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Clark S70F-12 Shifting Problem

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.
 

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IMP

Member
Site Supporter
At this time I have my IMP transaxle removed and shift cover plate off. If there is any measurements you would like me to make for you off mine I can easily do that. I do not have any of the issues you described happening in my gear case. Accept the one time I had this Imp in 3rd gear it popped out after a short distance. But I think my problem lies in the linkage external to the gear case. I have only driven my IMP a few miles total and I am now in the process of restoring it. Just let me know if there is anything you want me to check.
 

GSSD06

Member
Hi Imp, and thank you.
I should have written down the dimension between the 1st-3rd and 2nd-4th forks, and the gears in neutral but I did not.
My transmission is back together now.
If you do take any measurements I would appreciate them so I can document them and compare the next time I have the transmission apart, especially in case it does not work as I hope.

Another concern I have is what looks like a non-standard modification to the shift selector lever to allow it to move far enough to get into reverse. It looks like it has been done by hand with a grinder. Attached is a picture of it.

Does your shift selector lever have a similar relief?

My transmission would not shift into reverse unless this relief was installed facing towards the differential.
 

Attachments

  • Clark S70F-12 Shift Selector Lever.JPG
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  • Clark S70F-12 Shift Lever Assembly.JPG
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GSSD06

Member
Imp attached is a zipped AutoCad drawing of the gasket for the Control Cover.
If you do not have AutoCad and want a template for the gasket, let me know what format you want it in and I will see if I can generate one.
I put gasket material in my printer, printed out the drawing, cut it out, and it fits very well.
 

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  • Clark S70F-12 Control Cover Gasket.jpg
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  • Clark S70F-12 Control Cover Gasket.zip
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Bulldog1401

Anybody seen my marbles?
SUPER Site Supporter
You must have one hell of a printer to have it accept and move gasket material through it! I would have had to make it on paper and trace it onto the gasket..
 

Snowcat Operations

Active member
SUPER Site Supporter
Excellent post. I will be pulling my OC-4 c4 transaxle this summer. Its stuck in 2nd gear and the shifter moves back and forth with resistance but nothing shifts. I will be pulling the side plate first.
 

GSSD06

Member
Hi Bulldog I don't think it is anything special, just an HP 7130xi all-in-one scanner/printer/fax. I was not sure it would work that well, but I reverse rolled the material to get it to be flat and it went right through. The gasket material I am using is cardboard brown about 1/32" thick or so.

My large format printer is broken or I would have made the S70F-12 rear cover plate gasket too.
 

GSSD06

Member
Hi SnowOps have you looked at the shift selector lever to see if it is moving left-right AND forward-back?
Is there a chance that the shift selector has slipped out of the slot between the forks?
It is really easy to pull the shift lever out and you can see pretty well down into the hole to the slots in the shifting forks.

Be careful not to lose the shift lever registration pin down into the hole.
 

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  • Clark S70F-12 Lever Hole.JPG
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  • Clark S70F-12 Lever A.JPG
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  • Clark S70F-12 Snap Ring.JPG
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  • Clark S70F-12 Rubber Cover.JPG
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Snowcat Operations

Active member
SUPER Site Supporter
I disconnected the main shifter from the trans. I now have what you are showing. The small 6" shifter or so.

I will pull the shifter out in the morning and check. You have my curiosity wondering now. If it looks ok I will then pull the side cover off.
 

GSSD06

Member
I am able to shift the transmission by hand with just the short shift selector lever, made easier of course because I can rotate the input shaft to align gears.
If it is stuck in 2nd gear, then the inside fork (looking at the top of the forks through the shift lever hole) should be moved towards the rear of the snow cat. The center fork is reverse, so it sounds more likely one of the two outside forks might be stuck forward or back.
Good luck on what you find, I am very curious to hear the outcome.
 

