achris
More fish than mountain goat
- Joined
- May 19, 2004
- Messages
- 27,468
My drive is leaking oil. (1966-1991) (See next post for Gen II)
So, you have a Mercruiser 1 drive (MC-1), R, MR or Alpha One and it’s leaking oil. You want to know where from and how to fix it.
The first thing to consider is getting a few tools and some test equipment and running a pressure test. Most people also like to do a pressure and vacuum test upon reassembly of the drive prior to filling with oil too. Not such a bad idea.
Here’s a link to some pressure tester set-ups. The threaded fitting can be taken from a Mercury gear oil pump.
Pressure/Vac testers
The pressure test should help isolate the leak, but here are all the seals and ‘O’ rings in a drive and where the oil would most likely show if any of them leaked. This document is NOT a substitute for a pressure test, just a guide to help alongside it.
Drive-shaft housing (Top box)
Front seal and ‘O’ ring. If either of these leak, you’ll find oil in the bellows. Doesn’t matter which one is leaking, replace both on strip down. Check the seal surface on the yoke for a groove, and rust if the bellows have leaked water in the past. If you have rust and a groove you have 2 options. Replace the yoke, or fit a ‘speedie-sleeve’. Then a new seal in the seal carrier and an ‘O’ ring for the outer surface sealing. If the yoke is grooved but not rusted you will also have the option of setting the seal about 40 thou deeper in the carrier, thus it will run on a previously unused section of the yoke. Also be aware that replacing this seal (and ‘O’ ring) requires the bearing pack to be disassembled. If you have the earlier design, then it’s just 70-80lb-ft on the nut and you’re done (don’t lose any shims). If you have the newer style (or an older style that has been replaced) you’ll need to set up the nut torque using the ‘rolling torque’ method. You’ll need an in-lb torque indicator for that job too. Here is a video on the process of setting up that rolling torque.
Bearing pre-load/Rolling torque
Upper Drive shaft seals. These are in the bottom of the drive-shaft housing and oil will most like run out of the back of the gear housing, down in front of the propeller. It could also show up as oil leaking from the water drain hole on the front of the gear housing, especially if the drive has been stored in the trimmed UP position. To replace these seals requires the drive to be split, the input yoke and its bearings and drive gear to be remove (but not disassembled), the top cover removed and the upper drive shaft (with the driven gear on it) pulled out. Once these items are removed the seals can be driven out and new one fitted. Reassembly is reverse of disassembly, use a new ‘O’ ring on the top cover, and don’t forget the shims in the front of the housing.
Gear Housing (bottom box)
Lower shift shaft seal. Oil leak here will show at the water drain hole at the front of the gear housing. Once the drive is split, just use the special tool to unscrew the seal and bushing and replace them as a set. You can buy the seal on its own, but usually once the seal leaks the bushing is corroded enough to justify replacement. Also replace the ‘O’ ring under the bushing. No other disassembly is required to replace this seal. Just be careful as you lift the bushing (and seal) up to ensure the lower shift shaft stays in place.
Lower drive shaft/Water pump base seals. Two seals ‘back-to-back’. If these leak the most likely place you see the oil is from the water intake holes in the gear housing. To replace these seals just split the drive and remove the water pump assembly and its base. The seals are contained in the base, as is the ‘O’ ring around it. Again, replacement of the entire base is preferred. MC-1 and R drive use the same base, and it is a different base to the MR/Alpha One drives.
Propeller shaft seals/carrier ‘O’ ring. As with the water pump base seals, the propeller shaft seals are 2 seals ‘back-to-back’. If they leak, oil will be noticed dripping from the back of the drive, just in front of the propeller. Removal of the seals requires removal of the bearing carrier, which usually requires the destruction of the locking ring, a strong pair of pulling legs and judicious use of some heat. Once the carrier is out, nothing more needs to be done to the drive. Just replace the 2 seals and the carrier ‘O’ ring, ensure everything is clean and reassembly the gear housing. Use a new lock ring and put plenty of high quality waterproof grease, or the recommended sealant from Mercury.
