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Allison M20 Leaking

  • Thread starter Thread starter JCG
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J

JCG

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Jun 11, 2005
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73
Hatteras Model
46' CONVERTIBLE-Series I (1974 - 1981)
Took the boat out this weekend for a 12 hour fishing trip, tranny was full when checked the privious day. On the run home, 11 hours or so the port engine changed pitch and a drop in tranny oil drive pressure noted, pulled back to an idle and checked things out and it was low on oil, refilled and made it home with no other problem except oil in the bildge.

Spent the last hour reading about Alisson transmissions ( I love this site), and compared them to my observations. All of the leaks posted seem to be hoses, filters, or other items up high on the unit. I must admit it was hot in the engine room so I didn't get to involved in the looking process but I did not see any noticable leaks.

I would add that I knew the unit was leaking but thought it was a slow drip with nothing to do with operation, but that is certainly not the case now. I have looked for the leak in the past and always end up thinking it is coming out of the bottom somewhere. It is practically impossible to see anything going on there.

Any ideas on how to identify the leak on the under side? Or is there something I overlooked. I would guess it had leaked around two quarts, seems like a leak that big would be easy to see.
 
Sounds like a seal. Slow leaks get bigger and all of a sudden let loose.
 
FWIW,

I had a similar experience except in my case there was an obvious steady drip of tranny oil from a transmission filter. The OEM canisters can be tempermental and I have since switched them out for spin-ons and since then, no problema. As a matter of fact, I have gone to spin-on secondary fuel filters as well as tony athens spin-on fuel filters for mains and genny. Good luck,

Greg
1985 45C
Hat Time
Miami Beach
 
You could add that fluorescent oil leak finder to the oil, clean the outside of the case, including the bottom with one of the little steam cleaners, and then watch it with the blacklight. Check underneath with one a little mirror-on-a-stick. You'll see exactly where it's coming from.

Then the fun begins. I hope it is a hose or an old style canister filter, which takes 50 lbs of torque.

Doug Shuman
 
If you look around with a drop light or similar source and take your time, you'll find the leak....and a mirror on a stick as noted is a good idea....Most likely it's a hose or filter leaking...it could also be a pipe connection, say a nipple, as these sometimes crack from vibration and hose stress....
Another option is to place some clean oil absorbant pads underneath and run the engines a while...then just look for the drip/drip/drip on a clean pad....
 
OK, I found the problem.

The transmission is leaking out of the main seal from the drive shaft driving the propeller shaft. I can't see any leakage when the engine is running or not it only leaks when the the tranny is in gear and the shaft is rotating. I think this is the reason I couldn't find it earlier.

So, I found the problem now to address the cause. While looking very closely at the rotation of the shaft I think there is something out of alignment or bent because I can see some wobble at the stuffing box.

Next steps????

How can I check for bent components short of pulling them out and sending them to a machine shop?

How are the prop shafts and transmission shafts aligned (procedure/equipment)?

Can this seal be changed by just removing the drive hub? Is it possible to undue the coupling and slide the entire shaft aft to get clearance while in the water? (Note I have the coupled shafts.)

As you can probaly tell I don't want to pull the boat out because it's in the middle of the summer. In any case, my bet is that some serious gear is needed to pull the hubs and etc. My biggest concern is pulling the transmission, a bunch of work.

Thanks for your help!
 
Oh boy!!! Here we go...
Unless the seal is GAWN, get some "mechanic in a can". There are a bunch of seal softeners, which in effect is like a shot glass of brake fluid. It will soften the seal short term and may get you through the season. MAY is the key word here!
I doubt if you have enough room to pull the trans flange in place. On my 58 it wouldnt go. There are a few steps to doing this in place. At the very least, youll need to pull the exhaust risers. rear mount bolts and loosen the fronts, then rig a hoist and lift the back end up about 6 inches to give you the room for the puller and wrenches to get the hub off, plus pull the seal.
I wouldnt recommend pulling the trans unless you really need an overhaul. The trans weighs in at about 500 (per the book) and to jack the engine trans assembly, youll need to lift around 2500. Mine is 12-71s with a MH20.
Ill include some pics here, but PM me with your email and I will certainly go into this in detail.
You can break the coupling and indicate the shaft in place for runout, but they rarely bend at this end. Dont be surprised if the cutlass bearings are going south as well! I just went through this entire job last winter. ws
 
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These are some pics from an HOF member "FINALEE" (sp) of the job he did on his M20s. They are the pre cursor to the MHs. Look at the output bearing housings.
The torque wrench rig is just about what youll need to pull the flange and then re-tighten it upon assembly. Once the cover is off, you can change out the $75 seal with little difficulty. ws
 
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At the best, youd need to pull the shaft coupling also. I only had about 4 inches of free travel before the gland. The last pic shows the rig AFTER the floor was cut out to do the lifting. The pipes go down to the stringers for vertical strength. ws
 
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Is it leaking oil past the seal, or out between the coupling flanges? If the oil is exiting out between the outside of the trans flange and the seal you may be lucky and just have a bad seal. However if the oil is being slung outward from between the flanges and is dark red/rust colored it is most likely caused by the trans-flange to shaft retainer becoming loose. The retainer in question is torqued on at 300+ ft-lbs. When it becomes loose it is usually caused by a failure of the bearing to coupling mounting faces. In my case the root cause of the failure on both transmissions was sagging motor mounts. The problem wiped out both tyranny’s, all the cutlass bearings requiring a complete drive-line rebuild. I have lots more photos in excruciating detail if you need them, PM me if I can be of help and you want additional info.
 
Been wondering about you Mike! That other big boy must be taking precedence! Pardon moi, but I always get the 2 Finalies' mixed up but wanted to credit you on a few of the pics I posted. ws
 
Thanks for all the information. especially the pictures.

The attached file shows leak location.
 

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It appears from your last post the leak is between the outside of the flange and the seal. Given, this is the location I would do as Bill suggested and temporally swell the seal with some type of chemical. You could try to unbolt the couplings and move the shaft back a few inches to see if the retainer is loose. Like Bill described, I also was able to move mine back about 4”, which was just enough to check the tightness of the retainer. It’s held in place by high-torque and a 3/16” cotter pin. Once you spilt the couplings you can check if there is rotational movement between the trans output shaft and the trans coupling. If there is, long term your in for a rebuild. From the time I first id’d the problem to actually fixing it, I was able to put on an additional 250 hrs of run time. At no point did it fail to operate, it just leaked and vibrated excessively. But during the tear-down inspection the failed parts revealed that it was VERY close to a major failure of the bearings, the coupler splines and output shaft splines. For reference a coupler is about $1200, and the shaft is about $1800 if you can find them.

Bill thanks for the credit.... I just wish I didn't have to get the pics and the experience the hard way ;-). You’re right about the other big boy taking precedence. What a nightmare, I've got the worst of both worlds, steel to the deck line, and wood the rest of the way. So you can guess what’s going on, it's rust on the bottom and rot on the top. It makes me happy to be back working the Hatt. at least once she is done it's basic up-keep, and not a constant race to stay ahead of the next inherent materials related failure. Do you want a steel & wood boat cheap??
 
Hey Mike... sure would like to see A BUNCH OF PICS THO...!!! I'd consider a trade too! As is where is???
After the whole trans rebuild, it was found that the output bearing was bad (chipped/galled) and the 3 other main bearings were water/rust stained. Its smooth as silk now! Rebuild came in at $3500. Plus-plus-plus ws
 
Hey i know this is a old thread but am having a very similar issue right now. If you'd be willing to share any pictures or insight id greatly appreciate it. Also does anyone know where I can find a copy of the m20 service manual online
 
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