1992 Mack RD688s Steering Gear Drama

[KP2]

The steering gear was leaking by the input shaft.  I was told I could take the top off and replace the seals.  After removing the top I discovered the shaft had a groove in it.  On these forums the only option after finding the groove I saw was to buy another gear.  I tried putting JB Weld in the groove and turning it back down on a lathe, installed new seals.  It held for a few minutes then started leaking again.  Removed shaft again, installed wear sleeve and new seals.  I ran it about two hours and it started leaking again.  I searched for days and could only find one gear through Watts Truck.  Weller would reman one for me but they had to have mine first so that would be a drawn out deal.  So I have ordered the one through Watts.  It was $2500.  I called a few salvage yards and could not find one.  Several places I called wouldn’t even bother to answer the phone or call back after I left a message.

I had considered taking the shaft to a machine shop, have the groove welded up and then turned down but was not sure about the integrity of the shaft after doing that.  Anyone ever done that before?

Is there anyone on these forums with a RD688 that has had this same experience?  If so what did you do?

 

[mowerman]

if the 2500 includes shipping your not going to do much better i found an outfit in florida that sells rebuilt for around 900 plus shipping another 900 depending where you are is it a sheppard or a ross box???

[Vladislav]

Fixing the shaft condition could be the way to go. Lathe work is Ok if you dial the shaft's center well enough before cutting. Grinding in a machine is much better and preferrable option. Welding is no of good options since the shaft is (should be/ could be) made of high carbone steel. Which doesn't weld well and could easily crack through in the welded area later in service. So application of material (metal) on the area could be halvanic (chrome) or ultrasonic metal powder coating. I don't know which kind of technology could be found in your neck of the woods but strongly advise to get off of the idea to weld up.

Also an option might be cutting/grinding the shaft over the worn area to lesser OD. Than locate a seal with lesser ID (and the same mounting size) using a chart or catalog. For example Timken offers a wide range of industrial seals and their catalog is on-line. Not almost correct way of fixing things but could be applied.

[Geoff Weeks]

If it is a Shepard gear, I think the correct repair is to cut the shaft to a smaller OD and shrink a sleeve on where the seal rides. You could possibly have the sleeve made from hard chrome stock, making the repair better than the original. 

These repairs need a qualified machine shop that can do the work. It would take tooling that can cut the hard shaft and sleeve.

I was working on a car that had some brake parts that are NLA anywhere, my machine shop made a repair like the above and was surprisingly reasonable. You just have to think "outside the box" and remember anything made by man can be re-made by man.

 

[KP2]

10 hours ago, mowerman said:

if the 2500 includes shipping your not going to do much better i found an outfit in florida that sells rebuilt for around 900 plus shipping another 900 depending where you are is it a sheppard or a ross box???

I think with shipping both ways, they want the core back(1200 core deposit), it will be about 2700.  It is supposed to be a Sheppard.  The salesman said it was a genuine Mack reman.

[KP2]

8 hours ago, Vladislav said:

Fixing the shaft condition could be the way to go. Lathe work is Ok if you dial the shaft's center well enough before cutting. Grinding in a machine is much better and preferrable option. Welding is no of good options since the shaft is (should be/ could be) made of high carbone steel. Which doesn't weld well and could easily crack through in the welded area later in service. So application of material (metal) on the area could be halvanic (chrome) or ultrasonic metal powder coating. I don't know which kind of technology could be found in your neck of the woods but strongly advise to get off of the idea to weld up.

Also an option might be cutting/grinding the shaft over the worn area to lesser OD. Than locate a seal with lesser ID (and the same mounting size) using a chart or catalog. For example Timken offers a wide range of industrial seals and their catalog is on-line. Not almost correct way of fixing things but could be applied.

Thank you.  I was hesitant to weld then cut down the shaft because I thought it may weaken it.  I thought by putting the wear sleeve over the worn area and a new seal would do the same as cutting shaft down and putting a sleeve but I have no experience doing machine work.  

[KP2]

7 hours ago, Geoff Weeks said:

If it is a Shepard gear, I think the correct repair is to cut the shaft to a smaller OD and shrink a sleeve on where the seal rides. You could possibly have the sleeve made from hard chrome stock, making the repair better than the original. 

These repairs need a qualified machine shop that can do the work. It would take tooling that can cut the hard shaft and sleeve.

I was working on a car that had some brake parts that are NLA anywhere, my machine shop made a repair like the above and was surprisingly reasonable. You just have to think "outside the box" and remember anything made by man can be re-made by man.

 

Thanks.  It is a Shepard gear.  I really don’t know if the machine shop in my area could do it just right.  I know they cut down shafts for tractors but I felt more at ease getting the reman.  I have had other parts made but for some reason my gut said go reman.  

[Vladislav]

6 hours ago, KP2 said:

Thank you.  I was hesitant to weld then cut down the shaft because I thought it may weaken it.  I thought by putting the wear sleeve over the worn area and a new seal would do the same as cutting shaft down and putting a sleeve but I have no experience doing machine work.  

Right machine work presumes sharpness and accuracy with tolerances. Your bad experience with that speedy sleeve is the most prbably due to its imperfect fit. Which could be predicted by its very imperfect shape and even the design. A thin wall tube pressed over unevenly worn surface doesn't seem supplying perfect surface for a seal to seat against and work well. I honestly am surprized at all this technology is count as "good" and so widely used. And at the same time I heard from multiple guys that they use such a sleeve and got leak back again soon.

As less excentricity of the spinning surface is as less deformation of the seal's rubber is required to keep tightness. It's more actual for higher revs applications though so doesn't seem the matter of your issue. Possibly there was leak just between the sleeve and the shaft if you didn't apply sealer or epoxy in between. Or the sleeve's shape was so imperfect so the seal couldn't do its job at all. Or got worn or cut by a certain sharp edge. 

