NEW LIGHT SHED ON 4104 Fan Hub / Detroit Diesel front mount PTO drive coupling!

[dtcerrato]

After much research & conversation with innovative suppliers for the commercial/industrial marine industry particularly in the Louisiana area - I came to my own conclusion pertaining to the notorious unobtainium 4104 fan hub coupling that I'd like to share with the board. Please feel free to jump in & comment if you like... Okay it has been said on more than a few discussion that the dinosaur mechanical heavy weight fan taxes the engine output by as much or more then 35 hp. It has also been noted on the bus boards that the original equipment 4104 fan hub coupling as per Detroit Diesel should be replaced every 100,000 miles. WALA! There's a hint on why they have plagued some owner's (like myself & more recently Nelson among others on their failure. As a side note - at this point in time I wonder why such conservative overbuilt engineering would have accepted this "weak link" on a commercial vehicle that could put that kind of mileage on in no time! Besides the taxing hp of the fan is the fact that during a fast start or stop of crankshaft rotation the fan places shock loads on the hub much greater than taxing hp...
Now I'm going to jump to a similar part that is designed to do a bunch more than the OEM 4104 fan hub. The similar part I am referring to is the mighty expensive Detroit Diesel front mount PTO drive coupling that we have been seeing on eBay for quite some time. Their price tag is $1595 new which is somewhat of a bargain when Detroit Diesel markets their version to the commercial/industrial maritime industry at a price tag of $2045!
Recently on Nelson's post "4104 671 fan hub failing" I mentioned a used hub advertised on eBay for $600. This surprised me because I never seen the $1595 new hub offered on eBay as a used unit. When I compared the photos of the eBay hub to the 4104 hub it was evident that they were of different dimensions. After talking to a couple really informed & experienced two stroke Detroit guys from the source that markets the $1595 new hub on eBay their info about blew my mind! Their Detroit Diesel front mount drive coupling is designed to handle 75 hp! It is larger in diameter at the fan end & is 1/2" shorter overall which will not fit as is on the 4104 bus 671.
Their coupling is designed for vessels like tug boats, and fire boats (on water fire engine) to run accessory equipment & pumps requiring up to 75 hp. They also mentioned that the obsolete smaller bore, taller hub of the 4104 is designed for half that hp - WALA! another hint of why they fail... IMHO...
Their hub needs a machined spacer between it & the 4104 fan & the 6 fan mounting bolts holes need to be redrilled to fit the wider (larger) pattern of the beefier hub. With all that needed modification I decided to give it a go anyway. The $600 or make offer used unit after offer & a couple counter offers I got it for $350 & free shipping. Nelson nailed Scott's unit for an extremely fair price and rightly so because his bus was down for the count... Our hub is performing well but exhibiting some more "resilient" movement that is making me paranoid - especially road tripping around Alaska so at least IF it should fail - I hope we're as lucky as Nelson not to loose a radiator as we did twice from a failed but back in the 80's.

[chessie4905]

Get the hydraulic fan setup (Similiar to used on many newer GM coaches and buses for years) off an old look city bus with a 6-71. It's  been several years since we had one on our 4104, but I think the torus was the same as current ones. It uses a  casting directly off the front of block, and locates fan in same exact location as original fan.

[luvrbus]

Get the hydraulic fan setup (Similiar to used on many newer GM coaches and buses for years) off an old look city bus with a 6-71. It's  been several years since we had one on our 4104, but I think the torus was the same as current ones. It uses a  casting directly off the front of block, and locates fan in same exact location as original fan.

Some of the GM had a clutch pack that look like the torus but the operation was different I have one from a 4905,Dan found a front PTO for the 92 series I think, those are plentiful be interesting to see how he makes it work,I have 2 of those 1 short one about 4 inches long and the other is about 7 inches long.Dan I believe the drive from the engine is larger also I have 2 drives from the 92 when you start the project and need a engine drive I will send you 1 and you can try to adapt it     

[dtcerrato]

Clifford when you say the drive from the engine is larger - what does that mean? The PTO hub I purchased lists all the 71 series for fitment. Please clue me in on what you refer to as the engine drive - sorry but that over my head. Unfortunately I don't have an original hub to compare it to and wanted it for a spare while traveling so far from the home front. If I have to get into it on the road I'll deffinetly be asking questions. Spacers and machining the fan to fit the hub I'm cool with - didn't figure I'd be doing anything on the crank end of the hub. Thanks

[chessie4905]

Interesting.

