Late last year I discovered that I was overpowering my small block toploader’s 1 1/16” input shaft and needed to upgrade it to use the larger diameter 1 3/8” input shaft that is available from most manual transmission parts dealers. For details on swapping the input shaft, I created a separate tech article: http://www.fordmuscle.com/forums/tec...placement.html
This article describes how to modify the small block fork for use with the big block input shaft.
Although I could buy all the parts needed to update the transmission, the throwout bearing that was required to complete the conversion needed to be from a big block car. Much to my surprise, my existing small block clutch fork was too narrow to fit this new throwout bearing. Also, a factory big block Ford clutch fork was much too long to fit within my existing header configuration.
I made a quick call to David Kee and he told me what to do, as he has done many of these conversions. He suggested that I purchase a reproduction 428CJ fork and graft the forked end to my existing small block fork.
Below is a comparison of the 2 forks, with the typical small block fork on top and the 428CJ on the bottom:
I laid the 2 forks side by side and determined that they are practically the same except the part that sticks out of the bellhousing.
Before I did any cutting, I created a template out of cardboard that showed the shape of the original fork. Tracing it out was pretty simple and I marked a couple of key locations, such as the pivot area:
Once this was done it was time to get to work with the chop saw. I determined that the best grafting location was 1.22” from the primary pivot point and I cut the small block fork there.
I them moved on to the 428CJ fork and cut it so that it was 1/8” longer. The extra length allowed me to custom fit the ends for the best fit with the small block fork. A few quick swipes on the belt sander helped with the final fitting. I used my template to verify that the angles and length of the parts were correct. For strength purposes, it is important that all parts butt together squarely before welding:
Once I was satisfied that the butted fork pieces matched the template, I tig welded the parts together after practicing on the disguarded sections:
Interestingly, the material hardness of the original Ford fork and the reproduction was different so the practice run certainly paid off. I welded one side of the fork, letting it cool. Then I flipped it over to weld the opposite side and again let it cool. Note how tightly the parts fit together mechnically:
After all welding was complete, I cleaned the modified fork and gave it a generous coating of black paint. Here is the final product:
I installed the modified fork in my car and was able to successfully complete the remainder of the race season.