, where you've been a big help over the years. I've had a McCleod clutch in my '65 GTO for the past 31 years and 50,000 miles, and it's been bulletproof. High pedal effort, but a great clutch. It's been my experience with any make car that with aftermarket parts, there are often other problems that crop up due to fitment and design differences. OEM is almost always the most painless route!
Jeff

DansYellow66

I noticed on the last clutch I installed (a McLeod - I have them in two cars) that upon initial install, it seems that the new disc will result in long pedal travel for full release until the clutch disc is worn in, after which it seemed to require less travel to release cleanly. I guess it's kind of the same principal as breaking in new brake pads.

The point being that if you have problems getting enough pedal travel initially - just adjust it for minimum free play at the top of the pedal and after a couple hundred miles it may seat in just fine.

Good thread.

buns

Since my new Centerforce clutch has yet to be installed in my car, I thought I would test it using the procedure that Lars used. It is P/N CFT 361675.
The disc didn't break loose until it was .400" down. Here are the rest of the results.
.400" down-010" clearance
.450" down-.020"
.500" down-.025"
.550" down-.032"
.600" down-.050"

I still had my old Borg and Beck that came off the car so I tested it as well.
.300" down-.030" clearance
.400" down- .035"
.500" down-.050"

Desert Nomad

Lar's, how critical is the thickness of the flywheel when trying to adjust pedal free-play. In other words, is a slightly thinner flywheel due to refacing going to induce problems with travel and proper release of clutch?

JohnZ

I'm not Lars, but if your flywheel has had .050" of material removed during resurfacing, that moves the clutch .050" closer to the engine block, and you'll need another .050" of throwout bearing travel to generate the same disc clearance you started with.

Desert Nomad

Thanks John, it makes sense once you think about it.

hwcoop

I believe it!

I went with Hayes trusting thier brand name and thought it would be a good upgrade for my 454 build. I have had nothing but trouble trying to get some free play for the throwout bearing and so it isn't still grabbing at the floor.... Thanks for the in site, i'm about to pull the trans again(was going to try a different pivot ball) and will probably swap the whole assembly out

JodysTransmissions

Lars.

Great post and information......

We like to use the McLeod 16505 adjustable TOB vs the adjustable pivotball for setup simplicity. The 16505 comes with 3 spacers to dial in your clutch fork geometry and is the largest TOB we found on the market.

Your thoughts?

63Corvette

Great and informative article. Thanks Lars

GearheadJoe

Hi Lars:

Thanks, this is terrific info. I have two questions/comments:

1) Years ago I did some research on C2 Z-bars, and found that there were three different versions used on production C2s. As I recall, one version relocated one of the arms, possibly to help clear big blocks. I seem to recall that at least one of the three versions actually had a different effective ratio when comparing the length of the arms, which would alter the distance that the pressure plate travels. Do you have any info on that possible difference?

2) Aside from the amount of travel required to fully disengage the clutch, another parameter that I wish the manufacturers would publish is the actual amount of pressure required (on the clutch fingers) to achieve that. This would help with predicting the pedal effort. Do you know of any such parameter that the manufacturers actually make available? Is there a comparable, known value for the stock Corvette clutch?

JodysTransmissions

I cannot answer the static pressure for the Corvette clutches, but we are a McLeod Racing dealer and their clutches are rated by static pressure.

I'll choose McLeod's 26 spline 11" clutch series as an example.

McLeod uses the same 2400lb static pressure plate with their Street Extreme 75321 600HP, Super Street Pro 75221 500HP, and Street Pro 75121 400HP. McLeod just changes the disc material to up the HP capacity and rating.

McLeod uses the same 1800lb static pressure plate with their RST Street Twin 6911-07 800HP and the RXT Street Twin 6923-07 1000HP. Same thing, McLeod just changes the disc material to up the HP capacity and rating.

buns

From the 1966 Chassis Overhaul Manual:

327 10" disc 2100-2300#
427 10.5" disc 2300-2600#

MOXIE62

Lars, since you seem to know your way around clutches, can you recommend a soft clutch for a 300/350 1963 car. My left ankle is about to play out and I would like to install a clutch that takes as less pressure as possible and still function. I think it has an OEM in it now but not sure.

