8V71N rebuild - correct size hose for remote engine oil filters

[akeeva]

Akeeva is a 1968 50' MY with about 4,000 hours on her 8V71Ns. We bought her in 1999 and had our mechanic remote the oil filters in 2000.

On July 10 this summer we lost our port engine when a cylinder liner broke. Other problems with the engine became apparent; we are almost finished with a complete rebuild (ouch!). We also had the Allison pulled and checked out while the engine was out of the boat.

The injectors tested out fine. After some investigation, our (new) mechanic feels like he has figured out the cause of the liner breakage - the hose to/from the remote engine oil filter was too small (a size 12). Apparently there is a DD spec book that specifies the minimum hose size to use with remote oil filters on the 2-cycle DDs. In the case of an 8V71, the minimum size is a 20, considerably larger than the 12 in place. The book also says that the hose run to the remote filter should not exceed 9 feet.

Our mechanic's theory is that the hose didn't allow for the circulation of a sufficient volume of oil to adequately cool the engine. Excessive heat build-up caused the piston to seize and break the liner.

Just an FYI for all of you that have a 2-cycle DD with a remote engine oil filter.


Park
aboard Akeeva, in Seattle

[MikeP]

Of course the lines should be as per DD but what did the oil pressure gauge show during operation? If the filter lines restricted oil flow, the oil pressure gauge would have reflected a lack of sufficient oil pressure.

Also...If it has been running OK since 2000, it's a bit hard to believe that the size of the oil lines suddenly caused the problem. A too-hot condition would seem more likely related to the cooling system.

[krush]

While anything is possible, throwing a little bit of engineering sense at the reasoning makes me say I am doubtful the hose size caused your problem.

Like stated above (and assuming your oil pressure sender is in the block), if there was a flow problem, you'd probably notice a change in oil pressure readings.

[Canuck Dennis]

Interesting math....oil pump is pumping constant at 60 psi yet because of line size being too small, pressure at gauge reads low ??

[akeeva]

I'd ask my mechanic for more explanation of his theory, but he is out of state for 2 weeks. When we remoted the oil filters in 2000, we did both engines. The port engine (the one we are rebuilding) did have low oil pressure, at most 10 psi at warm idle. The starboard engine had "normal" oil pressure readings, and continues to run fine (as far as we know!).


Park

[Canuck Dennis]

dd'S 2 strokes are known to have low oil press. at hot idle..10 psi would not be too unusual....

[MikeP]

Guy at our previous marina had DDs that showed under 10PSI at idle - alarms sounded the whole time. He said it had been that way for 15 years!