Snowcat Operations

Active member
SUPER Site Supporter
OK. I pullled the shifter and Now I dont think it was in the slots. I pulled the side cover and Unfortunately I cant get out because of the body. BUT I dont think I need to. Im gonna reach in and shit her in Nuetral with my hands (Tracks will be chocked and wedged plus im on level gound). I will then aligne everythinhg in nuetral and see what she does. What diff fluid are you using? And to what level do I fill her up to? Thanks for the help. I'll be back in a bit to post picks and update you.
 

Snowcat Operations

Active member
SUPER Site Supporter
I out everything back together thinking I had it all the shifting forks lined up and the gears in nuetral position. I didnt. BUT now she shifts into reverse. So I will hold off and just yank the whole rear diff out when I have a clean shop to work in. THANKS!!!!!! I can fix her myself if nothing else is screwed up that is. I think when dumb shit transport scumbag jamed her in second I believe the shifter came out from the forks. I really needed to see what it was supposed to look like before I opened her up. Your pictures were excellent! Thank you. HUGE huge help!
 

Snowcat Operations

Active member
SUPER Site Supporter
pics

The last are showing the shifter (all sides no cut out for reverse)
The last one is of the tranaxle in reverse position.
 

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downriver david

New member
Having a shifting problem

Hi everyone. If you take a look at "arcticracer" pictures (Jan 2006) of the snow trac rescue from the ice in Alaska, you'll see the rig that I'm rebuilding. I changed the engine, and kept the same transmission. It's just about ready to go back to our cabin, but I'm having trouble keeping it in first gear. The shift pattern seems to work when not moving, but when I shift into first it pops out of gear. I'm not sure if I'm making a mistake in installing the plate where the shift level attaches. Can anyone help with this? Am I installing it wrong, or could the first gear teeth be worn?

thanks for any ideas

David
 

IMP

Member
Site Supporter
GSSD06, Here are the measurements, Hope its what you wanted. If not let me know. The shift plate gasket would be great. Can you save it Paint Shop Pro? My 6" shift arm had no grind marks in it
 

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  • Shift11.jpg
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GSSD06

Member
SnowOps, this picture of your shifting fork assembly looks like it is in 4th gear.
When you re-assembled, did you have all the forks centered and the three forks engaged with each respective gear?
If you assembled as the picture was shown and all three forks got engaged with their proper gear, then the shift lever would need to be inserted in the displaced 3rd-4th gear slot.
 

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  • SnowOpsDSCF0005.JPG
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Snowcat Operations

Active member
SUPER Site Supporter
That picture was taken just to show the shifters. I had all the shifters in Nuetral when I placed it inside. I had to have it straight up and down to get everything aligned in nuetral plus I was able to check it from the shifters hole. Where I messed up was inside. I didnt get the shifting forks into the fork grooves on the gears. Not much room to work with and I was doing it by feel. I will give it a shot again Monday. What oil did you use to put in her when you where finished?
 

GSSD06

Member
Thank you for the measurements Imp.
I have Paint version 5.1 that came with Windows XP
Try this zipped bit map image imported into Paint from AutoCad exported WMF
If it does not work, let me know and I will try something different.
 

Attachments

  • Clark OC4 Shift Plate Gasket.zip
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GSSD06

Member
Hi SnowOps, I used AeroShell Oil 100, "straight mineral oil for aircraft piston engines", "SAE 50 Aviation Oil" that I got at the local Shell distributor. It was in plastic one quart containers just like motor oil for the car.
On the opposite side of the transmission case from the shift plate is a plug you remove and fill until it reaches that level.
It is hard to see when mounted in the cat, but there is an arrow and a label indicating to fill to that level.
I have four quarts left from the last time, but I am not really sure I bought a case of 12.
I will be filling it up again in the next week or so and will keep track of the required volume.
 

IMP

Member
Site Supporter
GSSD06, The gasket image should work just fine, I just need to shrink it to the right size. Thanks and good luck
 
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