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Thiokol differentials

mtncrawler

Bronze Member
GOLD Site Supporter
OC-4, OC-12, and OC-15 steering differentials used in Thiokol Snowcats.
 

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Polar

New member
Thanks Mtncrawler! Those photos will help all of us who aspire to add another snow cat to our collection with easy identification. Perhaps another member can post contact info for parts/rebuild service for each of those rear differentials?
 

nutsster

Member
I would like to make a correction to the above statements.
The Imp uses a C-4 rearend, not a OC-4. The "C" stands for Clark, who was the manufacturer. The OC-12 differential was named "OC" for Oliver-Clark, who was also the manufacturer. I am not familiar with the -15 differential, so cannot comment on it. :confused2:
I hope this helps people looking for information on these differentials. I've seen a lot of confusion on how they work and how they are to be operated too. Some confusion is also found on the oil that should be used for band lubrication and heat reduction. Peterson's Equipment has probably done more repair and overhaul of these rearends than everyone else combined. In talking with them, they caution the use of lighter oils then what is specified. :confused: I have read the manuals and talked to operators for information on the proper handling of the steering systems. Let's get some comments from experienced people on how to operate and "not" operate the steering of these differentials.:thumb:
Thanks!
 

griz

New member
The first pic is interesting I have never seen an oc-4 diff with drop boxes. Was this to handle longer axles, more wieght for the Spryte?
 

mtntopper

Back On Track
SUPER Site Supporter
Drop axles will provide more ground clearance and change the gear ratios for better snow performance.
 

Snowcat Pat

Active member
The drop boxes for the OC-4 are chain reductions. The drop boxes for the OC-12 are gear reductions available in both 2.44 or 1.83 ratio.
I just finished rebuilding my 14th OC-12 and I think I'm starting to understand it.
I've tried several different oils in the last 18 years and I now use only Tractor Hydraulic Fluid brand Shell Donax TD, or sometimes Donax TC-30. Lots of these cats have a hydraulic system that combines the rear end oil with the hydraulic oil so it has to work for both. Change the oil often, when warm, and in the spring before storage.
Steering? Don't drag the brakes!

Snowcat Pat
Wallace Idaho
 

mtntopper

Back On Track
SUPER Site Supporter
Snowcat Pat said:
The drop boxes for the OC-12 are gear reductions available in both 2.44 or 1.83 ratio.
I just finished rebuilding my 14th OC-12 and I think I'm starting to understand it.
Snowcat Pat
Wallace Idaho

I am looking for a higher speed rear end set up for a late model LMC 1200 with drop boxes. A friend of mine has the low speed rear end with a top speed of about 10mph. I have been told the higher speed gear ratio will give you about 16mph.

Snowcat Pat, do you have any experience with the different gear ratios? Would you know a source and cost for the parts to do this changeover? My neighbor's home is a 12 mile commute each way and the higher speed would be used mostly on snow covered roads. We have looked at adding the overdrive that attaches to the back of the C6 transmission but that is only at best about 30% increase in top speed. The higher speed rear end set up along with the overdrive would make the 1200 LMC a 20mph cat without over revving the engine by my best estimate. Now, with this setup on snow covered roads would the LMC with the 300CI 6 cylinder pull that tall of gearing? :eek:
 

mtncrawler

Bronze Member
GOLD Site Supporter
OC-12 came with 2.44:1, 1.83:1, or as in the early 1200-B Spryte sometimes none at all. They also had ring and pinion ratios of 6.14 and 5.44. I don't know about the 1200-C Spryte but the A & B Models also ran both 9 or 12 tooth drive sprockets. The OC-4, or Clark Model S 70F-12, was available with 1.78:1 or 2.33:1 reduction in the A-Model Spryte's. Thiokol Imps did not have the drop reduction and instead used a 3 speed manual transmission upstream of the OC-4 in order to "gear down". A somewhat similar modification to some A-Model Sprytes is the addition of a Ford C-4 automatic transmission which allows the operator to leave the OC-4 in 3rd or 4th gear and let the automatic do the shifting. Differential output rotation is opposite with the reducted OC-4 so for instance if an Imp diff is installed in a Spryte the carrier must be removed and flipped over so the ring gear is on the other side of the pinion gear. Pat, all OC-4 reductions I'm familiar with are gears. Have you seen chain drive reductions?
 

Mainer

Boggie likes our museum
SUPER Site Supporter
Here's the back-end of my 61 Trackmaster. Just bought it and have it in storage...haven't had a chance to touch it or dig into it at all. Runs well as-is but I'd still like to do a chassis-off restoration. The previous owners went through a few 1/2 ton Ford pick-up back-ends and then went up to 1 ton and it's been fine. They pulled a 2,800 lb drag with it.
 

