Thoughts on adjusting an MCI air-assist clutch

[bevans6]

My bus has the typical air-assist setup that MCI seemed to use on most or all of their Spicer 4 speed installations.  It took me the better part of 6  months to finally figure out how to adjust it so that the air assist feature maximized it's actual assisting, so I thought that I would share this.  Someday some soul will do a search, maybe find this, and save themselves six months of aching left knee...

There are many things you could adjust on an MCI clutch, and only three things that you should adjust.  The basic linkage is all adjustable, is set at the factory, and if you don't touch it won't go out of adjustment.  If you do have to change linkages or joints, put them back to factory settings.  This is critical so that the leverage and force you start with your foot on the pedal is transfered as efficiently as possible to the arm on the side of the transmission that actuates the throw-out bearing that actually pushes on the clutch fingers.  Likewise, the clutch itself has to be the right one, and installed correctly, and it all has to be lubricated properly, and the joints all have to be in good condition.

The three things you can adjust are the height of the pedal, the static angle of the air-assist cylinder, and the free play between the clutch arm itself and the air-assist arm.  The important thing to know about how the air-assist cylinder works is that the farther the clutch actuating system can move, the more the air-assist can assist.  So the trick is to get the maximum movement.

The first thing to check is the pedal height.  It's adjustable, with a pinch bolt under the floor of the drivers compartment up behind the accessory air tank.  Set it so the pedal is as high as possible.  The higher it is, the more pedal movement you can get.

The second thing to check is the static angle of the air-assist cylinder.  It is a single acting 2" bore air cylinder that is fed from the accessory system via a manifold on the rear wall of the engine compartment.  Mine had a Tee-valve so that the air feed to the cylinder could be turned off if required.  I replaced it with a simple 90 degree connector.  On mine, the heim joints that allow it to swivel were rusted and siezed, not contributing to a light clutch in any way...  I did the requisite rebuild/replace to make sure it was good to go.

The thing with the air-assist cylinder is that it always has system pressure applied and so is always pressing with between 315 and 375 lbs of force.  You need to adjust the angle of the air cylinder so that in a static position (clutch pedal up fully) it is pressing directly towards the clutch actuating arm.  If it's exactly in line, all that pressure goes nowhere and the clutch throw-out bearing isn't riding on the clutch itself.  You adjust that with a 1/2" bolt and locknut on the bottom of the clutch actuating arm.  Crank the bolt in until the works are all lined up and lock it down with the lock nut.

The third thing you adjust is the free play between the clutch arm itself, and the clutch actuating arm that is moved by the pedal and linkage.  the idea here is that as the actuating arm moves, it creates an angle between it and the air-assist cylinder.  That angle lets the force provided by the cylinder to help lift the arm up, which in turn move the clutch throw-out bearing and presses in on the clutch spring.  The greater the angle, the greater the assist force.  So you try to adjust things for the greatest angle possible.  The connection between the actuating arm and the clutch is adjustable for free play with a handwheel that you access through the drivers side engine compartment door, it's a round wheel about 2" in diameter.  You grab it and lift to engage the adjusting mechanism. To start with, you grab/lift and turn it clockwise all the way until resistance is felt.  That is removing all of the free play.  Next, you back it off by one full turn counter-clockwise.  That's your factory setting.  What you can do now is fine tune that to your bus.  What I like to do is start the bus, leave the parking brake on, and put it in first gear.  I slowly let the clutch out, and feel for where it starts to engage.  I adjust the free play so that it engages with the pedal as close to the floor as is reasonable, making sure that you get full dis-engagement for best gear-shifting and sitting in traffic with the clutch in and not dragging.  With my clutch, I can clearly feel when the clutch is fully disengaged, the spring pressure goes higher in what can be felt as a step-up in resistance.  So I adjust for that, trying to get the maximum free-play in the linkage which results in the maximum angle of the air-assist cylinder and the maximum amount of assist.  If I get it right, the increase in pedal pressure from depressing the actual clutch is balanced perfectly by the increase in air-assist as the pedal goes down and the angle of the dangle increases, and I once again feel that I can put off installing an automatic tranny for  another little while...

Hope this helps someone sometime...

Briana

[Ed Hackenbruch]

Sure glad the PO put an auto tranny in the bus long before i ever got it.

[bevans6]

I guess I just love the romance of shifting the Spicer!  I really do enjoy the skill and the challenge, it's a big part of the fun for me.  The aching left knee, not so much...  this got the effort down to a reasonable level, comparable to my dodge pickup - not the same as, but in the same county at least!

Brian

[busshawg]

Well I for one sure apprieciate all that info. My clutch has been getting easier but I'm sure it could be betterm Thanks for all the info

Grant

[JohnEd]

Nice job Brian.

Thanks

[Ncbob]

Brian, you are a gentleman and a scholar, and there's darn few of us left.

