If This Don't Just Piss A Guy Off!

[Rob]

Well, when I purchased my 1977 R-795 it really leaked lubricant from the front rear thru shaft quite well. After consulting with Herb, (HK Trucking) I pulled the rear driveshaft from the truck, and removed the complete carrier from the housing. Before I did this, I checked the up and down movement of the shaft and there was none perceptible to me indicating good bearings. I then removed the yolk, and seal. There was already a sleeve on the sealing surface of the yoke but I replaced it with a new one anyways. I then cleaned everything up nicely, and put it back together with no further leakage.

Tonight I took the truck for a little cruise and noticed a whine, or howl that I'd never noticed/heard before. Upon parking back in my building, I noticed a drip from the same area that has not leaked since last spring. Now climbing under the truck, the top housing, (Mack double reduction) is very low on oil, and there is quite a bit of play up and down to the thru shaft. Obviously, I've destroyed a bearing or something. I'll pull it back apart within the next few days as I didn't have any time this evening.

I've got another "drop in" if needed, (4.17 ratio) so it is not a real big setback but I wonder what I could have done incorrect? I followed the service manual, and personal recomendations exactly.

Rob

[theakerstwo]

Well, when I purchased my 1977 R-795 it really leaked lubricant from the front rear thru shaft quite well. After consulting with Herb, (HK Trucking) I pulled the rear driveshaft from the truck, and removed the complete carrier from the housing. Before I did this, I checked the up and down movement of the shaft and there was none perceptible to me indicating good bearings. I then removed the yolk, and seal. There was already a sleeve on the sealing surface of the yoke but I replaced it with a new one anyways. I then cleaned everything up nicely, and put it back together with no further leakage.

Tonight I took the truck for a little cruise and noticed a whine, or howl that I'd never noticed/heard before. Upon parking back in my building, I noticed a drip from the same area that has not leaked since last spring. Now climbing under the truck, the top housing, (Mack double reduction) is very low on oil, and there is quite a bit of play up and down to the thru shaft. Obviously, I've destroyed a bearing or something. I'll pull it back apart within the next few days as I didn't have any time this evening.

I've got another "drop in" if needed, (4.17 ratio) so it is not a real big setback but I wonder what I could have done incorrect? I followed the service manual, and personal recomendations exactly.

Rob

Youi said that the top was low i think but you dont check the level in the top once you have put some oil in the top at the beginingf of fill up. the top will not stay any way.

[HK Trucking]

Rob, it sounds like that ball bearing at the back end of the thru shaft went bad.

As far as oil level, initially fill the upper & lower compartments (and the power divider on the front diff), then once in service just check the bottom compartment.

[Rob]

Rob, it sounds like that ball bearing at the back end of the thru shaft went bad.

As far as oil level, initially fill the upper & lower compartments (and the power divider on the front diff), then once in service just check the bottom compartment.

Hi Herb, that is exactly what I did. I only checked the top compartment as the oil was running from the thru shaft yolk seal again. The power divider and lower bowl are both right to the top.

I do have a bit of slop in the front driveshaft slip splines, and a little in the second joint. What series of driveline would this be with the cast bolt through caps? The rear driveshaft uses 1710 full rounds, but the first shaft is quite a bit larger.

Thanks,

Rob

[fjh]

Hi Herb, that is exactly what I did. I only checked the top compartment as the oil was running from the thru shaft yolk seal again. The power divider and lower bowl are both right to the top.

I do have a bit of slop in the front driveshaft slip splines, and a little in the second joint. What series of driveline would this be with the cast bolt through caps? The rear driveshaft uses 1710 full rounds, but the first shaft is quite a bit larger.

Thanks,

Rob

In my area the 417/ 442 are not a long lived ratios they tend to give out soon depending on aplication!

the thru shaft bearing is easy to change But pull the side cover and have a real close look at the top gear set before ya just asume its the bearing!And as stated the top end seeks its own level it was not low 1 to two knuckles down is common!

[Rob]

Thanks guys, I'm going to stage the truck in the shop this evening to take it apart. Citing that the rear through shaft bearing is bad, I've decided to take the whole assembly apart and check for worn parts I've got a spare "chunk" if worn too badly.

I would like to ask if there are any things to look for when dissassembling one of these things? The service manual looks pretty straight forward, and when replacing the seal the first time, I had no problems.