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Snowcat Pat

Active member
Mtntopper, Ive changed R&P's and reduction gears to suit what the customer needs. The final ratios are 9.96, 11.24, 13.27, or 14.98. You can figure your road speeds by the dia. of your sprocket. I once set up a Thiokol Spryte A that had a straight axle , steel encased OC-12, with the 14.98 ratio. The customer was hauling huge loads up steep hills and the gearing was not low enough. For this cat I added a 3 speed aux trans between the auto trans and the rear and it gave the low range for climbing, straight though for most work and a high range that gve it a top speed of 35 mph (down hill road). I changed alot of other things on that cat too (another topic). Last time I checked, R&P's cost about $1000 (new if they are still avail.) and a set of 2.44 DB gears are $1500 (used, w/exchange).

mtncrawler, Yes I have seen the chain reduction (I would not have said so otherwise) and had I once had a manual showing it, but right now I don't remember if it was a Spryte or an Imp. I chuckled when you mentioned the spool being reversed for the drop box rears. I ocasionaly get call from folks who rebuilt or had a friend rebuild their rear and they want to know why they now have 3 reverse and 1 forward gear!

Do any of you have a pair of OC-4 drop boxes with the 1.78 gears?

Pat Foster
Wallace Idaho
 

Snowcat Pat

Active member
"I once set up a Thiokol Spryte A that had a straight axle , steel encased OC-12, with the 14.98 ratio."

Oops, That should have read 6.14 ratio

I checked my notes on that chain reduction OC-4, I saw it at a govt. BPA boneyard way back in '85 and later in a manual that said they were on model 1404 Imp serial #'s 141-145.

I have only worked on the OC-4 without drop boxes, from an Imp. and those OC-12s from Sprytes and 2100s

Please keep this info coming, this is a great thread. I'm most interested in steering systems and suspensions for two tracK snow cats as I prepare to build my next machine.

Pat Foster
Wallace Idaho
 

Snowcat Operations

Active member
SUPER Site Supporter
Snowcat pat,
I have an OC-4 without drop boxes on my 1963 Thiokol Swamp Spryte. I am having one major problem and one minor. I am hoping you may beable to help. The trans is stuck in 2nd gear. I believe its 2nd anyway. The shifter moves fine but stays in gear. I suspect a broken shift lever in the trans. The second minor problem is the steering. pull the right lever and she turns with no problems. Pull the left lever and she turns but not like the right lever. So there are the problems any ideas? Hope you dont mind me asking.
 

Snowcat Pat

Active member
Hi Snowcat Operations,

I don't mind Q's at all.
You have been inquiring about this shifter problem for a while and I sent you scans from the service manual. Did you take the unit apart yet? I would do that first. Its the only way I can think of to determine if the shifter fork is broken.
The steering problem? Well I would not even drive the 'cat again until I had a look at the brake bands. If the bands, linings and drums are OK then a step by step check of the entire steering mechanism is required. Then also any differences in the left and right track and the surface you are driving on all can have an effect. Also differences in tire condition, size and inflation, wheel bearing condition, alignment of all the wheels etc.
Getting greasy, that where I start.

Pat Foster
Wallace Idaho
 

Snowcat Operations

Active member
SUPER Site Supporter
Snowcat Pat,
Thanks for the info. So you are the one I found up in Idaho. Thanks for those scans. I havent dug into her yet. Waiting till my current project is done. I was hoping for a easy solution of course (wishful thinking). I just started to think about the old Thiokol again since we have some new members bringing theirs back to life. Guess I will just have to wait a bit. Thanks for those scans I have them posted hear on the forums for another member.
 

Snowcat Operations

Active member
SUPER Site Supporter
My good friend Bob P. Found this Thiokol. Here is a pick of the rear end. Looks like an OC-12
 

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Bulldog1401

Anybody seen my marbles?
SUPER Site Supporter
Snowcat Pat, The lubricants you mentioned for the rear ends were tractor hydraulic fluid. I assume because of the fact that some machines mix the hydraulic system and the drive system like you said. The Imp does not. Are these lubes also correct for the oc-4? Thanks.
 

mtntopper

Back On Track
SUPER Site Supporter
With the OC4 rear end in the Imp, I have used and highly recommend the Shell Aero mineral oil that is used in airplanes. I used Shell Aero 50 and it works great in the Imp. You will have excellent braking/turning ability with very little wear on the brake bands. It came highly recommended to me from several people with a lot of brake steer snow cat experience. :thumb:
 

mtntopper

Back On Track
SUPER Site Supporter
I've tried several different oils in the last 18 years and I now use only Tractor Hydraulic Fluid brand Shell Donax TD, or sometimes Donax TC-30. Lots of these cats have a hydraulic system that combines the rear end oil with the hydraulic oil so it has to work for both. Change the oil often, when warm, and in the spring before storage.
Steering? Don't drag the brakes!

