The State of Head

The head of my Yanmar 1GM10, with my custom-fabricated valve-spring compressor
fashioned for me by a local machinist, Eddy.  Worked like a charm.

If you're going to rebuild an engine, it is pretty convenient that you can hold the head in your hand. Put aside the sludge factor and it would almost be cute.  Nothing this oily fits in that category, though, and when you get over that, the next emotion is awe.  This 9 horsepower engine turned turned over 110,000,000 times just on my Alaska trip.  Another 350,000 times last year... never mind all the other shorter seasons it has seen.  Half the horsepower of a John Deere lawn tractor, it has pushed a 6,000 lb boat 5,000 miles in the last couple years. Try riding your lawn tractor to New York and halfway back. Diesel engines rock, and this 20+ year old Yanmar has put in some time.

So why quit now? The hour meter has clocked a mere 2500 hours since the last rebuild (I'm assuming it has been rebuilt, but I don't even know that for sure).... That is very low for a diesel, and unless I can find the answer, I wonder if I can ever be confidant in it again?

I have the head off, and have extracted the valves, push rods, rocker-arms, and thermostat. Everything I've measured thus far is well within spec.  Some carbonization, for sure, but nothing outlandish.  It is early days, but I confess my greatest fear is NOT finding an obvious explanation for overheating and burning 2.5 liters of oil in 5 hours.

I will bring the block up from the boat to my workshop in the next days, and what I am hoping to find is obvious evidence of oil blowing by the rings and being burned in the combustion chamber.  If it isn't that, then the only possibility that I can conjure is the governor, and even at that, I have no explanation for how the governor could suddenly start behaving like a self-destructive despot intent on amplifying fossil fuel consumption.  No reason in that.

By the numbers:

Valve stems:
     Nominal at installation:     .2756
     Measured:                          .2740 (wear tolerance = 2717)

Valve Springs:
     Measured free length:       .4010   (wear tolerance = .3850)
     tilt value:    .533/.4010 =   .0133   (wear tolerance = .0350)
     Measured free length:        .4010
     tilt value     .203/.4010 =    .0051

Rocker arm bushing                .4728 (wear tolerance = .4724)


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