I do plan to use this truck as somewhat of a daily driver, so don't really want to take any chances by being "cheap", and having to repair something again.

Thanks,

Rob

[jerrymac]

Thanks guys, I'm going to stage the truck in the shop this evening to take it apart. Citing that the rear through shaft bearing is bad, I've decided to take the whole assembly apart and check for worn parts I've got a spare "chunk" if worn too badly.

I would like to ask if there are any things to look for when dissassembling one of these things? The service manual looks pretty straight forward, and when replacing the seal the first time, I had no problems.

I do plan to use this truck as somewhat of a daily driver, so don't really want to take any chances by being "cheap", and having to repair something again.

Thanks,

Rob

If you have never rebuilt a Mack differential, here are just a few things to keep in mind.

Make sure you retain the original shims from the pinion housing and spur shaft housing, as these are there to set the

gear pattern on the ring and pinion gears. If you toss them you will be a long time setting the patterns. If the shims look

good you can reuse them. If they are badly rusted/corroded make sure to replace them with the same overall total

thickness of shims. Also reuse the spacers on the pinion and spur shaft as a starting point for checking the bearing

preload on the subassemblies before installing them. The thickness of the spacers is stamped on the edges and this will

give you a good idea of what you might need if the preloads are too loose or too tight. Also, make sure that the little

tin tray with the magnets on it is still attached to the side cover. I have seen many of these crack around the bolt holes

and fall off. Although the ring and pinion gears will chew it up without much damage to the gears, this little tray does

serve a useful purpose. It collects the 'splash oil' from the pinion gear and then it drips down into the front housing on

the thru shaft. The thru shaft(depending on the age of diff.) has either a piece of wound metal strip, or a wound casting

on it that feeds oil to the power divider as it spins. If you find any metal in the axle housing, I would suggest installing

a complete bearing kit as you might not be able to see any possible cup/cone damage. Kit comes with all the bearings

and races , seals , gaskets etc. Also, while setting the preload on the bullgear, I always tap the race toward the gear with a hammer and punch as the spanner nut doesn't always seem to move it as easily as it should. Whew! Hope this helps.

[Rob]

If you have never rebuilt a Mack differential, here are just a few things to keep in mind.

Make sure you retain the original shims from the pinion housing and spur shaft housing, as these are there to set the

gear pattern on the ring and pinion gears. If you toss them you will be a long time setting the patterns. If the shims look

good you can reuse them. If they are badly rusted/corroded make sure to replace them with the same overall total

thickness of shims. Also reuse the spacers on the pinion and spur shaft as a starting point for checking the bearing

preload on the subassemblies before installing them. The thickness of the spacers is stamped on the edges and this will

give you a good idea of what you might need if the preloads are too loose or too tight. Also, make sure that the little

tin tray with the magnets on it is still attached to the side cover. I have seen many of these crack around the bolt holes

and fall off. Although the ring and pinion gears will chew it up without much damage to the gears, this little tray does

serve a useful purpose. It collects the 'splash oil' from the pinion gear and then it drips down into the front housing on

the thru shaft. The thru shaft(depending on the age of diff.) has either a piece of wound metal strip, or a wound casting

on it that feeds oil to the power divider as it spins. If you find any metal in the axle housing, I would suggest installing

a complete bearing kit as you might not be able to see any possible cup/cone damage. Kit comes with all the bearings

and races , seals , gaskets etc. Also, while setting the preload on the bullgear, I always tap the race toward the gear with a hammer and punch as the spanner nut doesn't always seem to move it as easily as it should. Whew! Hope this helps.

Hi Jerry, thanks for the advise. I always solicit "experience" before attempting repairs as I've found manuals through the years to be not perfect.

This does not seem to be too difficult by looking at photos and yours, and others advisments. I've never rebuilt this type of double reduction carrier before, and besides being heavy, it looks straight forward.

Is it best to manufacture a holding jig to secure the housing upside down to dissassemble, and rebuild? It appears this would be handy to handle the weight of parts. Is there a rebuild kit available for these units ready made, or go piece by piece?

While under the truck last evening, I did notice the large cover on the side with the fill plug on the front rear differential is aluminum, the rear is cast steel. Both carrier housings are cast though. I suppose one of the rears has been changed as the number of bolt holes are different that secure this cover. My other R models have the same type covers on both axles.

Thanks,

Rob