ZF5 Hard Clutch Problem Solved (full write-up)
ZF-5 Hydraulic Clutch Geometry: Stock Ford Parts
Hard Clutch Problem Solved
Craig (Tailpipe) Wilcox and Silas (Nevadasmith) Howell
The ZF-5 transmission swap has been a popular upgrade for many years now. As we are seeing more and more of these modern five speeds going into our rigs, there seems to be somewhat of a shortage of information about how to achieve the best results the first time around. This was my experience (and frustration) as I did the swap. The issue is not necessarily in swapping the actual transmission, but rather the hydraulic clutch system. For my application, as I always try to do when making conversions and upgrades, I used readily available stock Ford parts. I do this for two reasons; 1) junkyard are awesome and 2) I prefer to use parts that I can get quickly from the major auto parts stores (special order parts are hard to find on the trails).
Now I do need to say there are instructions out there that deal with installing a hydraulic clutch in the Early Bronco, but even following those, I was not achieving the results I desired (or that were correct with my equipment for that matter). Cory Walden (Desert Thrasher) has a very good and detailed article doing this conversion using Wilwood components that I am sure achieves great results, but his method left a lot to be desired when using stock Ford parts. This article is not to contradict Cory’s method, but rather to address another method when using stock Ford parts. You can check out Cory’s write-up at
http://classicbroncos.com/hydraulicclutch.shtml.
After the initial installation, the problem that I was facing was an extremely hard clutch. To make matters even worse, the clutch was more or less operating as an on/off switch; no feathering or control in terms of clutch engagement and disengagement. For my initial installation, I used the existing 24-year-old parts that came from the donor truck. Thinking this was the culprit of my issues; I pulled the transmission and installed a brand new clutch and pressure plate, new slave cylinder, and a new master cylinder. During the process of bleeding the system, I quickly found that my problem was not remedied at all. Enter Silas Howell (Nevadasmith). After posting my problem to the forum, Silas quickly answered with a solution that I had not read about nor heard mentioned to this point; geometry. Silas had the foresight to take measurements from the donor vehicles (mid 90’s F-250/350) with regards to pivot arm length, pedal ratio and stroke. To make things even easier, he then made the drawing that's attached. (PDF)
Following these measurements, you will be left with very nice, light clutch pressure that will make you think you’re driving a little four banger.
For the purposes of this write up and my particular set-up, the following parts are used:
1992-1996 F-250 5.8 or 300 clutch master cylinder with line
1992-1996 F-250 5.8 or 300 slave cylinder and throw out bearing assembly
1992-1996 F-250 5.8 or 300 clutch and pressure plate kit
When doing the ZF-5 swap, most end up mounting the master cylinder in the speedometer cable hole; this actually puts the push rod too low on the pedal causing an improper ratio.
For our method here, we both (Silas and I) made the measurements, welded and drilled on the bench. Since I have never been a big fan of pulling steering columns, I removed the pivot arm from the pedal box to retrieve the pedal.
Here is the step by step:
1. Remove the fresh air vent box from the cowl so you have access to the pedal assembly.
2. Remove the cotter pin on the right side of the pivot arm (closest to the accelerator) Picture 1
3. Drive the pivot rod out to the left (toward the clutch pedal ensuring all the plastic bushings are accounted for and not damaged).
4. With the clutch pedal removed, chuck it in the vise with the pivot rod and bushings in place.
5. Find the centerline (CL) of the pivot rod and mark or center punch. Take careful measurements, as this will be the base for all measurements. Picture 2
*For instructional purposes only* I’m using a 69 Mustang pedal for demonstration in this write-up (didn’t feel like pulling the whole thing a part again).
6. Following the measurements in the drawing, measure from the CL of the pivot rod down the pedal 2 5/8” and mark appropriately. This will become the CL of your extension tab.
7. Fabricate a steel plate (I used 1/4” steel) to serve as the extension tab from the clutch pedal. The tab should be measured and fabricated based on the measurements given in the drawing (mark but do not drill the hole at this time, see Note 2).
8. Determine the CL of the extension tab and mark it fully across the piece. This mark will be lined up with your pedal mark for welding.
