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How does it run once it is started? It is true that compression does rise as new rings seat, but I would wonder how much the seller really knows about the diesel, since I don't think the diesels use sleeves, but rather have normal bores in the block. If the rings have not seated and it has been run much at all, I would suspect that the overhaul or rebuild was not done right. I have been told that if a diesel starts well, the compression is probably not too bad. But there are other things that could contribute to it not starting well, including bad injectors and wrong timing. This time of year, the glow plugs should not need to be used--my 641D will start on the second or third turn down to the high 40's without using the glow plugs or the lower radiator hose heater. My 641D has many many hours on it and I do not think that the head has ever been off since it was built. There is a bunch of blow-by, yet it runs very well and has lots of power. It has seemed to me that running my 641D very hard, like doing heavy plowing, makes it run better than if I just have it do little jobs all the time and never work it. I would suspect fuel delivery problems if the tractor runs well after it is started and has decent power. I also would wonder how much it smokes after it is warmed up and when it is working hard. I do not have the proper tools to check compression on a diesel, and I doubt that many hobbiests do. As I understand it, it involves pulling the injectors and using a specialized compression testor that can read the kinds of compression numbers a diesel has--probably up to 400 or 500 lbs. Maybe taking it to a diesel shop or a New Holland dealer would be a way to get compression tested. If a tractor doesn't run right, it is worth lots less than if it runs perfectly. How much are they asking? Fixing a diesel can get very expensive, so maybe you can get the tractor very cheaply. But if it has been fooled with a lot, it could really be screwed up. I would pay accordingly or not buy it. Good luck.
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