Maybe it's worth for you to just spend a couple of hours in the evening to read on the web basic principles of fabrication of shafts. Engeneering is way not always a rocket since. Most basic points are easy enough to figure just by common logic.

[Geoff Weeks]

With the Shepard box the input seal is a high pressure seal, a speedy sleeve will not work at those pressures, as you found. 

[mowerman]

I still don’t know why the sleeve didn’t work and I was also surprised that you were able to get one 

[Geoff Weeks]

18 minutes ago, mowerman said:

I still don’t know why the sleeve didn’t work and I was also surprised that you were able to get one 

Speedi sleeves increase the diameter of the shaft by a few thousandths, not an issue for wheel seals and the like but a big issue for seals that have to hold back 2000 psi.

It has been a long time since I did a Shepard box, so I don't remember exactly how the seal is. Last box I had to send off for a input shaft repair IIRC it was $1200. Input shaft seal area and pitman shaft bushings are really the only thing that goes wrong on those boxes. 

There is a fitting on the side that you can pump grease in between the high pressure and dirt seal, to keep grit out of the high pressure seal area, if this is done regularly it helps keep the high pressure seal in good shape. The grease forces any dirt up and out. Just one pump every oil change is all it takes.

 

[Geoff Weeks]

Here is a better explanation as to why speedi seal will not work in this application.

Any oil seal, be it a rear main or a wheel seal, is vented to case pressure on the oil side. In a gear box or a rear axle that will be atmospheric, in a crankcase, it may be a few inches of Hg above atmospheric. 

Take the vent out of a trans or rear axle and put an air fitting in there and connect to 100 psi, would you expect no oil leak? most would say no, it would leak, now raise that pressure 20 fold! That is what the high pressure shaft seal on a Shepard box has to hold back. Stretching the seal a few thousandths over a speedi sleeve will damage it.

[Joey Mack]

I still havent seen where he removed the whole seal assemblies. The inner and outer seal with the snap rings that at the flat spring steel type that you remove with a pocket screwdriver.  the top dirt seal is not for high pressure.  

 

Unless I missed something in the story. If so.. forgive me. 

[Geoff Weeks]

On 12/28/2025 at 11:15 PM, KP2 said:

  Removed shaft again, installed wear sleeve and new seals.  I ran it about two hours and it started leaking again.  

 

I take this to mean he has had the thing all the way down. 

It is a known wear point on the Shepard box. I could re-seal once, and get some life out of the box (many years) but after that the shaft needs machine work.  Any of the two digit boxes and the 392 series/type are old enough that shaft wear is likely.

[KP2]

19 hours ago, Joey Mack said:

I still havent seen where he removed the whole seal assemblies. The inner and outer seal with the snap rings that at the flat spring steel type that you remove with a pocket screwdriver.  the top dirt seal is not for high pressure.  

 

Unless I missed something in the story. If so.. forgive me. 

I did remove all the seals down to a bare shaft.  I don’t remember snap rings.  I just bought this truck and it was leaking some around the shaft when I got it.  I thought it was change the seals and be done.  But I wasn’t aware that there maybe a groove worn in the shaft and that changed things.  I had a man helping me that has done this type of work all his life and thought we had it back right but apparently not.  

[Joey Mack]

Ok. So you did strip the input shaft cover inside an out.  Too bad it wont hold.  Im curious about your idea to weld it and turn it back down.  Good luck....

[KP2]

20 hours ago, Geoff Weeks said:

Here is a better explanation as to why speedi seal will not work in this application.

Any oil seal, be it a rear main or a wheel seal, is vented to case pressure on the oil side. In a gear box or a rear axle that will be atmospheric, in a crankcase, it may be a few inches of Hg above atmospheric. 

Take the vent out of a trans or rear axle and put an air fitting in there and connect to 100 psi, would you expect no oil leak? most would say no, it would leak, now raise that pressure 20 fold! That is what the high pressure shaft seal on a Shepard box has to hold back. Stretching the seal a few thousandths over a speedi sleeve will damage it.

When we put the sleeve on the shaft we ordered a sleeve to fit the sleeve and not the shaft so I wouldn’t think we stretched it  too much.  However I am not sure if the seal that was installed was meant for the higher pressure in the gear box.  

The groove in the shaft was large and I after I thought about it awhile I just decided to find a reman and not machine it.  Mistake maybe.  Costly yes.  

Thanks for the explanation.

[KP2]

5 minutes ago, Joey Mack said:

Ok. So you did strip the input shaft cover inside an out.  Too bad it wont hold.  Im curious about your idea to weld it and turn it back down.  Good luck....

Not sure what you mean about stripping the shaft inside and out.  When I removed the shaft we had to remove the bottom retaining “plate” and then the seals were under that with a washer that had a groove around the edge at the top of the seal area.  The seals outside were removed too.  I believe there was one seal and a dust seal outside.  

I ordered a reman gear through Watts Truck.  I tried to find one in salvage and other reman shops but never did.   Being the age of these boxes I don’t know what shape it would have been in anyway coming from a salvage yard.  

I was going to do machine work on the shaft, build it up and turn it back down.  After reading the comments on here, if I understand correctly,  it seems the correct was is to turn the shaft down to the bottom of the groove and reseal.  The groove was fairly deep so I don’t know about machining on it.  In the end I have to feel safe driving the truck and not worry about that shaft.  I am disappointed that I could not simply buy a replacement shaft.  If they reman the boxes why can’t they just reman the shaft?  Money? Liability?  Maybe combination of the two? 

[Joey Mack]

Good points at the end.  Too bad you cant buy a new input shaft....  feeling safe is tops..

[blackdog2]

I have several used and a rebuilt  ?