Today looked like a fine day to do a little project so here we go. Along the way we'll uncover some of the mysteries of these axles.
The simple fact is, poorly or improperly maintained
wheel drive shafts (or, axle) can lead to a
lot of trouble for the M151. If you are acquiring one of these trucks or if you are raising one from the dead, the health of the wheel drive shafts are among the first things you should check.
If the axle cannot efficiently change its length to suit the needs of the suspension, rotational movement becomes lateral movement (particularly upon compression.) That energy has to go
somewhere and it works hard against the
differential drive flanges which will eventually get out of spec and begin to wobble in such a way that would lead you to believe they are loose (which they will be, at that point) or, it works hard on the universal joints which can and will eventually fail. A failure of this sort can lead to a runaway axle which will beat the you-know-what out of the bottom of your truck and can potentially sever a brake line and you know what
that means....
no brakes at all!
So, here we have the right rear axle of my Alley Cat. Since we know this particular truck (which is being raised from the dead) hasn't seen much maintenance it is probably going to be worthwhile to really look closely at all the components of this axle. It looks nasty but it moves fairly free and it can change its length but only reluctantly. However, it feels spongy upon compression, like air is trapped inside and that can be an indication that something isn't quite right. So we're going to take it apart.
![Image](https://s19.postimg.cc/pzk40hc8j/IMG_4293.jpg)
Note that tape used to secure the loose
bearing cups (or caps, if you prefer) If these bearing caps fall off and hit the ground you'll be searching for lost
needle roller bearings for a while and there are 23 or 24 of them in there!
![Image](https://s19.postimg.cc/cyfvqujhv/IMG_4294.jpg)
And remember....
This is a good place to start. I keep a dull pocket knife in my tool kit for moments just like this. It's a great tool for scraping away old grease and debris. If the tape is off, secure those ends with your hand while you're manipulating it so as not to let those bearing cups get away from you. Getting rid of most of the junk will allow you to have a better look at the project and it forces you to slow down and contemplate the job at hand. A good mechanic is tuned in to what he's doing and works methodically.
If there is a great deal of grease at this end of the axle (meaning the end of the axle that has a grease zerk) and if that big glob of grease is located between the body of the u-joint and the end of the axle, there is a problem! For one thing, this means the axle has been overly lubricated at some point. We want to keep this end of the axle absolutely clear because, believe it or not, these axles have to breathe. More on that, later.
Some time ago we took a moment to go over this area with the wire wheel, lube it with PB Blaster, unscrew this cap, and clean up the threads. The shiny part is well-machined, polished and plated with what looks like chrome. If this area is compromised by pitting, it would be in your best interest to obtain a new shaft because the purpose of this plated area is to help create a seal.
With the cap unscrewed, take a moment to prepare to expose some greasy parts. This is when all those rags you've been saving are going to come in handy! Separate the two ends of the axle by pulling them in opposite directions. With that done, gently push the seal and the split washer out of the cap.
Gently separate the split washer and the seal. Occasionally you may find two washers where there should only be one. The seal is properly named a
Welch plug or
slip yoke plug. It rides against,
not within (as the term 'plug' implies) the face of the opening of the
slip yoke. It serves to seal in grease and seal out water and debris and it rides on the smooth surface of the aforementioned chromed area. Compression of the slip yoke plug by the threaded cap serves to create pressure to cause the plug make a seal.
The threaded cap does not have to be gorilla tight to achieve this.
The
slip yoke plug can be removed from the shaft by sliding it over the splines of the
yoke shaft.
CAUTION: This is extremely hard on these old plugs because they may not want to stretch.
If you choose to remove it, you will run the risk of damaging it. It is far better to clean it in place. The split washer is very malleable. It can be bent so as to be brought over the shaft for removal. It is also far better to clean this washer in place. But, both the plug and washer must be removed before you can remove the threaded cap. I elected to clean everything in place.
While you are wiping down the plug note that it has a very definite shape. It's not just a rubber ring. For a moment I'm going to engage in a bit of conjecture. If your plug is split and if you absolutely have to put the axle back in service you might try to clean it up so that it is absolutely free of lubricants and attempt to mend it with super glue. I know that Loctite super glue sticks to rubber like a champ! (Don't ask me how I know
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)
) Of course, using parts in good condition is always your best option.
