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Author Topic: Internal trouble in a DD 8v92  (Read 3303 times)
Mike in GA
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« on: April 19, 2011, 08:50:52 AM »

Here’s an update to a post I made this past February, when a Florida mechanic found a thumbnail piece of cast metal in my oil as he drained it. You can see the original post here:

http://www.busconversions.com/bbs/index.php?topic=18874.0

   A couple of weeks ago I delivered the bus to a shop closer to home (ATL), to  pull the engine for a transmission swap. I mentioned the metal piece to my mechanic (Howard Best’s son Christopher). With the engine on a stand, he drained the oil, dropped the pan, and found more cast metal pieces!
   It looked to him like something related to the gears on the back of the engine, so he pulled the alternator, power steering pump and air compressor, and took the cover off the rear of the engine.
   Long story short, he found that the idler gear between the crank gear and the cam gear had developed slop due to worn tapered bearings, and the vibration that caused had transferred to the cam gear, and eventually shattered the cam shaft end bearing.
   A member of this board with the handle “brando4905” hit the nail on the head back in February – signs of early gear train failure.
   No telling how much longer I would have been able to run, but if those gears had jumped a cog it would have been very bad damage – along the side of the road probably.  I got lucky. I hasten to add that I put on 600-700 miles getting home after the first metal piece was discovered, and neither then nor before the finding was there any change in performance or funny noise. A silent potential killer.         
   Makes me wonder at the wisdom of an in-frame on an older engine, where inspection of these crucial gear sets would normally go unnoticed!
   I am attaching a photo of the culprit. It shows the shattered bearing on the bench. The threaded rod was inserted so that a slide hammer could be used to extract the cam bearing from the block.
While waiting for a couple of new gears and the idler gear bearing set, the mechanic used the opportunity to inspect the rod and main bearings. They were showing wear, so he rolled in new rods and mains.
   The engine was brought back to life last Friday on a stand, and it sure sounded good. The 8v92 is set to be slid back into the bus this week, and I should be driving it away this weekend!
   So much better to deal with these things in a competent shop than along the side of the road!
Mike in GA
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Past President, Southeast Bus Nuts. Busin' for more than 12 years in a 1985 MC 96a3 with DD 8v92 and a 5 speed Allison c/r.
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« Reply #1 on: April 19, 2011, 08:56:20 AM »

  Yikes! That is very scary. Makes me want to go tear my motor apart right now and give everything a once over. I assume there is no way to inspect the gears without pulling the the rear cover???
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luvrbus
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« Reply #2 on: April 19, 2011, 09:12:35 AM »

That is what your good old gear driven 50 D alternator will do for you it was getting ready to break the cam and then all hell breaks loose gear lash was probably never checked and you are right about a inframe job just a band aid 

good luck 
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« Reply #3 on: April 19, 2011, 09:53:53 AM »

Not that I am an expert by any means, but I just finished buttoning my the back of my 8V71 so i have a couple of thoughts to put out.  The idler gear bearing is a dual taper roller and the rear cover/bell housing has a machined surface that actually puts preload on the bearing.  If the bell housing is removed, so is the preload and the bearing can go real loose.  There is a method in the manual for bolting a special tool on to the idler gear mount to apply the preload for testing purposes.  So maybe that is what your mechanic saw with the bearing being loose.  On the other hand there is no reason for it not to be real worn with the cam gear probably walking around a bit with the bad cam support bushing.

Harmonic or cyclic stress from the alternator gear drive, which I understand is a planetary gear system to increase the output speed to the alternator, could well have been the first effect cause of this failure, as suggested by Clifford.  Regardless, I think your mechanic deserves kudo's for diving in and finding the problem!

You can get at the cam gears if you remove the alternator belt pulley drive on the MC5, and the compressor.  You check for some things, but you can't really check the idler gear with the bell housing on.  You need to pull the cam gear off to see the bushing that broke up anyway.  You need the bell housing off to do that.  It's not a trivial job, not hard technically, just a lot of stuff to take off and some of it is very heavy - you need a crane or an engine hoist or several kinda patient friends...  And then you can't just put it all back together, you have to clean it all, paint it, rebuild the clutch, put on a rebuilt air compressor, change the seal in the alt belt drive, get new gaskets, clean off the old gaskets, get the flywheel refinished, change the wear sleeve on the crank, install a new crank seal, get the special bolts that have the sealant  on them so the bell housing doesn't leak, maybe put on a new power steering pump because you don't know how old that is (although that pump is a ton easier to change on the bus than the air compressor in a MC5).   Then you have to decide how much else of the engine you are going to rebuild while it's out...  this would be the story of my winter...   Grin

Brian

Cheers, Brian
« Last Edit: April 19, 2011, 09:55:26 AM by bevans6 » Logged

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« Reply #4 on: April 19, 2011, 10:42:01 AM »

For those non-mechanics, another benefit of finding a way to assist, or have close backup while you read the book and do it part of an engine pull yourself.  Because we did just pull my engine and take the end plate off, i can now reasonably understand everything Brian just said.   Grin Grin

much better than screwing around with some garage where you really don't know what they did, nor understand, and you didn't really see the parts before they replaced them.

