I haven't tried it yet, but everything I have read says that the larger engine will work just fine. I bought a 172 diesel engine to replace the 144 in my 641D, but haven't done the swap yet because the 144 keeps working fairly well. I am also not totally sure how good the 172D engine is, but I think I bought it for about what a rebuildable core would sell for. The 172 diesels are much easier to get parts for than the less common 144. The two engines look almost identical, but have different casting numbers. I have heard that the 6xx series uses somewhat lighter rear end components than the 8xx series, so really taxing the now more powerful 641 might result in breakage in that area. But I figure that if my 641D has held up to the punishment I have given it over the last 15 or 20 years, a bit more power probably will just make the rear tires spin, as they now often do. When I do make the engine swap, I plan to replace the clutch and throwout bearing while it is split and do a general cleanup of the whole tractor. One of the problems with the old engine is that it blows some oil out the breather and leaks out of the rear main, so as it is, the tractor is pretty messy looking. A buddy of mine has a steam cleaner, so we will probably give the tractor a once over with that before I begin. Once I determine that the replacement engine is good, I will also probably rattle can paint the engine, since it now is blue rather than the appropriate red. Good luck with the changeover, and please let us know how it works out for you.
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