39 40 series clutch adjustment mystery

[buick looks fine for 39]

Hey guys, I've come up with an issue I'm struggling to deal with. According to the shop manual the clutch pedal backlash should be 3/4 to 1 inch. In other words when the clutch pedal is depressed the pedal is only supposed to be pushed down 3/4-1 inch before resistance is felt. With my clutch release rod adjusted at it's maximum my pedal has about 1 3/4 inch backlash. Three quarters of an inch doesn't sound like much, but I'm concerned I won't be able to adjust my clutch properly as the clutch disk wears. Besides the shop manual states the proper adjustment is 3/4-1 inch and I don't like doing anything half-a$$ed. All my linkage is 39 40 series and I double checked each piece to make sure it wasn't mixed up with a later year or bigger series. I did change out my pressure plate  from an original diaphragm type  to a better 3 finger type. I think my problem is one of 2 issues. I compared an original diaphragm type pressure plate with the nos or old school 3 fingered pressure plate I'm installing and noticed the following difference. With both pressure plates placed face down on the floor there is 3/8 inch difference between my diaphragm pressure plate and the 3 fingered pressure plate. The throw out bearing contact surface of my diaphragm pressure plate is 3/8 inch  taller than the contact surface of my 3 fingered plate. That might account for some of my adjustment issues. My throw out bearing is nos 39 Buick directly out of a Buick box. According to my Buick Master Chassis manual the throw out bearing is interchangeable from 39 to about 53 40-50 series, but I haven't been able to find any part info for the throw out bearing sleeve press that is fit into the throw out bearing. From 40 to 50 years ago I kind of remember the diaphragm plate needed to be replaced with a 3 fingered pressure plate and a MATCHING throw out bearing. Did the throw out bearing for the 3 fingered clutch have a longer sleeve? I've talked to everyone I can think of including Seattle Brake and Clutch and I've come up with a goose egg. 

                                                                                                               Any help would be greatly appreciated

                                                                                                                                  Thanks 

                                                                                                                                    Leif

               

[avgwarhawk]

The amount of miles you will drive to require a clutch adjust will probably not happen for 20 years.  I have put on 8000 miles over the past 10 years on my 54 Buick 3 speed manual.  Never needed a clutch adjustment for wear.   If it working I would not worry about it.  Drive it!      

[Rock10]

How does it feel catching from off the floor?

[buick looks fine for 39]

Good point avgwarhawk, that's very true. My car is at the end of a complete 25-year restoration, and I'd like to find the problem before getting it on the road. Also, Rock 10 has an excellent question since the clutch pedal can rub up against the toe kick and it frequently happens. The sheet metal toe kick plate under the clutch and brake pedal is removed and there is no binding or catching anywhere.

 

                                                                                                    Great thoughts guys I really appreciate it

                                                                                                                     Leif 

[EmTee]

18 minutes ago, buick looks fine for 39 said:

The sheet metal toe kick plate under the clutch and brake pedal is removed and there is no binding or catching anywhere.

I think that @Rock10 was asking you to describe the point at which you feel the clutch engage as the pedal comes off the toeboard.  With excessive freeplay (essentially an extra inch of lash) my concern would be ensuring the clutch is disengaging properly since the throw is a bit shorter than specified.  So, can you feel the clutch 'bite' when letting off the clutch?  Along the same line of reasoning, do you get any gear clash, especially going into 1st or reverse?

[Bloo]

6 hours ago, buick looks fine for 39 said:

With both pressure plates placed face down on the floor there is 3/8 inch difference between my diaphragm pressure plate and the 3 fingered pressure plate. The throw out bearing contact surface of my diaphragm pressure plate is 3/8 inch  taller than the contact surface of my 3 fingered plate. That might account for some of my adjustment issues.

Close....

 

Any clutch you might want to put in there needs to be installed in the way it was designed (I'll get back to that). I hear some people use a Jeep clutch from the 70s. You are on the right track with your different measurements, but clamped down is what matters.

 

A pressure plate is designed to be bolted down with a specific "pull-down" I guess (because I don't know the correct term). For instance, if a clutch is made to be bolted down to a flat flywheel, and you use the pressure plate with a thinner-when-fully-compressed clutch disk, there would need to be a step up on the flywheel where the friction material contacts. The step makes up the difference in height. If you use a thicker disc, the place where the pressure plate bolts down would need to be stepped up.

 

Neither of those may apply to your situation, but you need to understand that. If there is not enough spring tension the clutch could slip. More importantly it there is to little OR too much, the geometry of the pressure plate is wrong. You might have some leeway here but I am sure it varies with pressure plate type.

 

For those of us who cannot engineer it from scratch, it is easier to just make sure the pressure plate is installed with the correct "pull-down", or at least the pull down for the known working application (make, model, year) the pressure plate was sold and intended for.

 

If you are using a flat flywheel, and a clutch disc and pressure plate that were meant to go together, and that clutch disc and pressure plate were intended for a flat flywheel, you are good to go on this part. I'm going to call this Important Thing #1.

