Dear Ole Mechanic;

1973 Chevrolet PickupSeveral months ago I bought an old 1973 Chevy ranch truck.  While I was able to drive it home, I knew that it would need some work--and it was cheap.  Cheap in both the purchase price and in the original equipment options.  It is a base ton and it was lucky to have two outside mirrors and a heater.  It has a 250 cubic-inch six-cylinder engine, a manual three-speed transmission and a long bed box.  It doesn’t even have power steering and the radio is a cheap aftermarket one.

I changed all of the lubricants, including the transmission, which is my problem.  After I started driving the truck around, it started making a squealing/grinding noise in low and second gears with the clutch pedal out (engaged).  I don't hear any noise in high gear, reverse or neutral.  In low and second, the noise will stop if the clutch pedal is pushed in (disengaged).  The oil level in the transmission has been rechecked several times and it is right where it should be. 

Here is the unusual thing.  The other day I parked the truck nose down on a steep hill for about 20 minutes.  After that, the noise quit for about a week.  When it started making the noise again, I parked it nose down again and it quit again--but not for as long.  Now the noise won’t go away no matter what I do.  I tried finding another transmission but no luck yet.  I'm nott going to buy a new transmission for a $600 truck.  What is wrong?   Thanks

Growling Tranny.
 

Dear Growling;

Wow, the year, make, model and a very good description of the problem--Thank You (I think).  The reason for the “I think” is your specific problem.  You are about to follow the thought process of a mechanic trying to diagnosis a problem from a letter.

Normally a noise in the lower gears but not high gear would indicate a problem with tje counter-shaft or counter-shaft bearings.  The problem with that diagnosis is that if there is a problem with the counter-shaft, the noise would show up in reverse as well.  Oops, you have no noise in reverse, so that’s not it.  Bad roller bearings between the input and output shafts would cause noise in the lower gears but it would be loudest in reverse and there would be some noise in neutral.  Oops, no noise in reverse or neutral, that’s not it either.  A bad pilot bearing or bushing would only make noise with the transmission in gear and with the clutch pedal pushed in (disengaged), so that ain’t it.  An output bearing would make some noise in all gears but would be loudest in high gear and get louder as speed increases.  Darn, no noise in high gear, that’s not it.  Reverse gear or reverse idler shaft bearings would make noise in reverse only--still not it.  About the only thing left is the input shaft bearing, but it should make noise in all gears and some in neutral,so that can't be it.  Since stranger things have happened, I guess it could be a side-load-sensitive noise in the input bearing, as there is a lot of side load on the bearing in low, second and reverse.  Dog-gone it, you said no noise in reverse, so that is not it.

And some people ask me why I retired.  It is things like this that remind me why I stay retired.  I don't doubt that there is a noise in the truck, but this is one of those times that require a mechanic to say: “It will need to be torn apart to see exactly what the problem is.”

Actually I contacted Growling and had some more checks made, like backing up in reverse for almost a block.  What came back was this:  “It does make some noise in reverse and there is some but not a lot of noise in the transmission when in neutral with the clutch out (engaged), but it goes away when the clutch is pushed in (disengaged).  There is no noise that I can hear in high gear.” 

Ah-Ha! With no noise in high gear but noise in all of the other gears, there appears to be a problem with the counter-shaft bearings.  There could also be a rough input shaft bearing but in any case, the transmission will need to be torn apart and all worn bearings and worn shafts replaced.  Either that or keep trying and see if you can locate a similar transmission in a recycle yard, formerly called a salvage yard or a junk yard.  A good yard will usually guarantee a transmission to the point that if the first one you get is bad, they will give you another one.  Be selective, as there was a transmission mounting change from 1972 to 1973 and a '72 transmission will not work in your truck but a '73 through '76 should fit without a problem.  If you find a four-speed transmission, be sure to get the drive-shaft as well, as the four-speed is longer than your current three-speed.  You will need to modify the floor in the cab as the four-speed is also taller.

While you have the transmission out, take a long, hard look at the clutch.  While you have the transmission out is the time to replace it, because the transmission must be removed to get to the clutch.  No need to pull the transmission twice. 

Followup:  The noise got so bad that the owner located a bell housing, a four-speed truck manual transmission, the mounting cross-member and a complete drive-shaft with carrier bearing out of a 1972 half-ton Chevy pickup.  His thinking was since there was only one year difference in the truck’s age, his being a 1973, things would fit.  However, GM made a definite model change for the 1973 pickups.  My friend asked me to ‘advise’ during the swap (yeah, right--and bring tools).  Things went something like this.

The 1973 had a frame-mounted cross member support at the rear of the transmission, but the 1972 cross-member supported things with mounts on the bell housing.  That would not work, as the 1972 transmission had no rear mount.  Also, the transmission on the 1972 had two internal and two external mounting bolts attaching it to the bell housing.  The 1973 had four external mounting bolts.  The 1972 bell housing could have been used, but it lacked a clutch bracket and there was no place to mount a fabricated bracket.

At this point, a newer (than 1972) four-speed transmission was located that had a rear cross member mount and had the four external mounting bolts required by the 1973 bell housing.  The only problem was that it was rusted up--BADLY.  OK, remove the good internal parts from the 1972 transmission case and put them in the newer four-speed case.  Oops, one bearing size change prevented the swapping of the good internal parts into the newer case.  The rear transmission cross member mount would fit the older 1972 case.  At this point the 1972 bell housing and transmission would work except for the missing clutch-bracket mount.  OR, we could use the 1973 bell housing with the clutch mount IF the two internal and two external transmission mounting bolts to the bell housing could be changed to the four external mounting bolts.  The problem was that the transmission was made in such a manner that a normal bolt could not be used even if the threads were drilled out of the transmission, because there was not room for the head of a regular bolt.  But if the threads were drilled out, an Allen-headed bolt would be small enough to clear the obstructions.  This would change the two internal mounting bolts into two external mounting bolts and the 1972 transmission could be bolted to the 1973 bell housing.

With that problem out of the way, a new clutch was installed and reassembly was started.  Darn, the rear transmission cross member was in a slightly different location due to the difference in lengths between the three-speed and the four-speed transmissions.  Other holes that were already in the frame and were thought to be in the correct place--WERE NOT.  With some hole elongation, 12 of the 14 cross member mounting bolts were installed.  Now to install the drive shafts.  Both the '72 and the '73 had two-piece drive shafts.  The complete 1972 drive shaft didn’t quite fit, as the carrier bearing was in the wrong location.  So, a bracket was made to move the carrier bearing--only to find out that the 1972 drive shafts were about two inches too short for the 1973 truck.  The 1973 drive shaft couldn’t be used, as the front drive shaft to the transmission attachment was a completely different design.  Now what?  Well the front part of the 1972 front shaft was adapted to the rear part of the 1973’s front drive shaft.  A special front shaft was made using a mix and match.  MJW (Make Junk Work).  Now the 1973 carrier bearing could be mounted in its original location and the original 1973 rear shaft used.  A test drive proved that the transmission transplant was successful.

After the truck was back on the road, a post mortem was preformed on the noisy three-speed.  The front input shaft bearing was in the process of grinding itself apart and it didn’t have much life left.  A new front bearing might have extended its life for awhile, but the small parts of metal from the bearing were spread throughout all of the gears and other bearings, so it was on its way out.

The above saga was played out over four days with as many as four people involved at different times.  The moral of this story is that when some one says that an automotive part will fit because “there is only one-year difference.” it should be taken with a grain of salt.

Herr Professor Nuzanbolts  

Herr Professor Nuzanbolts