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Clutch basic 101: A clutch basics thread for everybody

The following statements are my opinions but they are based on 26 years of wrenching on mitsubishis and 21 of those years on DSMs. This applies to 4g63 cars only. I am not commenting on a 420a clutch although some things do apply.

This is not meant to be just DSM specific but could be applied to many cars. That being said a few basic things need to be pointed out.

1. There are not AWD or FWD clutches. A turbo DSM uses the same clutch whether it's FWD or AWD. They do NOT use the same flywheel.

2. An NT flywheel is flat and is not the same as a turbo flywheel nor is the clutch the same. Turbos use a stepped flywheel and a 225mm clutch disk vs the NT 215mm disk.

3. Extended slaves do not work. They are a bandaid. Now before you go and bash me read this. If you install one and it makes a difference the following is why or why not. A clutch hydraulic system must be allowed to self adjust as a clutch wears. When the system is static there is no pressure on anything and the fluid is free to move. As a clutch wears the fingers on the pressure plate will change positions because the thickness of the disk is changing so the position of the fork changes. If a hydraulic system could not self adjust then as the clutch wears the system would become preloaded (similar to riding the clutch). DSMs do not work this way. You'll notice you should be able to push the slave back into it's bore easily unless one of the following exists.
a. The master has been adjusted so far you have effectively pressed the clutch pedal which closes off the valve in the master and you have a nonadjusting system at that point.
b. You have installed an extended slave rod and have bottomed the piston out in its bore so you have effectively limited its travel backward and preloaded the system. It's like you are pressing the clutch slightly before you ever press the pedal. Yes, this could aid in clutch disengagement but at the expense of the same types of wear you could expect from constantly having your foot on the clutch. Throwout bearing, crank thrust bearings, clutch pressure plate, fork, pivot, slave, and master all experience increased wear.

4. Shims under the pivot ball are a bandaid. I am not a fan but I can understand why people use them. It's is easily understood that the travel of the slave does not transmit in a straight line to the clutch. There is an arc involved and some trigonometry would help. If some angels/distances are changed one could see why people think it's necessary to add shims. The differences are most certainly real (flywheel step height machining for example) but the changes are very very small, thousandths of an inch. Could this be enough? Who's to say. Unless you have objective measurements of clutch disengagement travel, slave travel and throwout bearing travel I don't see how anyone could objectively comment. I do not use shims. I make damn sure the components I use elsewhere are up to spec and fresh. If you machined .020 off the flywheel and add a .020 shim to the pivot ball, that is your business. haphazardly adding various thickness shims is a bad idea, again, in my opinion.

5. Besides wearing a clutch can slip if it doesn't have enough pressure holding the disk and flywheel/pressure plate together. If you're a turbo car and the step height is too deep you just potentially reduced the clamping force of the clutch. If it's too shallow the same could happen in addition to possibly not disengaging. If you preloaded the clutch because the master is over-adjusted or you use an extended slave rod you could potentially be "riding the clutch" causing it to slip.

Now with those out of the way let's discuss how a clutch works and why we seem to have so much trouble. Any clutch design similar to a DSM (and many are) you have a pressure plate diaphragm spring system with internal pivots that provide pressure against the disk. Now here is where it gets interesting. We need stronger clutches to hold more power but clutch manufacturers do not just simply make the springs stiffer. It isn't practical as the system gets too stiff. We are talking about a system of springs and fulcrums etc. So what manufacturers do in addition to upping spring pressure is they change the pivot point to gain more leverage. What is the disadvantage to this? It requires more travel to completely disengage. Add this to a relatively narrow window of adjustment, worn components etc and you can see why so many DSMs have problems with clutch disengagement. This is unfortunately something we live with within the small surface area we have to work with. Eventually somebody came along with a twin disk clutch and changed all the rules. Lots more friction area within the same 225mm space.

Did you also know that there are different diameter slave bores available? Parts stores often list 90-99 ALL the same. Slaves are out there with either 3/4" or 13/16" bores depending on year, model and options. Mix and match NT and turbo etc and it's starts to get interesting. For the record on OEM there was a color code that may still exist. Red pistons are 3/4 and green ones are 13/16.

It is well known that our pedals are prone to wear but few know what wears or what it affects? On a 1g there are several bushings both on the pedal assembly itself and where the clevis pins goes through the clutch pedal lever. The largest wear point is usually where the clutch lever bolts to the pedal assembly. You can see this at the opposite end of where the master cylinder clevis attaches to the clutch lever. Usually, you can simply grab the two and twist them relative to one another. If this is the case a repair must be made. 2gs do not have this issues because their pedal design is completely different.

I wrote this because I am tired of people posting about how their clutch is doing this or that and there are no objectively statements being made and a lot of assumptions....for instance.

A new flywheel is the correct step height.
For what clutch? Even the same manufacturer. Are you sure? Did you measure? Did you measure with a caliper or a proper depth mic?
On a similar note, "my machinist made sure it was the correct step height". how do you know? Did you measure? If the answer is no you do not know, you assume. A good machinist should not mind if you ask for a depth mic to check the work.

My master and slave are new so they must be good? See the points above about different bore sizes. Not all parts allow the same amount of travel either. If a master gets pushed a little further before it closes the valve what happens? Less travel at the slave. I trust OEM and no other unless I had objective data to support they are the same.

I check the pedal and it looks good. What does that mean? Checked it how? Looked at it?

I'm all about helping people but some objective comments need to be present in order to do so.

Please, discuss, rant, comment etc. These comments are my own and are meant to provide information. Nothing more.
 
