The Stovebolt Site Forums The Shop Area Engine Shop Differential torque specs

I tried to find this on the search, but did not find the correct combination to trigger it.

What are the torque specs for the differential bearing cap bolts, and the cover bolts, for a 1939 3/4 ton pickup?

Thanks,

Gary

I have never seen torque specs on them. I think its probably the same rear as the early 50's. I usually give them 90 to 100. I try to get both the same.

Crenwelge,

I've dug thru as many of the old manuals as I can find on line, and none seem to list them. I did find one listing that says 55/60 for GM products, but it didn't specify which, and I suspect there are differences. I don't want to apply too much, and bust one off. I also have specs on bolts, and for different grades, but I don't know the grade of the bolts in the rear end.

I do appreciate your efforts, though.

Thanks,

Gary

Ah, the lessons we learn. I did my best to find the correct specs. They are all over the map. The bolt size, assuming it's grade five, calls for 55 lbs. I thought that would be safe. I also checked the depth of the bolt hole just out of curiosity. It's much deeper than the thread length. I even install new washers just to make things right. The first bolt clicked at 55 lbs. On to the second. It seemed to be tightening oddly, but it is a good torque wrench, so I continued. The darn thing busted in the hole.

A few choice words, and some head scratching, and I finally got the cap off where I could see the bolt. A slight bit protruded. I whacked it good with an impact screw driver. Much to my surprise, it came loose, and backed out.

Then on to the forensics. What I discovered was that the threads do not go to the bottom of the hole, and the new washers are not as thick as the old. The threads bottomed out, and the bolt was not torquing on the head. She busted, her.

So...I dug around, and found a good grade 5 bolt almost long enough, and dug through the pile, and found the old thicker washers. All is now in place and torqued to 55 ft lbs. I know that's on the light side, but I doubt I will be so lucky on the next busted bolt.

This is, by the way, my first differential experience. I now have a dial indicator, and know how to use it...sort of. Curiously, the differential was almost spot on after 71 years of mucking around the country. That speaks well for the engineers of olden times.

It really surprises me that it broke at 55. I always gave them around 90 to 100 and have never broken one. I don't know if I'll ever set up another 3/ton rear end, but I will keep the torque figures in mind.

Cren,

I think it was an anomoly. When the bolt bottomed on the end of the threads in the hole, and the head had not bottomed, it torqued on the stem of the bolt, and not on the base of the head, as it should. The stem subsequently twisted...until she broke, her. I was quite fortunate that a stub protruded, and it came loose with a good whack from the impact screw driver.

I think the issue is the depth of threads, and length of bolt, more than the actual torque pressure.

I would be happier if I could learn the grade of the originally installed bearing cap bolts, but that kind of info seems most elusive. I can't find bolt grade, and I can't find torque specs.

I looked in the parts book and it calls for a 9/16-12 x 3-3/8 with no grade specified. 90 to 100 lbs. may be too much torque for the application, but the bolt shouldn't have broken off because that is about normal for a grade 5 bolt. 3-3/8" length is a special bolt. I wonder if someone didn't put in a 3 1/2" and it bottomed. Or the last person to have it apart had an impact and grossly over tightened it. We have this happen quite often. Then when you turn them the other way, they break. All I have messed with in the last 30 years is rears for semis. They use grade 8 bolts for the bearing caps. I don't think a grade 8 would hurt on a 3/4 ton.

Cren,

The bolts in my unit where 7/16". It is possible that they had been changed, but I doubt it. All looked pretty much factory. I'll pull up my sheets, and see what they say. I think they are in the shop. It may be that the JE has a larger diff than the JD. I have it all buttoned up, and full of new fluid, so I won't be looking at the bolts again...I hope.

I just finished shackling the frame back on the axles. I will do the wheels/brakes, and then start on the engine. I have a fresh '58 235 sort of spread all over the shop. After I get it together, I will have to worry my way by the transmission. Somewhere in there, it will go into the frame.

night

There may be a difference in Chev and GMC. I was quoting out of the GMC 1939-1954 parts book and they call for 9/16" bolts. Out of curiosity, I looked in a Chev 1929 to 1953 parts book and it refers to the standard parts list of common bolts and doesn't give a specific size. I guess you just match up what you had.

In a lot of ways, the engineers of olden days really did have it all together. Much of what we find on the old rigs is really not that much different from modern stuff. I find it curious that there is a big gap, when it comes to the bolt specs and torque specs. It may be that they did not yet have the importance of that all straight.

Right now, I'm working on the brakes. Have you replaced any of the funky little slip clip things with modern circle clips? I see no reason why it won't work, the funky slip/bend clips are spendy...and probably should be replaced. I'm going to look for some, manana. I have the second set of brake shoes awaiting me. 20 bucks for a set/axle. Probably not the best material, but a quite decent price, I think.

I use the originals as long as they still feel like they have metal left in them. If not, I have used plain old external snap rings. Its probably not recommended, but I'm from the generation that you do what you got to do. I am assuming you have Huck brakes. My preferences is to replace them with Bendix brakes of the mid 50's.

crenwelge,

I, too, am from that age, and from that remote ranch. I picked up a few snap rings (what the store had in stock), and will look over the old push/bend things. Some are reusable.

I never thought of up-grading to Bendix. Probably a good idea, but I have the Hucks, now, and they will last a long time. They may out-last me.

I have been honing the master, and it is currently about 18 thousandths over. The piston with rubber almost falls through. I got a brain storm that chroming them might be the answer. Both shops in town have stop the chroming process, and I was told that it would cost as much as a new mc.

I will compare prices, and order one. Too much time wasted already.

night

I don't know about chroming them. They can be sleeved which is a permanent fix. If new master cylinders are available, its probably your best bet. Its always one of those decisions. You have to keep honing until the pits are all gone or it will fail right away, but If its .018 too big, you will worry about every time a tree jumps out in front of you. .018 is only 2 thou less than exhaust valve clearance on a GMC.

Yes, I am pricing new master cylinders. As to the funky little slip-on clip things, I just noticed that Chevs of the Forties lists modern circle clips along with the original style. I guess it's acceptable...except to the purists. Mine will be a mixture of both, and which ever is closest at hand. I have worried one set of shoes on the rear, but without the cylinder. I am gaining.

I also see a '53-'54 "Double Duty & Hydramatic" listed that looks a lot like the mounting pattern on the '39, and is a bit more than 1/3 the price. I will dig through Napa and Advance Auto, and see what they have to say about it. It may be a much cheaper options, and it may be...wrong in some way. I will find out.

Gary