My starboard transmission oil pressure has started dropping significantly when I shift, then recovering. It also takes longer to come up to pressure than the port. What do I need to fix ? Something is off, this isn’t normal behavior for it. Boat is a 47 with standard 8v53 detroits and gears.

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My port engine oil gauge is acting in a similar way. It was explained to me that the manual pressure gauges have some type of backflow valve in them so the oil doesn't run back down into the engine when it's not running. If this isn't working, and the oil flows back, then upon startup it takes some time for the oil to make it's way up the copper pipe to the gauge, before you see a reading. 

I'm not sure that would explain the large pressure loss during a shift, I suppose it depends how fast the oil could flow back? My boat drops to 250 when I shift (both engines, either direction) and I recall a post where Jim R said that is normal operation.

Jim Rivas once told me that pressure drop when shifting indicates an imminent failure of something or another I just don’t remember what it was. I’d rather find out before it goes bang.

Ron made the comment that if the pressures are different in F / N / R, then that is an issue - I'm hoping that's what the reference was about.

        "The important question is if the pressure is the same in neutral, forward, and reverse.

         Impending clutch pack failure will usually show itself as a pressure drop in forward or

         reverse in comparison to the other positions."

The usual failure mode in a Twin Disc transmission is for one of the two seal rings to fail on a clutch pack piston. This will cause a leakage flow that will be observable as a reduced pressure while the clutch pack is  fully actuated in comparison to the neutral running pressure or a healthy clutch pack, all observations at the same rpm.

The rated minimum operating pressure is 270psi@1800rpm. This is the running pressure, not the observed pressure while shifting. The running pressure will usually be 300-320psi@1800rpm.

If the clutch packs are healthy, the observed pressure at the same rpm will be the same in reverse, neutral and forward. A moderate pressure drop while shifting is normal.

Assuming no external oil leaks on the transmission, the usual prime suspect for a Twin Disc 506 that is suffering from sluggish shift behavior is to check the pump suction strainer that is behind a cover located on the rear of the transmission at the bottom. The recommended service interval is to clean every 1000hrs, every oil change..

Many people, including mechanics, are not aware of the existence of the strainer so it is commonly neglected.

The other issue that sometimes occurs is that the relief/pressure regulating valve spool can get sticky and not compensate for low pressure as well as a regulator that is not sticky. This may result in slightly extended shift times.

You might also want to compare the transmission temperatures between Port and Starboard to verify that they are close to the same. Unequal temps may indicate a heat exchanger issue.

The Twin Disc design is very robust and should easily go 10,000hrs before needing an overhaul. The main operational concern should be if the transmission pressure is at least equal to or greater than 270psi@1800rpm and not noisy,


Steve said:

Ron made the comment that if the pressures are different in F / N / R, then that is an issue - I'm hoping that's what the reference was about.

        "The important question is if the pressure is the same in neutral, forward, and reverse.

         Impending clutch pack failure will usually show itself as a pressure drop in forward or

         reverse in comparison to the other positions."

Copy and pasted into a permanent file in my Chris Craft directory for reference. Thank you Ronald.

Ronald Zick said:

The usual failure mode in a Twin Disc transmission is for one of the two seal rings to fail on a clutch pack piston. This will cause a leakage flow that will be observable as a reduced pressure while the clutch pack is  fully actuated in comparison to the neutral running pressure or a healthy clutch pack, all observations at the same rpm.

The rated minimum operating pressure is 270psi@1800rpm. This is the running pressure, not the observed pressure while shifting. The running pressure will usually be 300-320psi@1800rpm.

If the clutch packs are healthy, the observed pressure at the same rpm will be the same in reverse, neutral and forward. A moderate pressure drop while shifting is normal.

Assuming no external oil leaks on the transmission, the usual prime suspect for a Twin Disc 506 that is suffering from sluggish shift behavior is to check the pump suction strainer that is behind a cover located on the rear of the transmission at the bottom. The recommended service interval is to clean every 1000hrs, every oil change..

Many people, including mechanics, are not aware of the existence of the strainer so it is commonly neglected.

The other issue that sometimes occurs is that the relief/pressure regulating valve spool can get sticky and not compensate for low pressure as well as a regulator that is not sticky. This may result in slightly extended shift times.

You might also want to compare the transmission temperatures between Port and Starboard to verify that they are close to the same. Unequal temps may indicate a heat exchanger issue.

The Twin Disc design is very robust and should easily go 10,000hrs before needing an overhaul. The main operational concern should be if the transmission pressure is at least equal to or greater than 270psi@1800rpm and not noisy,


Steve said:

Ron made the comment that if the pressures are different in F / N / R, then that is an issue - I'm hoping that's what the reference was about.

        "The important question is if the pressure is the same in neutral, forward, and reverse.

         Impending clutch pack failure will usually show itself as a pressure drop in forward or

         reverse in comparison to the other positions."

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