[luvrbus]

Clifford when you say the drive from the engine is larger - what does that mean? The PTO hub I purchased lists all the 71 series for fitment. Please clue me in on what you refer to as the engine drive - sorry but that over my head. Unfortunately I don't have an original hub to compare it to and wanted it for a spare while traveling so far from the home front. If I have to get into it on the road I'll deffinetly be asking questions. Spacers and machining the fan to fit the hub I'm cool with - didn't figure I'd be doing anything on the crank end of the hub. Thanks

Dan,we will wait and see what you have ,maybe I can get a photo I know the ones I have take a different drive from the, damper ,rated for 75 hp it probably is for the 12v71 not many 71's could handle 75 hp from the front of the crank

[dtcerrato]

Got the PTO. It's beefier. Crankshaft end bolts up the same. Fan end has a larger diameter bolt pattern and the unit overall length is 1/2" shorter. A spacer & fan machining seems to be in order. Our original hub is doing okay but needed piece of mind while traveling to have a backup. Clifford what you said about the puller tool is interesting & was mentioned on another board. There's a shop in Louisiana that is having a spacer machined for a customer with the same needs.

[luvrbus]

Got the PTO. It's beefier. Crankshaft end bolts up the same. Fan end has a larger diameter bolt pattern and the unit overall length is 1/2" shorter. A spacer & fan machining seems to be in order. Our original hub is doing okay but needed piece of mind while traveling to have a backup. Clifford what you said about the puller tool is interesting & was mentioned on another board. There's a shop in Louisiana that is having a spacer machined for a customer with the same needs.

Those Kent/Moore pullers pop up on Ebay for under 50 bucks,I bought one for the 6-71 and 8v92 for less than 50 bucks a few years ago,they are so easy to have made. I don't use mine for the 6-71 coupling much anymore but do use the puller on the 8v92 to remove the rubber mounted crank pulley from buses,you want to pull from the inside shell not the outside shell on the coupling   

[dtcerrato]

Clifford can we get the part # To that Kent Moore tool? I can't imagine how many of those hubs that must have been pulled from their outer flange...

[dtcerrato]

The beefier PTO hub I purchased has no internal threads for a puller so I assume it's pulled by it's outer flange?

[luvrbus]

The beefier PTO hub I purchased has no internal threads for a puller so I assume it's pulled by it's outer flange?

Nope it has a push flange with 3-3/8 bolts to push it off installed just for that ,if the flange is missing you use a split bearing puller that goes around the shaft and pull on the tool.On your bus it may not have enough room for the push flange,anyways you never pull on the outer shell if you want the coupling to last.People destroy the pulley on Eagles (1500.00 bucks if you can find one} by pulling it from the outside ,same with cam damper on the 8v92 people pull on the outside and wonder why it came apart going down the road but you can roll dice it is your money   

[dtcerrato]

Resurrecting this post with some progress & pictures - (reference beginning of this post for review)- well some months later carrying the larger PTO hub - it finally made its way to a machine shop along with the original fan & an old oem hub (for machining an extra needed spacer out of it). $500 in machining and $350 for the used serviceable PTO hub on eBay. We'll have $1000 wrapped up in it and probably will out live us. The photos are of "dry fit" components to check for interferences (if any) - not yet .
Another thing I'd like to bring up is all the conversation here and lots of elsewhere about the desire to go to a lighter fan. In adding up all the numbers pertaining to weight on the front end of the 4104 crankshaft - the total is over 85 lbs. 53% of that total weight are in just the vibration dampers. Only 16.4% of that total weight is in the oem (rather light @ 15 Lbs.) mechanical fan. I guess where I'm going with this is IMHO it's a waste to try for a lighter fan. fwiw The only geometry that changed besides the larger/stronger fan (PTO) hub is the fan comes 1/2" closer to the engine & still fits fine inside the radiator shroud. Please forgive me mister engineers of yesteryear but unobtainium forced me to do it.