In The Shark Tank – Can Your Rod Really Be Too Big?

Can Your Rod Really Be Too Big?

Just when I thought I had everything taken care of on my 1985 Euro 928, I decided to investigate why I always had difficulty getting the car into first gear or reverse when it was cold.  The clutch always seemed to drag.  It was so bad that I would actually start the car in first gear or reverse with the clutch pedal fully depressed.  This was not an ideal arrangement.  Whenever I needed to back out of a parking spot, I would always have to shut the car off, put it into first gear with the pedal fully depressed, then re-start and head out.  If you were on the fall color tour, you may have witnessed this first-hand.

The first thought when one has a dragging clutch is that there is air in the hydraulics.  I bled, power bled, and even reverse bled the system to no avail.  I replaced the rubber line between the master and the slave as it looked pretty bad and was bulging.  Still, there was no improvement.  After some research I found that the issue might be the master cylinder, which made my heart sink a bit.  It is one of those PITA things nobody wants to do on a 928.

The position of the 928’s clutch master cylinder is not one that endears itself to anyone.  It’s nestled underneath the brake booster and next to the fender.  It is virtually impossible to see or get at even with the engine out, let alone with it in.  A few are able to perform alchemy and get it installed without removing other items.  Most, however, resort to pulling the brake booster and master cylinder to gain access.  I did neither.

I was “lucky” because a previous owner must have already replaced the clutch master cylinder, because I found an access panel cut in from the driver’s side wheel well.  I would never advocate cutting a panel to access the master cylinder, but since this is not a show car, so I’m okay with it.  Plus it’s already there, so why not take advantage of it.

web-My lucky access panel

I opened the panel and pulled the unit out.  What I found is that within the past couple years, the replacement clutch master cylinders are slightly different from the original equipment.  Them master cylinder has a depth-limiting rod on the piston.  With the current inventory, these rods are longer than the original equipment.  This prevents you from getting a full stroke and fully disengaging the clutch.  With some minor surgery you can quickly correct this design flaw.

I should point out that it appear only the pre-1987 928s seem to be affected by this.  The master cylinder is the same for all years (except some very early 1978 cars), but in 1987, Porsche switched to a single disk clutch that may be more forgiving to the changes in the master cylinder.

web-The piston-rod length is 80mm.  That is 5mm too long

To disassemble the master cylinder, all it takes is the removal of a snap ring.  The piston and rod comes right out.  The 928 community found that on the original units this plunger plus rod length was about 75mm.  This one measures 80mm.  5 mm doesn’t sound like much, but it is, especially on a dual disk clutch like on this car.

With a cutoff wheel, I took 5mm off the end and smoothed it so there were no burrs and chamfered the end like the original.  I also put a small groove on the end.  This is to allow fluid to enter and exit the cylinder when the pedal is fully depressed.

web-It is important to put a small groove in the endweb-After some minor surgery the length is now 75mm

The next step is to take a couple coils off the return spring.  Without doing this, you could end up with a coil-bind situation and run into further problems.  Re assembly is pretty straight forward, but it helps to have an extra set of hands because you are fighting the return spring.  My son helped me.  Then you need to re-install and bleed the system.

web-Also take a couple coils off the return spring so you don't coil bind it

At this point, it is critical to check the routing of the infamous “blue hose.”  This hose, besides being blue, carries fluid from the brake fluid reservoir to the clutch master cylinder.  If it is routed in such a way that traps air from escaping the system, it may never bleed properly, and you may never get the clutch to operate.

web-Here you can see the clutch master cylinder

I was able to bleed the system 90 percent in a few minutes.  It can take a little time for all the air to find it’s way out of the master.  The pedal will fall to the floor several times.  You just have to pull it back up a few times manually.  Eventually it will return on its own.  Once you can get it into gear, a short drive exercising the clutch (and getting a little heat into it) will finish the job and the pedal will be back to normal.

Since this operation, the clutch on the Euro works great and I can easily put it into any gear.

web-My two 928s.  The one on the lift is my 1991 GT, the other is my 1985 Euro-spec

If you have any specific questions about 928s or suggestions for future articles, please send me an email at andrewmolson@yahoo.com or call my cell at 734-837-7908.  The 928 group gets together on the second Wednesday of each month for a beer night.  If you are interested, send me a note and I can get you on the mailing list.


Comments are closed.