9. Ensuring you keep all marks aligned, weld the extension tab to the clutch pedal. Picture 3
10. Now this is where it starts to get tricky. Since the master cylinder will not be mounted in the existing speedometer hole, we need to measure, mark, and drill the new location on the firewall. The master cylinder location will be approximately 2” above and slightly inboard from the speedometer hole when all is said and done. I found it easiest to partially install the pivot rod with the CL marking facing inboard (towards the accelerator) and the clutch pedal to make my measurements. I took my time making these measurements and I suggest you do as well unless you don’t mind your firewall looking like a piece of Swiss cheese (measure twice, cut once). Again, referring to the measurements on the drawing, measure and mark the location 2” below the pivot rod CL on the firewall. I found this to be easiest to make a vertical plain mark on the firewall that reflected the CL of the pivot rod. From that mark, I measured down 2” and marked. In order for this to work, the clutch pedal must be in its final position. Before drilling, I held my push rod in place to determine if the hole marking was good and to determine if the firewall marking was accurate. Of note, the push rod will be mounted on the right side of the tab, or inboard of the pedal. If you measure correctly, the mark (or CL of the master cylinder) should end up to the right, or inside, of the pedal mounting bracket. Notice the relationship between the speedometer hole and the actual mounting position of the master cylinder; displayed are manual brake and hydroboost applications. Pictures 4-7.
11. Once the master cylinder mounting position has been determined and marked, drill a pilot hole through the firewall. I drilled my pilot from inside out, but you could transfer the mark to the outside and drill from there.
12. With the pilot hole drilled, use a 1 ¼” hole saw to make the final mounting hole.
13. After the hole is drilled, install the master cylinder in place with the push rod installed and verify the extension tab mark, adjust if necessary. Despite my careful measurements and constantly rechecking everything, I ended up off somehow and had to make a slight adjustment. Regardless, everything still worked appropriately so try it out prior to throwing parts through the wall. Picture 8
14. Remove the pivot rod and clutch pedal and drill the hole for the desired mounting mechanism. I chose to use a ½” piece of all thread and plug welded it in place. This will fit the stock push rod eyelet or you can build or buy a custom heim joint adjustable push rod (Note 3). This is also a great time to clean and lube up that pivot rod that hasn’t seen the light of day for 41+ years.
15. With the master cylinder in place, mark and drill the two holes for the mounting bolts. Using appropriate fasteners, mount the master cylinder in place and connect the push rod to the pedal.
16. Fully reinstall the clutch and brake pedals, ensuring that all the plastic bushings are in place.
17. Finally, bleed the system and enjoy a Ford Focus like clutch!
* Note 1* For our two applications, I am running hydroboost brakes and Silas is running manual brakes. Unfortunately, I no longer have my power brake booster assembly to use to check fitment, so at this time we are unsure how these measurements will work with a stock brake booster. In order for this to be a viable option for all of us, we would like to know if it is possible to achieve these measurements with vacuum assisted brakes.
*Note 2* If you noticed in the pictures I have an firewall reinforcement plate (black firewall). It is a ¼” piece of flat steel cut to the same shape as the stock plate. I did this more so for the hyrdoboost, but I know there were some issues with firewall flex in the 1990s F Series trucks so I added it to help both. Silas does not have an extra plate (yellow firewall), and does have firewall flex from the pressure and will be adding the reinforcement plate. This additional ¼” plays a factor in the extension tab bolthole. If you do not have a plate, your measurement should fall between 1” and 1 1/8”. To avoid any error it is important to check the final mounting location of the push rod with the master cylinder in place and the pedal in the full rest position.
*Note 3* For those who would like to build an adjustable hiem joint push rod the following parts will be needed:
McMaster Carr P/N 90268A325 (coupling nut ¼”-28)
McMaster Carr P/N 60645K121 (rod end ¼”-28)
You can also check out
www.mcmaster.com for any size or style you choose.
We hopes this help some of you solve this problem. It may sound hard, but it is actually quite simple. Feedback is welcomed, especially with regards to vacuum assisted brakes. This is by no means the end all be all for this modification, just a method that worked for a guy that "tried it all".
