Can we talk about the Detroit Bull Gear?

One of our local had one go on his 12.7 and took the block with it!

 

I was draining the oil out of ours 3 years ago and found a gear tooth stuck to the magnetic drain plug. It turned out it was from the idler gear (between the bull gear and cam gear), so we replaced those 3. This was on a reman Detroit with only 200K on it. The tooth had a small indentation like someone had pried on it at one time, don't know why, but that's what it looked like. Its a fairly big job to change, I helped a mechanic do it at our place, I think it took us about 7 hours to tear apart and another 7 to put it back together. The cam gear cover on the upper drivers side is another place to open up to check, that's how we discovered where the broken tooth came from. I think it takes a gasket or is it just sealer, can't remember for sure, its definitely not an o-ring.

 

You remove the cam cover to check cam gear backlash and your right it has a gasket

 

The front part that you have to take off to get to the bullgear is sealed up with an anerobic sealant. Mechanics told me you have to use what Detroit specifies or it will leak. This is not a job for the orange formagasket! I've done 2 of them, getting ready to do my 3rd in a month or so. You have to have a shop hoist to pull that cover and replace it. That or 1 BMFD that can pick it up by hisself

 

Had one go ...Started Knocking down freeway in Winter pulled off and right into TA ...Mech on duty said its Bad Towed to shop ...1 week to fix ....soon as it started knocking I let off power. Ran Fine just before it happened ...Bang ! it was over at 1 mil miles Didnt blow motor apart ...just bad gears inside ...Lucky!!

 

After my truck hit a million miles I don't need much more of an excuse than doubt to replace anything original.

 

I got 1.7 million out of mine

 

I worked on them from day one and I changed one he ran into a train and chipped a tooth. Every gear problem I have ever seen was because when a component, usually the air compressor, was changed the gear was damaged, breaking teeth. When the head was installed the back lash on the idler, was not set properly.
The gear on the Detroit is held with a set of preloaded, bearing and they are not the kind of thing you just set also all the talk about people having done it, they forgot the nut being left hand thread and most torque wrenches now do not do left hand torque to 600 lbs.
The replacement bearings have spacers so that when the nut is torqued the bearing preload is correct. Same idea as the idler on the 2 cycles.
Cat uses the same design with bushings those I have changed and had broken retainer discs.
I know of trucks that had well over three million miles and never a problem with the gear