 

And now we get to the meat of the matter. When installed to it's designed specs as outlined in the previous paragraphs, the contact surface for the throwout bearing might be at a different height than the original. This also matters, because if it is wrong the angle of the clutch fork will be wrong. That is by far the most important thing we are going to be talking about. I am going to call the fork angle Important Thing #2.

 

OK, if the fork angle is wrong, what are we going to do about that? Since it is the height of the fingers (or whatever the pressure plate has for the throwout bearing to contact) that is wrong, the thickness of throw out bearing is the best way to approach it. In the musclecar era, GM had a whole raft of different thickness throw out bearings. A few are still available. Ford had maybe four(?) with their attachment method. Last I checked 2 were still available. There are also aftermarket adjustable-thickness GM bearings. I don't know if any of these later bearings will adapt back to 1939 Buick easily.

 

If you can't get the fork angle back where it was before with an available bearing, another possibility is that you could shim or change the fork pivot to move it in or out. On some cars these are threaded and can be shimmed, or a different height of pivot ball can be installed.

 

You can't usually make huge changes here, but if you do, make sure nothing is going to hit. The trouble with this is that the end of the fork that attaches to the linkage will be fore/aft compared to where it was before when the fork angle is right. It might take part of your adjustment range for wear away. This is probably better to just be used for fine tuning after correcting most of the problem with a throw out bearing.

 

Generally speaking, you want the fork arranged so the throw out bearing end is angled back when the clutch is engaged, and the throw out bearing end is forward with the clutch disengaged. There could be exceptions. If you can figure out how Buick had it arranged with the original clutch, do that.

 

All of these pieces of clutch linkage pivot around something, so they do not behave in a linear way. The "gear ratio" changes as they move. In the case of the clutch fork the pivot is the ball. With the fork angle set up as I have described, there is a little movement at first, lots of movement when the fork is straight, and then less movement again as it see-saws past the center.

 

Lots of movement means harder to push. Less movement means easier to push. It's just like gear ratio. If the fork angle is wrong, you could easily get into a situation where the pedal gets radically harder to push the closer it gets to the floor. It is really annoying to drive something like that. On the other hand you could get into a situation where the fork is already past center and as you push it, it gets easier and easier and easier. A tiny bit of "easier as you push" is desirable, but if the fork started out with the bearing too far forward, there probably wont be enough total motion to disengage the clutch, and the gearing gets slower the farther it goes. The more you push the worse it gets.

 

As I mentioned before all the clutch linkage pieces, z bars, pedals, etc. change gear ratio as they move. It is very hard to estimate how the ratios all multiply and change with motion just by looking at the linkage. An adjuster in the linkage might fix pedal position, but it doesn't fix anything else.

 

The best info I have ever seen on setting up clutch forks was on novak-adapt.com, buried somewhere where it is tough to find. I'd link it but I can't find it. It's not what comes up when you google, although that page is useful too.

 

EDIT: https://www.novak-adapt.com/knowledge/clutches-etc

 

EDIT2: This is for Jeep engine conversions, but a good starting point for anything.

 

Quote

Install the transmission on the clutch housing. The release bearing must be able to move away from the diaphragm spring approximately 1/16 to 1/8" (this is the "free play"). At this point you should be able to move the release bearing back and forward with the fork. The bearing, when against the clutch, should leave the release fork positioned at 4 or 5 degrees LESS than a right angle with the engine centerline and allow it to be moved away from the clutch 1/16 to 1/8". If this condition does not exists, do not install the assembly into the Jeep until it does. If not, you may need a different clutch release bearing or pivot or you may have the wrong fork.

 

Edited November 29, 2022 by Bloo (see edit history)

[EmTee]

That was a nice clutch linkage summary tutorial - thanks! 

 

The clutch on my '38 Century is adjusted for the pedal lash called-out in the shop manual and works fine.  In fact, the pedal pressure is fairly light (IMHO) and consistent through its travel.  I can easily see how most anyone could comfortably drive the car back 'in the day'.

 

My GP requires significantly more pedal pressure to disengage the clutch.  I plan to replace the clutch disk this winter as the engagement point is pretty high and the disk is about as thin as I want to let it get.  I'll definitely take a good look at the linkage and fork angle as I disassemble it and compare new parts with the old ones.  I'm hoping it only needs a disk, but I'll check the wear surfaces on both the flywheel and pressure plate when I get it apart and go from there.  Likewise with the throw-out bearing.  A little less initial pedal pressure would be nice, but it's not imperative.  Maybe the pressure plate that is in there now will tell me something (e.g., if it is a 'performance' vs. 'stock' item).

[Rock10]

My brother had a 66 Vette with a 427 in it.

I think it had a performance pressure plate and clutch.

It took so much effort to push it in and hold it in that at stop lights, my leg was shaking.

Edited November 29, 2022 by Rock10 (see edit history)

[peterjohn72]

You have questions, I would call falcon clutch in deer park. They have gone above and way beyond to help me out with my 41. 

[buick looks fine for 39]

Photos of my clutch assembly with the clutch disengaged. Clutch pedal is not depressed. The throw out bearing is completely in the pressure plate cavity. Only the throw out bearing sleeve can be seen. Does this look right?

