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Author Topic:   Lower Unit Check Procedure

normo posted 11-28-2009 11:49 AM ET (US)
I'm a back yard mechanic looking for a used 50hp motor. Could someone explain how to perform a lower unit check.
Thanks

jimh posted 11-30-2009 09:50 AM ET (US)
The initial inspection of an outboard engine gear case is to
--drain gear case lubricant.
--inspect for signs of water intrusion
--refill gear case with fresh lubricant.
If there is no sign of water intrusion you can reasonably conclude the gear case seals are intact. A further test can be performed by applying a slight positive pressure to the gear case and observing for any leakage. Be very careful to not use too much pressure. I recommend you obtain the service manual for the particular model of engine you wish to test in order to learn the recommended procedure. Too much pressure will blow out the seals. The procedure may also recommend testing with a slight negative pressure as well. Again, follow the procedure in the manufacturer's service manual for the particular engine you are testing. You will need some test equipment; details are given in the service manual. I don't think that most outboard engine owners own all of the test equipment needed to perform this type of testing, nor do they routinely test their outboard engine gear case in this manner. Most people take a cue from the condition of the lubricant and leave it at that.
Martino posted 12-04-2009 08:06 AM ET (US)
Also make sure it goes into forward and reverse smoothly, and doesn't jump out of gear. If possible, test on boat to confirm this. Look for excessive build up of metal chips and shavings in the oil, or on the drain plug if magnetic.
adlert posted 12-04-2009 09:43 AM ET (US)
Also rotate the prop shaft while pushing forward on it to get a feel for the front main bearings that carry the bulk of the load. They should feel smooth. Side to side play should be minimal, front to back play is often evident - specifications would have to come from a shop manual
At least a visual check should be made for prop shaft straightness. Stare straight at the end of the shaft - often there's a slight dimple in the center of the shaft you can focus on, then grab a propeller blade and give the shaft a spin (in neutral). Look for any wobble.
Check for bent/broken skegs.
Draining and refilling the gearcase is obviously the best way to go but because it's messy, time consuming, and the owner may not want you to do it, I often simply crack open the lower drain plug and see what comes out. If the engine has not been run or even cranked in awhile, any water in the gearcase will have settled to the bottom and be first to come out. I always ask my sellers to please not crank the engine prior to my check-out visit just for this reason. The gearcase is then checked first before ever spinning the engine for the various other checks needed.
If the seller has just changed, very recently changed the fluid, obviously determining gearcase health via fluid inspection is futile. Gearcase fluid will turn dark as it's used. It will also start to emit a fairly strong, foul odor. This is all normal and expected.
The pressure and vacuum checks JimH referred to are an excellent way to assess gear case seal health. Typically 3-5 psi is used to check the seal lips against their mating shafts. 15 psi is usually used to check the seal case seal integrity (outer metal ring of the seal). All pressure and vacuum readings should hold steady for 15 minutes. Drive and propeller shafts should be spun some by hand during the test. Some shop manuals skip the 3-5 psi test and skip straight to the 15 psi test. This can have the effect of pressing a lip-to-shaft seal location into sealing when under normal operating pressures, it would not seal properly.

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Author	Topic: Lower Unit Check Procedure
normo	posted 11-28-2009 11:49 AM ET (US) I'm a back yard mechanic looking for a used 50hp motor. Could someone explain how to perform a lower unit check. Thanks
jimh	posted 11-30-2009 09:50 AM ET (US) The initial inspection of an outboard engine gear case is to --drain gear case lubricant. --inspect for signs of water intrusion --refill gear case with fresh lubricant. If there is no sign of water intrusion you can reasonably conclude the gear case seals are intact. A further test can be performed by applying a slight positive pressure to the gear case and observing for any leakage. Be very careful to not use too much pressure. I recommend you obtain the service manual for the particular model of engine you wish to test in order to learn the recommended procedure. Too much pressure will blow out the seals. The procedure may also recommend testing with a slight negative pressure as well. Again, follow the procedure in the manufacturer's service manual for the particular engine you are testing. You will need some test equipment; details are given in the service manual. I don't think that most outboard engine owners own all of the test equipment needed to perform this type of testing, nor do they routinely test their outboard engine gear case in this manner. Most people take a cue from the condition of the lubricant and leave it at that.
Martino	posted 12-04-2009 08:06 AM ET (US) Also make sure it goes into forward and reverse smoothly, and doesn't jump out of gear. If possible, test on boat to confirm this. Look for excessive build up of metal chips and shavings in the oil, or on the drain plug if magnetic.
adlert	posted 12-04-2009 09:43 AM ET (US) Also rotate the prop shaft while pushing forward on it to get a feel for the front main bearings that carry the bulk of the load. They should feel smooth. Side to side play should be minimal, front to back play is often evident - specifications would have to come from a shop manual At least a visual check should be made for prop shaft straightness. Stare straight at the end of the shaft - often there's a slight dimple in the center of the shaft you can focus on, then grab a propeller blade and give the shaft a spin (in neutral). Look for any wobble. Check for bent/broken skegs. Draining and refilling the gearcase is obviously the best way to go but because it's messy, time consuming, and the owner may not want you to do it, I often simply crack open the lower drain plug and see what comes out. If the engine has not been run or even cranked in awhile, any water in the gearcase will have settled to the bottom and be first to come out. I always ask my sellers to please not crank the engine prior to my check-out visit just for this reason. The gearcase is then checked first before ever spinning the engine for the various other checks needed. If the seller has just changed, very recently changed the fluid, obviously determining gearcase health via fluid inspection is futile. Gearcase fluid will turn dark as it's used. It will also start to emit a fairly strong, foul odor. This is all normal and expected. The pressure and vacuum checks JimH referred to are an excellent way to assess gear case seal health. Typically 3-5 psi is used to check the seal lips against their mating shafts. 15 psi is usually used to check the seal case seal integrity (outer metal ring of the seal). All pressure and vacuum readings should hold steady for 15 minutes. Drive and propeller shafts should be spun some by hand during the test. Some shop manuals skip the 3-5 psi test and skip straight to the 15 psi test. This can have the effect of pressing a lip-to-shaft seal location into sealing when under normal operating pressures, it would not seal properly.