Last edited:

my dreamt dsm

15+ Year Contributor
3,304
16
Jan 29, 2007
Somewhere In, California
Thank You Guru's for chimming in. So my question is I have been having clutch issues ever since I installed a new act flywheel and 2100 kit.
"I" seem to be slipping around 1800rpm - 3000rpms in gears 1-4. With that being said a known member on here with a reputable shop (name disclose) said it was okay to just install and go. As you mention above I did not measure anything and just install taking his word and experience into consideration. Is there a way to go around this or I have to redo everything?
Another question is ever since I install my act light flywheel I will bog every time I start the car from a stand still position and even during a launch (note my poor 0-60 times). Is that due to the flywheel or improper clutch install?
 

Jon Lane

15+ Year Contributor
386
21
May 1, 2004
SW, Florida
In a static position with no pressure on the clutch pedal, should the slave cylinder apply any pressure to the throw out bearing? There is a spring in the cylinder to keep the piston extended but the amount of preload from that spring seems excessive.
 

pauleyman

DSM Wiseman
8,426
2,946
Nov 19, 2011
oklahoma city, Oklahoma
You should be able to press the slave into its bore with your thumb easily.
 

Jon Lane

15+ Year Contributor
386
21
May 1, 2004
SW, Florida
You should be able to press the slave into its bore with your thumb easily.

I can. Just want to be sure that that constant pressure is normal and not considered harmful to the center main thrust bearing.
 

1990TSIAWDTALON

Moderator
9,868
5,743
Nov 14, 2013
Independence, Kansas
Another question is ever since I install my act light flywheel I will bog every time I start the car from a stand still position and even during a launch (note my poor 0-60 times). Is that due to the flywheel or improper clutch install?
The more weight in the flywheel/clutch assembly, the more energy stored to get the weight of the car moving. I like stored energy. The light weight flywheel will give you that problem. The only way to get around it is to give the flywheel more energy...ie rpms or run a motor that has a bunch of torque down low. I haven't found a good combination of the 2 yet, in modified dsm's. Maybe some of the other guys have tho, but lets talk about that in a different thread.
 
Last edited:

GST with PSI

DSM Wiseman
2,704
1,454
Jul 27, 2005
San Diego, California
For you ACT users:

90% of ACT clutch setups will shift like shit (or not at all) at or near redline for the reasons Paul mentioned above. The 10% that do shift fine are the result of luck, or extensive modification to the clutch system. There's no easy way around it.
 

Dsm panda

Proven Member
234
57
Dec 8, 2017
Franklin, Wisconsin
So my question on that gst with psi and wiseman is what do you use or recommend I'm fairly new to the dsm life but act seems to have become somewhat of a standard. I don't know a whole lot about the cluch system so thank you for starting this thread.
 

tay97gst

10+ Year Contributor
476
32
Dec 20, 2011
Plaistow, New_Hampshire
So my question on that gst with psi and wiseman is what do you use or recommend I'm fairly new to the dsm life but act seems to have become somewhat of a standard. I don't know a whole lot about the cluch system so thank you for starting this thread.

Southbend clutches have a good reputation.
 

Vincent Pandolph

Proven Member
81
12
Apr 2, 2021
Townsend, Massachusetts
I'm using a South bend ss2200 with a kevlar ceramic hybrid disc. I didn't ask about step height ,nor did I know to ask. The clutch before was OEM with 100k on it. With all the parts brand new all I can think of is I didn't get all the air out of the clutch lines. I've never encountered this problem before, I've changed lots of clutches on many different cars. Always turn the flywheel, bolt on the new stuff and go. This car is a 1990, I had a 91 20 years ago and changed that clutch with an aftermarket from advanced. Bolted in and go. Never this low to the floor crap.
 

twicks69

Supporting Vendor
4,103
1,408
Mar 12, 2004
Milwaukee, Wisconsin
New clutch disengagement should be 0-2" from floor.
 

Vincent Pandolph

Proven Member
81
12
Apr 2, 2021
Townsend, Massachusetts
New clutch disengagement ought to be where the driver is most comfortable using it. But I don't think anyone wants it on the floor. My ? Is how to get it off the floor when I'm already at max adjustment. People are saying extension is only a band aid ⛑️, I kind of agree. What in the actual f is really the problem? I will try bleeding the system again, and push the slave rod in while I'm doing it so air doesn't get trapped in it. Should I de-adjust the pedal height before bleeding the system?

I'm going to review the procedure in the Haynes manual also
 

twicks69

Supporting Vendor
4,103
1,408
Mar 12, 2004
Milwaukee, Wisconsin
New clutch disengagement ought to be where the driver is most comfortable using it. But I don't think anyone wants it on the floor. My ? Is how to get it off the floor when I'm already at max adjustment. People are saying extension is only a band aid ⛑️, I kind of agree. What in the actual f is really the problem? I will try bleeding the system again, and push the slave rod in while I'm doing it so air doesn't get trapped in it. Should I de-adjust the pedal height before bleeding the system?
Adjust the master cylinder rod into the pedal assembly several revolutions before bleeding and adjusting. You could be over-adjusted and not filling the fluid capacity of the clutch master cylinder resulting in less fluid movement. You could be having an issue with the inner piston seal on the clutch master bleeding off pressure as well. Or you could be having movement from the pedal assembly being worn, or have old brake fluid in the system that is moisture contaminated. Or your flywheel step height is not within spec.

There are numerous variables that can cause disengagement issues.

As well, South Bend Clutch SS and SSX pressure plates should be adjusted for a pedal range of 0-2" from the floor on full movement. I don't know how I can tell you that any other way. If you want it to disengage really high, well, then you will be over-adjusted and will damage the clutch as it only has that range of disengagement before you bend the pressure plate diaphragm spring fingers and wreck the clutch. If you want something with a really huge disengagement range, then you will need a clutch with a different fulcrum ring position for the pressure plate that has significantly higher pedal feel for the same pressure plate spring rate.
 
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