Other than a cracked housing, the only other place oil could be leaking from is the quad seal ring between the drive-shaft housing and the gear housing. Unless you have just had the drive split (for a water pump service for example) then that seal usually does not leak, but don’t dismiss it out of hand.
Just one more thing. I mentioned a vacuum test earlier. Here’s why (and why I have a ‘vac’ side on my test gauge).
Standard lip seals generally only seal fluid in one direction, the ‘lip-side’. As the pressure increase (to a reasonable level) the lip will be pressed down on the shaft and forms a tight seal. And this brings up another issue. A seal may be good at higher pressure, but leak at very low pressure. To that end, I recommend pressure testing at 3 or 4 psi as well as ‘full’ pressure (between 10 and 15psi). And I like to spray a little 'soapy water' around the seal areas while they are under pressure (both low and higher pressure). A very slight leak may not show quickly on the gauge, but a big bunch of soapy bubbles is a dead give-away.
If fluid is pushed from the ‘non-lip’ side, the seal will allow the fluid to pass at a much lower pressure. This is why there are ‘back-to-back’ seals on the lower drive shaft (water pump base) and the propeller shaft. The inner seals’ lip faces the oil and stops oil leaking out of the drive. The outer seals’ lip faces out and stops water passing into the drive. A pressure test only tests the drives’ ability to stop oil getting out. The vacuum test tests the drives’ ability to stop water getting in. I usually pull about 9-10 inches of vacuum. On older drives you may get a vacuum leak at about 6 inches from the lower shift shaft seal. At 6 inches, I accept that as ok, any less gets it replaced. I expect a new one to hold around 10 inches easily. You will also note that there is only one seal at the front of the drive-shaft housing, facing in (to the oil). That’s because the only thing on the other side of the seal should be air (in the bellows), hence there is no need of a second seal.
Chris.....
BTW. 'Back-to-back' seals must have grease pushed into the cavity between them, or the outer one will burn up.
So, you have a Mercruiser 1 drive (MC-1), R, MR or Alpha One and it’s leaking oil. You want to know where from and how to fix it.
The first thing to consider is getting a few tools and some test equipment and running a pressure test. Most people also like to do a pressure and vacuum test upon reassembly of the drive prior to filling with oil too. Not such a bad idea.
Here’s a link to some pressure tester set-ups. The threaded fitting can be taken from a Mercury gear oil pump.
Pressure/Vac testers
The pressure test should help isolate the leak, but here are all the seals and ‘O’ rings in a drive and where the oil would most likely show if any of them leaked. This document is NOT a substitute for a pressure test, just a guide to help alongside it.
Drive-shaft housing (Top box)
Front seal and ‘O’ ring. If either of these leak, you’ll find oil in the bellows. Doesn’t matter which one is leaking, replace both on strip down. Check the seal surface on the yoke for a groove, and rust if the bellows have leaked water in the past. If you have rust and a groove you have 2 options. Replace the yoke, or fit a ‘speedie-sleeve’. Then a new seal in the seal carrier and an ‘O’ ring for the outer surface sealing. If the yoke is grooved but not rusted you will also have the option of setting the seal about 40 thou deeper in the carrier, thus it will run on a previously unused section of the yoke. Also be aware that replacing this seal (and ‘O’ ring) requires the bearing pack to be disassembled. If you have the earlier design, then it’s just 70-80lb-ft on the nut and you’re done (don’t lose any shims). If you have the newer style (or an older style that has been replaced) you’ll need to set up the nut torque using the ‘rolling torque’ method. You’ll need an in-lb torque indicator for that job too. Here is a video on the process of setting up that rolling torque.