Here is a view of the splined shaft from the end. This machined area is called a
slip spline. Note that one of the splines appears to be missing at the 11 o'clock position. It's not actually missing, of course. The gap creates a
keyway. This serves to ensure that the other end of the axle shaft can only be installed
one way, so that it is in the proper "phase" (or alignment).
Here are the splines inside the opposing end of the axle shaft. This component is called a
slip yoke. See the big wide spline? That is the
key. The key goes in the keyway. So, these two components can only go together one way. It is impossible to get these two ends back together while out of phase!
Let's have a look at the health of the universal joints. It's time to carefully remove those bearing cups. Set them aside in a secure place. You can clean them up now or later. Later might be the better option if you find the U-joints to be in need of replacement. Repurposed chip dip containers are my weapon of choice when it comes to this sort of thing. If you intend to replace the u-joints and if the old ones are
not excessively worn, at least
SAVE the old needle bearings and their cups...maybe even the whole thing. Why? Because occasionally, Murphy's Law happens and you'll need something just to get you home. Ask me how I know. They're a great addition to your Oh-S**t Kit (which is another topic altogether).
Here is the bearing cup with a rubber
lip seal. Like wheel bearing seals, the lip seal serves two purposes - to keep grease in and to keep debris and water out.
Don't use a U-joint with a lip seal that is compromised. If you do, you are risking a great deal, starting with the eventual failure of the u-joint and when that fails you will be risking the loss of your brakes if the runaway end of the axle takes out your brake line. There's a helluva lot riding on that little bit of rubber!
With the bearing cups removed we are now ready to locate the ends of the semi-circular clips, called
snap rings, that serve to hold the bearing cups in the
bores of the
yoke cross hole. These snap rings ride in the
snap ring grooves machined in the bearing cups. Here's the end of one, located at the point of the knife. See the little shadow?
Use a punch or similar object to engage the end of the snap ring and tap the snap ring straight out of the snap ring groove. Despite all the dirt and debris this shouldn't take much effort. If it won't budge you might have to tap the bearing cup inward, toward the body of the u-joint.
Don't tap the snap ring all the way off unless you want to risk losing it.
Just ease it the rest of the way,
straight off. You might also just trap the ring between the screwdriver and your thumb. If you don't manhandle this part and if you don't lose it, it is reusable. Most new u-joints will come with their own set of snap rings.
![Image](https://s19.postimg.cc/xbyqxi983/IMG_4309.jpg)
Again, secure those parts and set them aside.
A little PB Blaster never hurts. Dose the part and set it aside while you work the other end. We'll soon be removing these u-joints entirely. Zapping it with the wire wheel also helps to remove dirt which helps to let the PB Blaster in.
The part of the u-joint upon which the needle roller bearings ride is called the
trunnion. Wipe it down and inspect it for irregularities. It should be smooth and even without flat spots or other evidence of wear. In this case, we have what is called,
brinelling. See the striations or lines on the long axis of the trunnion? That's not just leftover grease from removing the bearing cup. Those are wear marks! Essentially, grooves are being worn into the trunnion of this u-joint. When I turned the bearing cup on the trunnion, I could actually feel the needle roller bearings going up and over the high spots in the trunnion.
Brinelling is a result of several causative factors such as - excessive continuous torque loads, seized slip yoke spines, excessive driveline angles and sprung or bent yokes. Seized slip yoke splines would be the most common factor for the M151 series, right? So this u-joint is done for..
Look closely and you'll even see some evidence of high heat.
Ok, let's get this clanker out of our axle. Take note of the flat surface area machined into the yoke. It encompasses approximately 3/4 of the circumference of the bearing cup in the bore. Below that machined surface is the
shoulder of the yoke. Now time to decide how best to remove the bearing cup from the yoke.
