6-71 Thrown Flywheel

[lwbclark]

My '58 4104 (with a DD 6-71 and V730D) threw its flywheel and I'm wondering if anyone has experience with the causes / consequences of this.  I'm looking at rebuilding the engine but am wondering if the trans was likely damaged as well.  I've heard that this is a known issue on these engines and that there's a mod to the flywheel / crank fasteners to reduce the chances of it happening.  I'm considering the likelihood that the failure was simply a stress failure of the flywheel bolts versus another problem in the drivetrain.  Has anyone seen / experienced this and what are your thoughts?
Thanks
Clark

[chessie4905]

I believe there is are DDA service bulletin. The two dowell pins are removed and crank holes are tapped for bolts, thus now  having 8 bolts instead of six to retain flywheel.

[Ron4104]

Had a new clutch put in our 4104 in florida back in about 1996. Everything was great for a couple thousand miles but the flywheel came loose in Tennesse. We ran it loose to a garage in Knoxville. Pulled in and when we tried to back the bus into shop it would not move.  and Owner pushed it in the shop with his jeep. His mechanic took everything apart. The owner had 20 some school buses and 2 charter coaches. He did not have anyone he trusted to drill out the broken bolts because they are in the end of crank. He knew the terminal manager at Crete trucking who he said would be able to do the job. He came after work removed broken bolts put in new and that repair lasted a long time. He said the procedure for tightening the bolts and the proper torque was very important.  He also used a Detroit recommended lock tite. I'm sorry I don't remember the tightening procedure or the torque lbs. but any good old 2 cycle detroit mechanic would know or it should in the book. It was only common with the 671 because many people didn't do the right procedure. His statement not mine. My total bill for the job was $0.00. The owner and the terminal manager fell in love with the 04 and wanted it back on the road.

[luvrbus]

Remove the dowels, tap the holes,buy the step head grade 8 bolts for the flywheel use a good grade loctite, tighten to torque and it will hold, that flywheel is one heavy sucker so it ever moves watch out here come trouble

good luck   

[Scott Crosby]

What Clifford said... 



Whole process is documented on my website

[lwbclark]

Thanks all. This is about what I'd read.  I assume that this mod is now a standard on rebuild engines? Any ideas on the likelihood of the trans surviving a flywheel separation without internal damage?
Clark

[TomC]

The V730 is a unique transmission in that the flywheel is not bolted to the torque converter. Instead the V730 has what is called a quill shaft-a small drive shaft that connects the flywheel to the input shaft of the transmission. There is a good chance that the transmission did not get damaged. You might want to replace the quill shaft just in case. Good Luck, TomC

[bevans6]

Detroit calls for International Compound No. 2 on the threads, which is a special lubricant paste, not Loctite.  The lubricant ensures the bolts stretch the correct amount when the bolt is done up, which pre-loads the bolt and lets it develop it's designed gripping strength.  You torque the bolts all to 50 ft lbs, to draw the flywheel on and take up all slack in the joint, then twist the bolt another 90 degrees to 120 degrees.  That is what creates the correct pre-load.  If the crank has been modified and there are helicoils installed in the two dowel holes (per the manual the modification requires helicoils) then don't use loctite in the helicoiled hole.  All you will do is glue the bolt to the helicoil, and you'll have the devil of a time getting it out again.

[luvrbus]

Detroit calls for International Compound No. 2 on the threads, which is a special lubricant paste, not Loctite.  The lubricant ensures the bolts stretch the correct amount when the bolt is done up, which pre-loads the bolt and lets it develop it's designed gripping strength.  You torque the bolts all to 50 ft lbs, to draw the flywheel on and take up all slack in the joint, then twist the bolt another 90 degrees to 120 degrees.  That is what creates the correct pre-load.  If the crank has been modified and there are helicoils installed in the two dowel holes (per the manual the modification requires helicoils) then don't use loctite in the helicoiled hole.  All you will do is glue the bolt to the helicoil, and you'll have the devil of a time getting it out again.

There is a service bulletin on the upgrade, the bolts you buy from DD come with a special loctite and it depends on the dowel size if a heilcoil is used or not,some 6L71 and 6v71 engines have a stepup dowel fwiw 

[eagle19952]

Part number 1#5198563

[Geoff]

My guess is that someone over torqued the flywheel bolts and they broke.  Forget the dowels and all that noise.  It is very easy for someone that is inexperienced to realize when a bolt is stretching by over torque that the bolts are ruined which make them weak and fail.  When I worked in  DDA shops this was commonly known.

--Geoff

[Oonrahnjay]

My guess is that someone over torqued the flywheel bolts and they broke.  Forget the dowels and all that noise.  It is very easy for someone that is inexperienced to realize when a bolt is stretching by over torque that the bolts are ruined which make them weak and fail.  When I worked in  DDA shops this was commonly known.

--Geoff

     Yeah, people think "nuts and bolts are simple".  Brian Evans's post above lists these as being "torque to yield" installation.  They're even trickier to get right than just a plain torque, so I would guess that you'd see still more screw ups by inexperienced "mechanics" than before.

[eagle19952]

I am not sure i would ignore the update ... 

[luvrbus]

I am not sure i would ignore the update ... 

It was a problem they had to have many failures for DD to look for a answer