Bearing pre-load/Rolling torque
Upper Drive shaft seals. These are in the bottom of the drive-shaft housing and oil will most like run out of the back of the gear housing, down in front of the propeller. It could also show up as oil leaking from the water drain hole on the front of the gear housing, especially if the drive has been stored in the trimmed UP position. To replace these seals requires the drive to be split, the input yoke and its bearings and drive gear to be remove (but not disassembled), the top cover removed and the upper drive shaft (with the driven gear on it) pulled out. Once these items are removed the seals can be driven out and new one fitted. Reassembly is reverse of disassembly, use a new ‘O’ ring on the top cover, and don’t forget the shims in the front of the housing.
Gear Housing (bottom box)
Lower shift shaft seal. Oil leak here will show at the water drain hole at the front of the gear housing. Once the drive is split, just use the special tool to unscrew the seal and bushing and replace them as a set. You can buy the seal on its own, but usually once the seal leaks the bushing is corroded enough to justify replacement. Also replace the ‘O’ ring under the bushing. No other disassembly is required to replace this seal. Just be careful as you lift the bushing (and seal) up to ensure the lower shift shaft stays in place.
Lower drive shaft/Water pump base seals. Two seals ‘back-to-back’. If these leak the most likely place you see the oil is from the water intake holes in the gear housing. To replace these seals just split the drive and remove the water pump assembly and its base. The seals are contained in the base, as is the ‘O’ ring around it. Again, replacement of the entire base is preferred. MC-1 and R drive use the same base, and it is a different base to the MR/Alpha One drives.
Propeller shaft seals/carrier ‘O’ ring. As with the water pump base seals, the propeller shaft seals are 2 seals ‘back-to-back’. If they leak, oil will be noticed dripping from the back of the drive, just in front of the propeller. Removal of the seals requires removal of the bearing carrier, which usually requires the destruction of the locking ring, a strong pair of pulling legs and judicious use of some heat. Once the carrier is out, nothing more needs to be done to the drive. Just replace the 2 seals and the carrier ‘O’ ring, ensure everything is clean and reassembly the gear housing. Use a new lock ring and put plenty of high quality waterproof grease, or the recommended sealant from Mercury.
Other than a cracked housing, the only other place oil could be leaking from is the quad seal ring between the drive-shaft housing and the gear housing. Unless you have just had the drive split (for a water pump service for example) then that seal usually does not leak, but don’t dismiss it out of hand.
Just one more thing. I mentioned a vacuum test earlier. Here’s why (and why I have a ‘vac’ side on my test gauge).
Standard lip seals generally only seal fluid in one direction, the ‘lip-side’. As the pressure increase (to a reasonable level) the lip will be pressed down on the shaft and forms a tight seal. And this brings up another issue. A seal may be good at higher pressure, but leak at very low pressure. To that end, I recommend pressure testing at 3 or 4 psi as well as ‘full’ pressure (between 10 and 15psi). And I like to spray a little 'soapy water' around the seal areas while they are under pressure (both low and higher pressure). A very slight leak may not show quickly on the gauge, but a big bunch of soapy bubbles is a dead give-away.
If fluid is pushed from the ‘non-lip’ side, the seal will allow the fluid to pass at a much lower pressure. This is why there are ‘back-to-back’ seals on the lower drive shaft (water pump base) and the propeller shaft. The inner seals’ lip faces the oil and stops oil leaking out of the drive. The outer seals’ lip faces out and stops water passing into the drive. A pressure test only tests the drives’ ability to stop oil getting out. The vacuum test tests the drives’ ability to stop water getting in. I usually pull about 9-10 inches of vacuum. On older drives you may get a vacuum leak at about 6 inches from the lower shift shaft seal. At 6 inches, I accept that as ok, any less gets it replaced. I expect a new one to hold around 10 inches easily. You will also note that there is only one seal at the front of the drive-shaft housing, facing in (to the oil). That’s because the only thing on the other side of the seal should be air (in the bellows), hence there is no need of a second seal.
Chris.....
BTW. 'Back-to-back' seals must have grease pushed into the cavity between them, or the outer one will burn up.
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