Old school method (and by this I mean
really old school method) would have you supporting the shaft and rapping on the shoulder of the yoke with a hammer. provided the yoke isn't too beat up. Hopefully you'd elect to use a softer metal hammer or a dead blow hammer for this (if you had one) and if you
don't have one....
get one. The effect of rapping on the shoulder of the yoke is to walk the bearing cup out of the bore owing to a combination of inertia and the cruciform body of the u-joint acting somewhat like a slide hammer on the inside. With the bearing cup exposed you can then grab it with pliers and twist it free of the bore. I am not a fan of hammering on parts I intend to reuse but I will say that in a pinch, this is one way to do it. Another option is to use a hammer and socket but today, I am going to use my brand new press that cost me a mere $89.00. (Harbor Freight)
In this case, the plates provided with the press weren't going to be my best option for supporting one end of the yoke while pressing on the bearing cup on the opposite side. I couldn't orient the plates to provide sufficient and safe support to the yoke. So, I started with pressing the bearing cup to the point where it was flush with the edge of the yoke just to see if it was going to move without a lot of trouble. Sometimes it pays to sneak up on these things. Well, Gentlemen, let me say this.....a press makes this job the easiest thing you'll do all day.
And it having offered no real resistance, we see the bearing cup has been moved down so that it is now flush with the side of the yoke.
I went nearly the rest of the way by using a large socket to support the yoke. It allowed sufficient space for the bearing cup to come out the bore and past the face of the yoke.
I will admit, a
better way to do this would have been to ensure that the grease zerk was facing up, right? In other words, I should have pushed out the bearing cup that is
opposite the grease zerk. We can see that, had I attempted to push the u-joint further, the grease zerk would have ended up crammed against the inside of the yoke.
The bearing cup need not be pushed completely out of the bore. It is advantageous if you do not push it completely out. At this point this one is really only being held in by the lip seal and can be removed by my fingers which is better,
because if it drops out we risk having needle roller bearings everywhere.
The lip seal took a beating in the process of removal. We really aren't concerned about that because this u-joint is never going back in service but don't get in a hurry to throw these old u-joints away. It doesn't hurt to keep a few used needle roller bearings on hand just in case you lose some from an otherwise serviceable u-joint!
With the bearing cup removed it is a simple matter to remove the u-joint from the yoke.
This looks important
(and it IS)! I love crazy little contraptions like this.
Here we have a spring loaded one-way valve. It can be removed with a 7/16" wrench. It allows pressure (and the excessive amounts of grease someone might have put in there
![Embarassed :oops:](./images/smilies/icon_redface.gif)
) to escape from the void between the end of the spline of the yoke shaft and the interior of the slip yoke. (And properly lubed, you shouldn't have grease squirting out of this vent) This facilitates venting during the compression (shortening) of the axle while at the same time allowing the assembly to remain well-sealed. Genius!
Here we have another axle (actually the left axle of the Alley Cat) that very clearly has it's vent completely obstructed. I can barely extend it and compressing it is nearly impossible.
Clearly, excessive amounts of grease can create more problems than they solve
so lubricate responsibly and keep this device clear! Allow this vent to be covered in grease and it will not vent properly and your axle won't compress and lengthen properly! And, if the grease is so old that it has hardened, it won't vent at all. Such a small thing, and yet, so important.
Here is the yoke shaft coming out of the blast cabinet. I could have just as easily wire-wheeled these parts and the results would have been nearly as good. If you choose to blast these parts, first, degrease them completely and
do NOT blast the surfaces of the bore or the splines or the chromed areas! Mask, or plug those areas where you don't want abrasives. As you can see, the edges of my masking tape are quite tattered. Not only did I have the bores masked off, I had my big, fat rubber glove-covered fingers in the bores while I was blasting that area.
The slip yoke, blasted, primed and finally in paint! That odd-looking vertical thing is a wood plug I made from a dowel rod and I used it to plug the threaded hole for the one-way valve (which has been removed.) I've come to like the Rust-o-leum Painter's Plus Gloss Black paint. Goes on thick, it's not prone to runs and it's a little slow to dry but you can handle it after several hours (especially in this Texas heat) after 24 hours, it's as hard as a rock. Good stuff!
Here we have the yoke shaft coated in primer.
Everything is hung up to dry. This is as far as I'm going with this for now....until the new paint dries and hardens and some new u-joints arrive.
In examining the other universal joint, I found more problems, and a mis-matched bearing cup from who-knows-where. So today, two bad u-joints went on the scrap heap and our parts got cleaned, checked and recoated. We are preparing this axle for some nice new parts. It'll be better, and safer if the day ever comes when I get to actually drive the Alley Cat.
Cheers,
TJ