Snowcat Pat
Wallace Idaho

I agree with Snowcat Pat on using a good quaility hydraulic oil in the OC12.
 

Thiokol2track

Bronze Member
SUPER Site Supporter
I've used KUBOTA UDT (universal drive train) in several oc-12 rears, its for wet bands ,and for hydrayulics that use the rear for a reservior. I don't know if its the best oil, it meets the specs in japan..:confused2:
 

Snowcat Operations

Active member
SUPER Site Supporter
On my OC-12 pictured on page one of this thread you can see all the dirt attached to the oil on the oc-12. The whole top plate was loose. Which is why it was leaking oil. Each bolt needed to just ever so be tightened. There were a few real loose ones. My question is, is there a gasket that I can buy for the top or do I just use a tube gasket material?
 

Snowcat Pat

Active member
Well, thanks for asking. In a non-integral , no filter and no cooler system I like the Shell TC-30. Its made for some Caterpillar standard that is suposed to be good to their brake and clutch lining and not eat bronze (chemical reaction of some kind). I once read that Aeroshell had that problem but I think that was a long time ago. Same maker, go figure. TC- 30 is smoother than the Donax TD which will sometimes cause brake band chatter when the oil gets gets hot. If you have a system cooler thats not a problem. TC- 30 doesn't heat up as fast but is noticably thicker in cold temps. Thats why I don't use it in hydraulic systems because its too thick, though it is compatable. It steers good at all temps. They also make a TC-40 and TC-50. I like the extra protection of the TC-40 in the spring. A generic version is called Powertrain-30 (40, 50).

In a non- hydraulic system I've tried 50 weight engine oil and the steering is very sluggish untill the oil heats up, then it overheats. I don't think it lubes that well as it gets really foamy. I get more metal on the magnetic plug with it. I've tried 15/40 engine oil and it acts a little better than straight 50 weight. These are more expensive than Donax-TD or TC-30

The manual says a synthetic called Conoco DN-600 is recomended for the LMC 1500 and can say that it works great in them at about $200. per oil change plus filters. Mobil SHC 627 works well too, $300 per change. These oils are best for The late model machines with coolers, filters and combined rear end and aux hydraulics. In a non hydraulic system I would like to try Mobil-One engine oil as it about the same as the SHC formula but multi vis. though still spendy.

I won't use Type -F automatic trans fluid as is sometimes recomended. The stuff turns black and smokes at 180 degrees F, and is thinner than water, maybe it works good in Antarctica.

Finding that balance between good steering and low wear, thats all.

Anyone else?

Pat Foster
Wallace Idaho
 

Bulldog1401

Anybody seen my marbles?
SUPER Site Supporter
Thanks Mttopper. I have heard of the shell aero but was hoping for a source of supply that was more readily available. I have only seen it advertised on airplane web sites. If I have to I will buy it there but hoped for something I could get at tractor supply or something. Thanks.
 

Snowcat Operations

Active member
SUPER Site Supporter
Now here is another dumb question. How often do you change the fluid? Once a season is over? I figure on 600 hrs per season.
 

Thiokol2track

Bronze Member
SUPER Site Supporter
Snowcatops, There are no dumb questions....just dumb mistakes. :rolleyes: Looks like you are adding yet another thiokol to your collection !!! Congrats!:applause:
 

samwe

Member
Here's the back-end of my 61 Trackmaster. Just bought it and have it in storage...haven't had a chance to touch it or dig into it at all. Runs well as-is but I'd still like to do a chassis-off restoration. The previous owners went through a few 1/2 ton Ford pick-up back-ends and then went up to 1 ton and it's been fine. They pulled a 2,800 lb drag with it.

How does that work? I assume there is an engine, and I see 2 coupled rear ends, but I am lost after that.
 

Mainer

Boggie likes our museum
SUPER Site Supporter
How does that work? I assume there is an engine, and I see 2 coupled rear ends, but I am lost after that.

Driveline sequence is: Engine to transmission to power divider to twin drive shafts to drop boxes to tracks.

These were more maintenance and slightly more challenging but not that much more so than the updated single-shaft model. The Air Force used these successfully in the field and made the decision to distribute them to several locations around the globe...that's for this twin-shaft design after testing and use in North America.

So, the diffs we see on these units have complexities and costs and this soluiton does as well. I have both and they are just both very interesting! You have a piece of history which you may wish to update ... or leave original but if you're changing a bunch of other gear out then it may not make any difference. The power divider uses 24 clutch disks to achieve the slippage/catch necessary. It's the same type of system in the old crawler tractors.

Here's a diagram.

Good stuff!
 

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socal4t12

Member
need to chat with you about this rearend configuration. ive got a 1961 thiokol 4t10 with a similar dana 70 set-up. i need help.
 
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