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After replacing two head gaskets, had water in the oil again after 50 hours of use. This time we removed the head and took it to a machine shop for pressure testing and milling if need be.
The head is warped. The machine shop asked how much can they mill before the valves contact the pistons? That is my question, too. I do not know the clearance spec for the Y485T engine. Is it 0.0015 or more or less? Could the clearance be 0.0025″? The machine shop estimates that they will have to take off 0.0012″ to clean the surface and get rid of the warp. Where can we find such specs?
Is there a reference for how much clearance there is between the valves and pistons? Or a spec or reference for how much you can mill without structural weakness and overlap between the valves and piston head?
No problems were detected as a result of the pressure test.
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You can take lead readings to determine the actual clearance between valves and piston, and head and piston, then you can determine how much material you can safely remove. It your post you are using the numbers one-and one-half thousandths of an inch and two-and-one-half-thousandths of an inch. That's not very much at all. If the machine shop used an accurate straight-edge, they could determine easily enough how much material to remove safely with a feeler gauge. I would try a different machine shop, their lack of knowledge kinda scares me. Usually a 0.005″ to 0.015″ cut is sufficient to eliminate any warpage without issues.
Also, when tightening the head, do it in two stages following the sequence each time. First, torque to 75% of the full value. Second, torque to 100% of full value. Make sure all threads are clean.
Account deleted.
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Got the head back from the machine shop; they milled off one ten-thousandth of an inch. It has been cleaned and pressure tested.
The order of head bolt tightening is to start with the two center bolts and then tighten those on each side of the middle ones moving in opposite diagonal directions until they are all tightened.
Torque specs are to tighten 137-157 Nm, that is 101-116 ft-lbs according to my Jinma Operation Manual.
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A little late for a reply, but I replaced the head gasket on my 485 several years ago and did the 2 stage ramp up torque tightening. have put around 300 hours on the tractor since with no problems. Wouldn’t hurt to go to 3 stages as Dave suggests-purpose of the exercise is to make sure the pressure on the gasket is about the same over the entire sealing surface.
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