I have an old 49 Farmall Cub that I am fixing for my father. I replaced the bearings in the transmission. The old manual I found online says the countershaft bearing retainer should be torqued to 35 ft lbs. The top clutch shaft retainer says to torque to 20 ft lbs. Before I new this I torqued both to 20 ft lbs because of a bolt size/torque diagram I found said this is the proper torque for a 5/16 bolt. Will I have problems? Why would there be different values?
The stronger the grade of steel a bolt is made from the higher the torque it can be done up to. 35 ft lbs is a bit steep for any grade of 5/16" bolt though. I've never seen anything that size in an ordinary engine needing more than high
20s ft lbs. A grade 12.9 high tensile metric 8mm would perhaps just stand that much torque but you don't usually find that high a grade except in aftermarket bolt sets for race engines.
30 to 35 ft lbs is about what I'd associate with a normal 9mm high tensile big end bolt (grade 10.9) and perhaps 24 to 28 ft lbs for an 8mm one.
No point using a bolt chart if you don't know the grade of steel of the bolt and also the strength and thread length of the parent material it's screwing into.
Here is an interesting chart listing torque values for Holochrome SHCS's, which does call out 38 ft lbs for a 5/16" fastener. Seems very high to me, but there you go. I've recently started using Holochromes for engine assembly work on my race engines, since you can get them in a bright plated finish and they look stunning on a new race motor. I just ordered some, about 25 cents each for 5/16" by 3/4" which isn't cheap, but customers love a great looking motor, I find. I still use ARP for the critical fasteners.
The 35 ft-lb figure may be intended for high-strength bolts, although if they're threaded into a casting it's not likely. Did the bolt torque chart specify coarse or fine threads? Coarse threaded bolts will often have lower figures than fine threaded. Are the higher-torqued bolts loaded primarily in tension, and the lower-torqued ones primarily in shear? Aircraft bolt torque charts will reflect that difference. I had an old Farmall Cub I sold 12 years ago. Still miss it.
I won't claim to know this specific application, but the 35 ft/lb sounds high for a 5/16, when you consider that our torque specs for a
3/8"-16 were 35 -42 ft/lb, grade 8 bolt.
We had some torque specs in certain applications that seemed much higher than one would expect, and the auditors could be quite picky about them. As far as I can remember, the only 5/16 bolts we had were the oilpan on the engines, and they were somewhere in the 14 -18 ft/lb range. (With a note to be retorqued after enine test, something that I never remember seeing done. The pan gaskets compresssed and the torques would go down below 10 ft/lb, but never seemed to come out on their own.)
I am not sure what grade bolts were in there but I put grade 5 back in. (new bolts) The threaded portion the bolts go into are in the transmisison casing itself which i assume is cast iron. I was more afraid of stipping those out than anything.
I recall that farm and industrial stuff often uses pretty good quality fasteners. if a torque call-out of 35 ft/lbs is in the book, then a grade 5 bolt is not adequate. I would use L9 or Holochrome type SHCS in tension for that kind of requirement. If this is holding something on that you want to stay on, then you pretty much have to do it right.
If you think you know more about the engine's design and requirements and bolt theory than the guy who designed the engine then by all means carry on. If you replace high tensile bolts with pieces of shit and do them up to half the required torque and it all falls apart don't be too surprised.
However, having bits of knowledge that you may not, having spent some
12 years working in Case transmission plant, and he DID say "in the transmission", here goes. Rule number one, tractors, especially old ones are WAAAYY overbuilt. Bearing retainer, which I will assume is a retainer holding the bearing in a blind bore, or a bore/ counterbore in the transmission housing, and probably four or more bolts, is probably overkill to the extreme. I believe he said it was the intermediate shaft, and the only thrust forces on it are going to come from the helix of the gears, assuming they're not spur cut gears. The
35 ft/lb sounds like it was intended to be self locking, no lock washer, not a common practice, but IH did a lot of things that were not common practice.
A further guess, the retainer is cast iron, and the bolt pads are probably about the same thickness ad the bolt diameter, meaning that if anything breaks, it will be the cast iron beneath the bolts. Also meaning that the weakest point, even with grade 5 bolts is going to be the cast iron. I'd have to say that if he used a lock washer to prevent it backing off, torqued to the 20 ft/lb, it will probably last longer than what is going to be the next thing in the transmission to go, more than likely a bearing somewhere, or the teeth off a gear. Tractors of yesteryear were made with being able to field service them in mind, and they knew that more than likely the service was going to come from a farmer with hardware store parts. Chances are that the bolt size and length were picked to be able to use something they already used in an other application, if it was a higher grade, in the lots they buy in, price difference isn't going to even make a dent, where having several different similar bolts with different numbers will make keeping the inventory cost more than using a higher grade than is really needed. This applies to older tractors, the newer ones, manufacturing cost is a higher priority than availability, reliability or serviceablity. Then we wonder why the small farmers are selling out in droves.
When you've looked at some 40 transmissions a day, five days a week, for such things as split lockwashers, missing bolts, etc., for more than 12 years, perhaps I'd give you a little credibility on that. The favored retainers for bolts inside the transmissions were actually keepers, the kind you bend up to hold the head, second to that was the lockwasher recessed into a counterbore, last one was a star washer. Safety wire in most apps inside the gearbox is not cost effective, takes too much time fishing for the bolt head you can't see, and loctite requires parts free of oil. Parts free of oil are usually not free of rust in a very short time.