Oh, and btw, we didn't see any issues with gear lash (another term i had no clue about previously).
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Tom
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« Reply #5 on: April 19, 2011, 02:53:13 PM »

You guys should see a DD when the alternator takes one out not pretty and Tom when you set a differential do say the lash looks good lol the DD has a adjustment for a reason and fwiw they have a gear lash setting on the water pump also

good luck
« Last Edit: April 19, 2011, 03:19:19 PM by luvrbus » Logged

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« Reply #6 on: April 19, 2011, 02:58:34 PM »

I have seen that very thing happen.  A friend has a 4905 and the 50DN grenaded and took out 3 of the gears on the back end, and exploded the bell housing. He put in new gears and a new bell housing, and buttoned her back up. another 100 miles down the road and more trouble, seems it bent one of the cams as well. bad news all around. glad I have a belt driven.
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« Reply #7 on: April 19, 2011, 03:06:35 PM »

Mike is rolling the dice on that one I hope it last a long time for him

good luck
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« Reply #8 on: April 19, 2011, 03:09:30 PM »

  Do you have to entirely pull the engine to change the clutch, pull the rear housing, etc.. Or can you slide it back far enough to pull it all apart?
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bevans6
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« Reply #9 on: April 19, 2011, 05:01:31 PM »

pull the engine.

Brian
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« Reply #10 on: April 19, 2011, 05:09:15 PM »

It's not a problem isolated to the ole Detroits either. I have a friend that owns a salvage yard and his backup payloader with a Cummins dropped a gear to it's compressor and lightened his wallet by 10 grand!
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« Reply #11 on: April 19, 2011, 06:55:31 PM »

50DN's are legendary for causing gear train failures.  Besides the above mentioned by our buddy Clifford, be sure the alternator bearings and drive adaptor bearings are in good shape.  If those pieces drop down through the gears, it's good night.  If you don't know what kind of shape they are in, say you inherited a bus with shaky maintenance, it's a good idea to replace them.  We did all our unit overhauls in house and figured that a 250.00 kit and 6 or 7 hours labor at 75,000 mi. instead of 250,000 was cheap compared to an engine.  Moving to the other end of the engine I have seen a few cranks break on 92's due to the vibration caused by the A/C compressor.  The OEM compressor is usually removed in our hobby though. Just food for thought, don't ignore that alternator.
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« Reply #12 on: April 19, 2011, 10:07:48 PM »

When I bought my MC-9 back in August, it had a bad oil leak where the 50DN was mounted to the engine. I bought it from a pretty large Charter Company that operates 70 buses out of Virginia. The Charter Company has a nice huge shop and several mechanics working 2 shifts so they pulled the alternator and adapter to change the gaskets before I left for home. When they pulled the alternator, they noticed the gear was cracked. They installed a new gear and then re-installed the 50DN with all new gaskets.... Since I was several hours from home, I had no choice but to sit there and wait on all this to be done but that was probably the best 8 hours I have ever spent. I can only imagine what would have happened if that gear would have broken on my way home.
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« Reply #13 on: April 19, 2011, 10:42:44 PM »

When I bought my bus from the PO, the speedo was showing about 67,000 mile-which is good since the bus company changes the speedo at engine overhaul time.  But-the overhaul was a down and dirty one. At around 71,000mi, I had the engine pulled apart and the block bored .010 over to take .010 outside oversized cylinder liners-but still using the same pistons.  About 15,000 miles later, I had Don Fairchild turbocharge the engine.  We kept the same pistons, and he checked the oil pump since it was out of the bus.  The drive on the oil pump was about ready to go-who knew with all the overhauling being done.  I highly recommend pulling the engine apart to check all engine parts for wear and reassemble with new gaskets. Will give good piece of mind-no matter how well it runs or sounds.  Good Luck, TomC
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« Reply #14 on: April 20, 2011, 08:04:06 AM »

Forgot to mention that the alternator drive gear on the engine side has to be perfectly aligned.  There is a special alignment tool used to do this. The tool is rather expensive but must be used.  If anyone is doing a backyard teardown, buy or borrow the tool or a drive failure is sure to occur.  I believe the tool cost me around 300.00 from MCI, and that was about 15-20 years ago.  There is also a support bracket used for the alternators that use a drive adaptor.  It cradles the alternator underneath it and provides support between it and the flywheel housing to reduce vibration.  Sometimes it is not reinstalled by previous operators. 

Another tip on alternators is to make double sure that the positive lead is securely tied by a wire clamp.  If the eye separates from the cable and it comes in contact with the alternator case, 24V can burn a hole in the case and either start a fire or cause pressurized oil to pump out with the resulting loss of oil pressure.  Most of the old buses I have seen just have this cable swinging in the breeze.  It needs to be securely tied down.
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