I have a 1992 Mazda Navajo a friend left for me in my yard. He had a steam and pressure event-cooling failure somewhere. I took out the spark plugs and found that the left side piston nearest the driver was partially filled with coolant. All other cylinders were clean and dry. I have removed the left head and used Magnaflux “Spot Check”. The block is OK/Clear. The left head has a crack that is 2 1/2" long and goes within 1/4" of the cylinder. I am going to replace the left head and valves. The valves look beautiful, no burning etc.
Question. A friend says I have to redo both heads so the engine won’t be “unbalanced” even though only one head is damaged…he is an electrician. I, as a ships engineer on Skinner Uniflo Engines, think I only have to replace the bad head and do the valves on it. [Neither of us are really auto mechanics]. Need advice. Is it OK to do the cheap way with only one side (190,000 miles on the vehicle), or do I have to bite the bullet and finish disassembling the engine, get one new head, but have them both refurbished. By the way, the gaskets are set up so one side can be done at a time without effecting the other. $$$ is important being a retired disabled guy with fix $$, however If I must I can save for months to go a more expensive route.
As a DIY er you can go ahead and just do one head. As shop that wants to protect themselves from doing any free work would never agree to do one head more so on a “why should I take ANY risk”.
Be aware you are taking a risk,not a risk of a unbalanced condition but a risk that a problem exists right now on the other side or soon will.
I would be nice to do just the one and get another 50K out of the engine but it is a risk,that would not be so good to get it back together and have a problem on the other side.
Are you doing any work on the engine (reboring, honing, milling block surface, or anything) while you have it opened up? If it’s simply replacing a head with an identical unit, I don’t see why you can’t replace just one. Given the number of miles, and the fact that a head gasket may have failed (or was it definitely just the crack in the head?), it might be a good idea to pull the other head and just check the condition of everything. Any idea why the failure on one side? The valve cover and head gaskets may be getting near the end on the other side, anyway, and it will give you a chance to properly torque the head bolts.
If the new head is a remanufactured unit, you do need to know if it’s been milled flat. To keep compression balanced on the two sides, you’d either have to mill the other side’s head, or use a different gasket on the work side.
Consider the top end job on both heads as preventative maintenance. What will you do with the truck while the first head is off? I’d do a full top end job if you plan to keep the truck for several years. I realize that this is twice the cost of one side, but the truck is free, isn’t it? You could save money by finding a surplus head and have it refurbished. New valves, new springs all around. Maybe you should take the second head off and inspect it first. It may turn out that you want 2 surplus heads.