GSSD06
Member
I removed the OC-4 or C4 or S70F-12 differential from my Thiokol Imp to check it over before placing it back in service. I found the shifting fork assembly plate had cracked and been brazed. Even though it had always worked fine, I located a replacement, and when I installed it and turned the input shaft by hand, I noticed that in 1st and 3rd gear the transmission sounded fine, but when in Neutral or any other gear, there was the distinct sound of gears partially meshing.
I removed the shifting fork assembly and reinstalled it several times and after spending a fair amount of time to determine how the transmission worked, It dawned on me that the problem must be in the 1st and 3rd gear shifting fork Neutral position because only in 1st and 3rd gear was the problem gone.
To confirm, I moved the 1st and 3rd gear shifting fork slightly in both directions with the shifting fork assembled to the transmission and found that when moving the fork foward towards 3rd gear the gear meshing stopped. I even found that if I removed the one countersunk mounting bolt and moved the entire shifting fork assembly forward against a few mounting bolts that the gear meshing was gone.
I also checked the input shaft to see if it was tight in the housing, thinking that possibly the misalignment was not in the sliding gear but the mating gears. The shaft had little or no end play and there was no visible signs that the shaft had ever moved.
I removed the safety wired bolt holding the 1st and 3rd gear shifting fork onto it's shaft, but the shaft had a drilled location for the bolt, so no way to adjust the position of the shifting fork on the shaft. Even if there were an adjustment here, that would have misaligned the slot where the shifing lever fits into all three shifting forks.
It was the last adjustment I wanted to consider, but I decided to use the universal thermal adjusting tool (oxy-acetelene torch) to bend the 1st and 3rd gear shifting fork forward a small amount. After carefully measuring to determine the current alignment of the shifting fork I convinced myself that it was, if anything, misaligned to the rear and a simple bend would suffice instead of a compound bend to keep the shifting fork parallel with the grove in the mating gear.
After two minor adjustments totalling about 1/8", the shifting of the transmission seems to work well. Checking as well as possible through the fill and drain holes with the shifting forks assembled, the gears appear to align properly in all gears.
Having been around some of the earlier Thiokols that used this transmission as the only one in the machine, I was always surprised how rough some operators were trying to get the transmission into different gears, especially reverse. A person almost needs to see the transmission apart to have a feeling for the best way to shift without causing damage.
I hope this might help someone else in the same situation.
I would also be happy to hear if anyone has a better solution than heating up and bending the shifting fork. There is of course a concern that heat treating may have been compromised.
I removed the shifting fork assembly and reinstalled it several times and after spending a fair amount of time to determine how the transmission worked, It dawned on me that the problem must be in the 1st and 3rd gear shifting fork Neutral position because only in 1st and 3rd gear was the problem gone.
To confirm, I moved the 1st and 3rd gear shifting fork slightly in both directions with the shifting fork assembled to the transmission and found that when moving the fork foward towards 3rd gear the gear meshing stopped. I even found that if I removed the one countersunk mounting bolt and moved the entire shifting fork assembly forward against a few mounting bolts that the gear meshing was gone.
I also checked the input shaft to see if it was tight in the housing, thinking that possibly the misalignment was not in the sliding gear but the mating gears. The shaft had little or no end play and there was no visible signs that the shaft had ever moved.
I removed the safety wired bolt holding the 1st and 3rd gear shifting fork onto it's shaft, but the shaft had a drilled location for the bolt, so no way to adjust the position of the shifting fork on the shaft. Even if there were an adjustment here, that would have misaligned the slot where the shifing lever fits into all three shifting forks.
It was the last adjustment I wanted to consider, but I decided to use the universal thermal adjusting tool (oxy-acetelene torch) to bend the 1st and 3rd gear shifting fork forward a small amount. After carefully measuring to determine the current alignment of the shifting fork I convinced myself that it was, if anything, misaligned to the rear and a simple bend would suffice instead of a compound bend to keep the shifting fork parallel with the grove in the mating gear.
After two minor adjustments totalling about 1/8", the shifting of the transmission seems to work well. Checking as well as possible through the fill and drain holes with the shifting forks assembled, the gears appear to align properly in all gears.
Having been around some of the earlier Thiokols that used this transmission as the only one in the machine, I was always surprised how rough some operators were trying to get the transmission into different gears, especially reverse. A person almost needs to see the transmission apart to have a feeling for the best way to shift without causing damage.
I hope this might help someone else in the same situation.
I would also be happy to hear if anyone has a better solution than heating up and bending the shifting fork. There is of course a concern that heat treating may have been compromised.