How Much Gear Rust Is Too Much

This evening when I pulled a side cover off one of my spare M37 transmissions I discovered it had taken on some moisture and therefore some of the gears have picked up some surface rust.

It seemed to come off with WD40 and steel wool with no apparent pitting BUT, there is some discoloration on the teeth. It's hard to tell exactly what damage was done without stripping the transmission. .

I've always blindly assumed any amount of rust would ruin a gear but maybe not???

A few years ago I had a Jeep transmision that picked up some water that pitted the gears so bad it looked like pieces of chrome flaked off the teeth --- it was toast.

This M37 transmission doesn't have the pitted chrome signs, just discoloration although closer inspection may reveal some minute pitting.

Where do you draw the line on what is acceptable and what isn't?

I would render the thing still usable. No pitting is great- the discoloration is prolly just the 'stain' leftover from the rust etching, so to speak, the gear. the m37 should be soft enough on power to not have that be much a concern.

Most gears, and bearing races too, are surface hardened and the depth may vary by process; i.e. flame hardening is not as deep as case hardening as a rule. A good way to tell is; if the gear teeth have lost their bright sheen and are starting to look kinda frosted, then they're worn through the toughened surface and will begin to wear faster. Doesn't mean you can't get more miles out of the box but it would probably be smart to find a replacement to put away for the day when you're tired of listening to the increasing gear howl and decide the old box finally has to go. From best to worse it goes; Bright gear tooth meshing surfaces, discolored surfaces, frosted surfaces, pitted surfaces. Chipped or missing teeth is a problem with the clock ticking; once things get that bad it's easy to do major damage if a broken tooth finds it's way in between two good, tightly meshing teeth.
Another point to remember is the color of the discoloration; Rust or brown discoloration is not anything to worry about and may even polish itself out as the box is run. HOWEVER, if the discoloration is straw gold to blue in color, it's from the gear being run hot and if that's the case then the gear or shaft should not be trusted to give too many more miles before failure due to the gear having been heated to the point of softening. If a manual gearbox runs around, let's say 250 degrees, than it's about halfway to the 500 to 650 degree annealing temperature of steel; the temperature at which carbon steel starts to lose it's hardness (temper).
With bearing races; you definately need to change out if they have a frosted look to the surface where the rollers or balls ride. The extra clearance will let the bearing chatter and that tiny but steady hammering will further pound it out.

Thanks for the insight.

This is some valuable information about a subject thats always been evasive.

My experience says run it and do not worry. I have been into lots of old tractor transmissions, etc. that have sat idle for a long time and had worse rust than you describe. They had no problems even when worked in the field. I would wager a good bet that many people own vehicles with worse rust on the gears they are driving and do not know it, having no problems. I bought a set of 4.10 gears for a Mopar rear axle that had a ring gear with rather deep pitting on the pressure side of the teeth. We ran the gear set for four years behind a 440 six pack motor pushing a 4800 pound car into the 13.4's in the quarter mile with no problems. The set is now in another car with a 440 with dual four barrels. That should be a good test, and proof of what I am saying. You might want to change the oil in the trans after say 1000 miles to get any rust residue out, then refill with good oil and add a can of STP. There are those who will say STP is a waste of money, but again I can tell you from experience. they are not right.
Regards Marti

My CJ5 had water in the front when I got it. Enough to kill the bearings. The gears were coated with rust. I cleaned them and put in new bearings and a locker. About 15 years later it is still going strong. It was the front axel but I use it about 40% of the time when driving. A lot of that is on road. I pulled the cover about 2 years ago and the teeth had shined up and did not show any weird wearing.

Yep, same with mine. The transmission I currently have on my truck was full of water at a stripping yard (Howard's yard in KY). I rebuilt it-replaced all the bearings, wire brushed the gears and case. It is still running and performs great.

Several years ago I found a set of gears for a cat 631 scraper laying out in the weeds at a customers yard. We beadblasted the gears up and put them to use in a water tanker. Diff ran 5000 hrs until a brg failed from no lube. We reused the gears. .Shined up like new. Just change the oil a bit more often.