[buick looks fine for 39]

Wow, you guys put a lot of time and thought into your responses, and I really appreciate it. My car is not running, and I probably didn't make that very clear. I've been in the process of getting the engine ready for startup when I came across my clutch issue. Man, Bloo did a great job with his reply. He is correct when he mentioned the location of the clutch fingers being most important when the pressure plate is bolted to the flywheel. I knew that but I got so danged confused trying to figure things out I forgot this important point. Bloo is also right with his discussion of linkage geometry. I didn't think of checking my clutch release pivot and that turned out to be correct. My clutch does operate smoothly, and I do have 1/16-1/8 throw out bearing free play which is more critical than clutch pedal back lash. 

 

I just sent photos of my clutch assembly for additional information 

 

                                                                                                                     Thanks

                                                                                                                          Leif

 

 

[edinmass]

After fifty years of servicing car we have a mantra in our shop. “Stock is trick”! 
 

 

[EmTee]

On 12/1/2022 at 10:16 PM, buick looks fine for 39 said:

Photos of my clutch assembly with the clutch disengaged. Clutch pedal is not depressed. The throw out bearing is completely in the pressure plate cavity. Only the throw out bearing sleeve can be seen. Does this look right?

Here are a couple of photos of the clutch fork and throw-out bearing on my '38 Century.  My clutch pedal freeplay/lash is set to the 3/4" ~ 1" as specified in the shop manual.

 

[buick looks fine for 39]

Thanks EmTee for your photo. Comparing my photo with yours it looks like my yoke is closer to the transmission than it should be at the pivot point. I'll check it out!

 

                                                                                           Thanks

                                                                                             Leif

[EmTee]

Just remember that my Century has the 320 with the bigger transmission.  I don't know whether or not that makes a difference with regard to the relative position of the fork at rest.  I know the linkage is probably different.  I was mainly looking at the position of the throwout bearing with respect to the pressure plate.

[buick looks fine for 39]

Yep, the forks between 38 60 series and 39 40 series are different but your throw out bearing seems to be in the same position as mine. Your fork seems to be at a different angle than mine. Thanks, EmTee for the photo it does answer some questions.

                                                                                                                    Leif

[buick looks fine for 39]

My clutch problem may have been solved. After spending 30 hours double checking all my linkage for non-39 40 series parts and any other possible issues. I took peterjohn72's advice and sent my clutch assembly to Falcon Clutch. Apparently, my pressure plate was set up for the wrong application and the clutch fingers were nearly 1/2 inch out whack. Rather than trying to make my rockwell clutch work in my car I'm going back to one of my stock diaphragm pressure plates. Apparently, stock is indeed trick. 

 

I am constantly reminded by how many valuable resources we're losing every day. Not too long ago I could take a pressure plate to Seattle an hour away and have it checked out in minutes. Now it's a struggle finding anyone with the tools or ability to figure it out.

 

                                                                                                                 Thanks

                                                                                                                      Leif

[EmTee]

Glad that you got it sorted-out.  Hopefully, now you can get back to building those left leg muscles! 

[buick looks fine for 39]

My leg muscles are great, it's the muscle between my ears I have a problem with

[Ben Bruce aka First Born]

9 hours ago, buick looks fine for 39 said:

My leg muscles are great, it's the muscle between my ears I have a problem with

 

 In my case, the lack thereof.

 

  Ben

[39 Again]

Leif

I happened to be looking for an old picture of when I disassembled my engine and ran across this pic.  It made me think of your post about the clutch position.  I don't know if this will help at all, but as you know, mine is a 39 model 40 and as far as i know this is original.  The pic was taken while mounted to the motor stand with the bell housing still in place...clutch released.  The second pic shows the linkage as it was removed so you can get an idea of the adjustment.  When it was running before tear down, the clutch lash seemed ok.

 

Talking about clutches, I wasn't thinking when I removed my pressure plate from the flywheel 3 yrs ago and I can't find where I match marked them.  The flywheel and PP are balanced at the factory and should be matched according to my manual at disassembly.  I can't find any pics that show any matching identifications to cheat.  Anybody know how important it is to have them matched?  I could probably find some way to have them balanced to the original position before installing my new clutch disk.

Thanks,

Tom

PS.  Let me know if 2 pics don't load.

[EmTee]

3 hours ago, 39 Again said:

I could probably find some way to have them balanced to the original position before installing my new clutch disk.

Do you know anyone with a wheel/tire bubble balancer?

[buick looks fine for 39]

Thank you for thinking of me and the pictures. They confirm my throw out bearing fork is not sitting where it should.

 

Because my engine was undergoing a complete rebuild I had my pressure plate and flywheel balanced together but it may not be that important. Old straight eights are a low rpm engine and balance on the back end is probably not as critical compared to a higher rpm engine. The last dude in your engine probably didn't bother with balancing either. Do you plan on reinstalling your pressure plate as is or are you going to have it serviced? 

 

                                                                                                         Thanks again

                                                                                                               Leif