GearheadJoe

Thanks, this is helpful, but I'd like to be sure I understand what "static pressure" refers to and whether this number fully addresses how the pedal effort will compare to other clutches.

I suspect that "static pressure" is the pressure that the springs apply to the pressure plate when the clutch is fully engaged. If so, that would not necessarily predict the pressure required on the clutch fingers to fully release the clutch. That pressure (the pressure required on the fingers to release the clutch) is what directly translates to pedal effort.

If the effective lever ratio of the clutch fingers is different between two clutches that have the same "static pressure", the pressure required on the fingers will be different. Another side effect is that travel distance of the throwout bearing will be different, such as Lars has found.

I suppose that if two clutches have the same static pressure *and* the same .580" travel distance to open, then the pedal effort would the the same.

My main concern is trying to be able to look at the clutch specifications and predict, in advance, how the pedal effort will compare to a stock Corvette clutch. Buns has posted that the stock Corvette 10" clutch had a static pressure of 2100-2300 pounds. Presumably the finger travel to fully release it was about .580" as reported by Lars.

It would seem to me that if the McLeod pressure plate with a static pressure of 2400 pounds also requires only .580" to fully release the clutch, then the pedal effort of the McLeod clutch would be only about 4% to 14% higher than the stock clutch, based on a direct comparison of the static pressures.

I can't recall what clutch Kiesler sold me with my TKO 600 kit, but the pedal effort of that clutch is WAY higher than a stock clutch. I'd like to replace that clutch with something closer to the stock pedal effort, but I want to know in advance what to expect in terms of pedal effort, rather than waiting until the clutch is installed to find out.

croaker

Yeah what a let down I did the same. Haven't swap it out gotta get some mileage out of it. Mine is adjusted to the end of the rod. I used a Hays years ago was great really grabbed

Desert Nomad

I am in the process of changing out my clutch for the same reason, inadequate travel. Following Lar's procedure, I checked the three clutches that I've installed over the past six years:

1st original which came with car, manuf. unknown but I think maybe Centerforce: Obtained .050" @ .650 distance, which is unacceptable.

2nd which was a Sach clutch: .050" @ .680 distance, unacceptable.

3rd which was a Luk clutch: .050" @ .680 distance, unacceptable.

After Lar's demo, I contacted McLeod Manf. In fact, I went to their factory which is only about 25 miles from here and talked to them personally with Lar's procedure in hand. They assured me that their entry level clutch p/p assy would fully release at .360". So I purchased their 11" entry level clutch assy. took it home and performed the same test as Lar's. By god it certainly did release as advertised. I just got my flywheel and p/p assy back yesterday from balancing. I'll be installing here in the next day or two or longer as I ain't the fastest mech when it comes to wrenching on the Vette, but will keep you posted on the outcome. Based on my test results, I got high hopes that its gotta work. Cars a '67' 427/390. I am using an adjustable ball stud also.

Desert Nomad,

DansYellow66

I have a 11 inch McLeod clutch in mine and it works well. I do have to carefully adjust it for minimal free play at top so that it doesn't start to engage until a bit off the floor. But the short pedal travel distance in these cars is a real issue. The rubber bumper can be trimmed slightly to allow a bit more travel but then the pedal rides higher which is a problem. Clutch selection pretty critical - let us know how it works out.

EMD645F3B

Great Info. I went through the very same thing 2 years ago on my 67BB.
Sacs #5552 was completely unusable. Finally was able to get a LUK 049 to work with removing the floor matt, trimming the rubber bumper and setting the free play so small it makes me nervous. Even with that, clutch engages right off of the floor. I performed the same tests on my press and came up with .563" to just to release the disc and turn with slight drag, no free space. If your set up does in fact work, can you provide part numbers for the entry level pressure plate and disc

Thanks

C4toC2

Really wish I had read this set of posts prior to buying my clutch cover. The Hayes clutch I purchased was out of spec for the travel on my C2 as well. I will likely go McLeod to fix it. In the interim I welded up some bar stock that allowed me to lower the pivot point on the clutch control bar by an inch with out drilling a second hole in the bar. This did require flipping the clutch peddle push rod over 180 degrees so it would pass easily through the fire wall.

Anyhow - a really nice post - thanks.