Well, an overhaul is eminent... but I fear there's not going to be anything "partial" about it. Over the weekend I ran a compression test which showed about 300psi on all 3 cylinders. Spec for this engine is 498 +/- 14psi. Leak-down testing revealed that all three cylinders are passing substainial air past the rings and some leakage past the exhaust valve on No. 1 cylinder. So, the tractor has been pulled out of service and is parked in the barn awaiting a tear down.
I talked to my JD dealer to see who they use for machine work and then called that machine shop. He said he has done hundereds of these little Yanmar diesels. He said that it's been his experience that the vast majority of the Yanmar's he's worked on had either broken rings or broken pistons (collapsed ring lands) both of which usually take out cylinder walls. Since the only oversized pistons for this engine are 0.25mm (0.010in), which aint much, he usually has to overbore the block, sleeve the cylinders, and then bore the sleeves back to standard bore.
Assuming that this machinist is correct (I haven't torn the engine down yet), I called up my Yanmar parts distributor and started getting prices on parts... pistions/rings, rod bearings, main bearings, cam bearings, valves, gaskets etc. Again, if the machinist is correct, I'm looking at somewhere around $1,600 for parts and machine work.
My plan is to start by removing the head and pulling the pistons with the engine still in frame to do some inspection and/or measurement. I'll go from there depending on the results of the inspection.
You have to be careful about overboring these little engines as there is not much material between the cylinders. I had a quote on a 3TNA72 and they would not bore it past the .25" over .:confused3: