View Full Version : Gear Repair - how?

07-01-2008, 06:20 PM
I've got a couple of these transmission shafts with pitted flanks on some of the teeth. What are the options for repair? They're case hardened originally, and perhaps wouldn't need to be again since these transmissions realistically aren't going to get another 50,000 miles on them. Would hard facing be feasible or advisable?

They could be milled or ground after surface repair, but I'll have to cross that bridge later.



07-01-2008, 08:21 PM
Are those gear teeth? They look like a spline for some kind of slider.

07-01-2008, 09:11 PM
When bits flake off the load surfaces like that it means that the metal is failing by fatigue. I've never heard of any way to repair that.

J. R. Williams
07-01-2008, 09:47 PM
Grind off the old damaged teeth and build up the area with a good weld and machine new gear teeth and case harden. The next question is - what is the condition of the mating parts?? I doubt they are any good.


07-01-2008, 10:19 PM
Are those gear teeth? They look like a spline for some kind of slider.

That's first gear on the layshaft. It's possible that the transmission shucked a bearing and some of the trash did some damage though the other gears I removed don't show similar problems.

I'm trying to avoid re-heatreating the whole thing because I'd like to avoid warpage if possible. The seating surfaces for the other gears are already ground.

They haven't made these since the mid fifties, so a new replacement isn't an option, and since they're getting more scarce I'd like to figure out how to preserve this.

07-01-2008, 10:35 PM
Just fill the transmission with sawdust and oil. It'll run nice and quiet.:D

07-01-2008, 10:37 PM
It may be one of those applications where you would have less trouble by leaving it alone. It has been working this way for a few years and it is still performing okay? Are you going to "fix" it or just create another problem that is more serious? As is teh gears may be less than ideal, but still more than adequate.

07-01-2008, 10:53 PM
I have to agree with the let it alone. I believe that wear is from pressure and/or neglect. It took fifty + years to get there, it should last as many more with care and perhaps the benefit of some moly or similar additives to the gear oil.

I doubt there is anything that can be done in the way of building up that would not create more problems than solutions. What is the transmission from?

07-01-2008, 11:15 PM
I'd agree with the "leave it alone" advice unless there are compelling reasons other-wise.

If it were me, I'd have it "metal-sprayed" where a whole range of materials are available. This spraying can be both very precise and effective with minimal heat or other effect on the shaft.

Applying heat would be my last option as it may create more and worse problems than it solves.

There have been others here in the past who are really well-informed as regards metal spraying.

This "Google" list may help.

07-01-2008, 11:17 PM
I'm with the "leave it alone" crowd, but I'd also run synthetic oil in it when you put it together. Use a magnetic drain plug if possible and change the oil regularly if you use it at all.

07-01-2008, 11:17 PM
I doubt that putting more metal on top, sprayed or otherwise, will do much good. The metal underneath will continue to spall out.

It won't work any better than putting a fresh coat of paint on top of rotten wood.

07-02-2008, 12:44 AM
Leave it alone. Juergenwt

07-02-2008, 01:13 AM
Build a new shaft and gear out of titanium.

:D I just had to through that in after reading all of the "leave it alone" replies. Looks to me like alot of work that could go horribly wrong to repair a minor issue. I hate putting stuff back together that hasn't been properly fixed, but sometimes you just gotsta do it!

If it is something you absolutely need replaced and back to original condition, what about calling up a gear company and getting a bunch of those gears made and then design and turn the shaft and make it into a two piece assembly. Not sure how complicated the rest of the shaft is, but the parts that I can see could be whipped out between a lathe and a horizontal mill pretty fast.

Like others have said, that flaking is very bothersome. If the teeth were just uniformly worn, building up the surface could work but seeing those flakes indicates fatigue failure like rantbot said. We see it all the time in our FSAE cars. Its bad on case hardened surfaces. What happens is either caused by many many hard accelerations or caused by putting too much power through the gear during an acceleration and the teeth flex just a little bit. When they flex a little too much, the hard case shell cracks and then the next time away the chip is pinched out and it leaves that cragly surface behind. To me, its an indication that the whole gear has had a rough life and building it up may not help much.

Bob Farr
07-02-2008, 02:14 AM
That looks like a Triumph part. If so, I can give you a couple phone numbers to sources in Michigan that might have a better looking replacement for a lot less work and money than welding, milling and heat treating will take. Drop me a PM if you want the info. Bob

07-02-2008, 11:15 AM
The part is a Jaguar XK-120/MK VII type JH transmission shaft so one could expect that drivers will push them on occasion. Unless I'm sure the car will be owned by a little old lady from Pasadena who plans to be buried in it, I wouldn't feel right sending out a rebuilt transmission that I know has "issues". I've wondered too about replacing just the gear, but it's a hollow shaft with a needle bearing just under the gear so there's not a lot of stock to work with.

Reverse engineering and making it new might be an option. I've been doing machining (lightening gears) for a friend who does the rebuilding. He thinks rebuilds with a little extra work can command a couple thousand dollars in the market so there might be room for a few hundred spent on the shaft.

Actually, if anyone knows about any of these transmissions just lying around I'd appreciate a pointer. It would seem that there ought to be a small surplus of transmissions after engines have been blown up or cars wrecked, but so far I haven't stumbled on that particular elephant graveyard.


07-02-2008, 11:43 AM
I think you would do a lot better to deepen the annular groove a bit instead of drilling those holes for lightening. A deepened undercut won't make a difference in that sort of gear but those holes may give rise to stress concentrations a bit too close to the teeth.

07-02-2008, 12:09 PM
i'd say leave it alone. At the most, I'd send it out to have it shot-peened and then run something like Bel-Ray 140 transmission fluid in it. Unless you are drag racing, you probably won't ever have any trouble with it.

The way I see it, the "best" is the enemy of the "good."

07-02-2008, 12:15 PM
Another consideration is that it is quite likely that the tooth form is not a standard involute gear. Auto transmissions can be a law unto themselves, and even if a successful overlay of some sort could be added in the worn area, duplicating the original tooth form will be problematical.

Perhaps some of out British correspondents can offer a source of repair parts.