I've been down this road a couple of times, having the engine out of the bus etc., but the clutch assist was always down on the bottom of the list.  But my 75 year old left leg (the right leg is only 72) is paying a price I can't afford.

Oh, I've read all the articles about "hot rodding" an MCI clutch assist but they didn't amount to a hill of beans. After reading your comprehensive  blow by blow description I will print it out and have it in hand when we take the bus to our Hero in Rabun Gap, GA prior to our return to FL in the fall.

Your attention to detail and desire to share your hard won information with the rest of us earns my thanks if not the same from many more 5 drivers.

The best to you and your travels,

NCbob

[bevans6]

Well shucks...   Call me if it works for you, the proof of the pudding is in the eating...

One thing to check first, and that I kind of mentioned but neglected to emphasize, is check that the tee-valve that feeds the air-assist cylinder is open and is actually feeding air, and that the hose to the cylinder is not collapsed.  I recall Vivianne saying that the valve on her clutch was turned off one time.

Brian

[Tony LEE]

Another thing worth doing is to pull the assist cylinder right out, disassemble it and clean the bore and the seals and pivot points and lubricate everything before putting it back together and refitting. On mine, that was all that needed doing to change the clutch from "oh, no. Why did I buy this #$R%& heap" to something that Betty could drive without problems.

[Dave Siegel]

Hi Folks,
I have been reading this post with very serious interest. Our coach is a 1948 GMC Silversides with a Spicer 4 speed. My left knee is now completely made of titanium and is stiffer some days than others. But it is definitely painful whenever we are stuck in traffic and I have to hold the clutch in for any length of time.

Is it possible to locate these MCI parts and install them on our bus so that I can have some sort of help shifting this neat old coach?

If so, does any one have a complete parts list that I could use for a shopping list?

Thanks for not laughing. (Too much)

Dave Siegel

[bevans6]

here is a retro-fit system that could probably be fitted to your bus, they say they can fit any bus.  The MCI system needs the MCI linkage to work, it would probably be hard to retrofit to a different bus.

http://www.hb-industries.com/air.htm

Brian

[Dave Siegel]

Thanks Brian. I will contact them. The photo that they show for a 4104 looks just like the set-up on our coach. It will probably work. Thanks again.

Dave Siegel

[jwavc1]

Wow, Brian (or briana) thank you,  thank you!

I just purchased an mc5a 1964  at an incredible price but unable to run it or look inside any of it.  I completely lucked out in that I got a real gem for almost free. The cost of a pair of 8ds is my only repair that cost anything and bus purrs, sits up and shifts excellent. Now that I have the bus airing up (tow truck disconnected main line from compressor) the clutch some time returns from the floor slowly. I've tried pulling up with my toe but doesn't seem to help.

This and powersteering are my next obsticals towards road worthiness. I mostly work on my own vehicles (play mechanic) and the web is the key for keeping my 2007 Yukon on the road with 345k on it. With the MC5 (that I love the style of and can't believe I have) the internet has been very sparse. I'm guessing the majority of the units left are possibly owned by a generation not big on the internet of things. I get that...  I'm a tech guy and believe I'm missing out on a more peaceful time before my generation (I'm 55).

With that said, your info on clutch linkage adjustments is well written and seems very clear. I have limited time to work on this machine and look forward to going thru what you have very clearly laid out here.

Again,   Thank you


Also, if any one has a layout of this systems or the power assisted steering they could forward I would appreciate it.

My background is audio video and control system so if I can help any of you please don't hesitate to ask.

[ol713]

   HI;
   My clutch has always been "heavy".   I need to get under the bus and have a look at
   what is going on.  I have even changed the seals in the air assist with no change in
   clutch pressure.  It has been a few years since I looked at the air assist, but I will
   read and re-read your post to see if I can get a lighter clutch.  My left knee is also
   suffering from "old age"
   How about a few photo's?  Not sure about the "angle of the dangle"
                                       Thanks,   Merle.          (mc-7)

[chessie4905]

Does the air assist cylinder use full coach pressure or is it reduced? If reduced, you could increase pressure. They used to sell a kit to add air assist to the 4104, which ours had. You could adjust the pressure to make the clutch lite like a car. There was a motion sensor that was welded in time the clutch apply rod. Movement detected here applied air to assist cylinder. I'm guessing the components came from another use. Also the more the disc wears, the stiffer pedal gets as apply springs get more and more off center.

[bevans6]

The clutch uses full air system pressure.  There was a on/off valve on mine, rear bulkhead manifold that feeds the clutch cylinder, that I removed.  Inside the spare tire compartment, up on the roof there is a 6" or so long fairly heavy spring that is the pedal return spring.  It helps lift the pedal up.  If you lift up the access hatch at the rear of the bus you can clearly see the main clutch lever on top of the gearbox.  If you undo that joint between the pull rod and the lever you can assess the ease of movement of just the pedal/linkage/pullrod for stiffness when it's not attached to the actual clutch mechanism on the transmission.  Lots of lube points in there as well.