Another cracked head!

My 55SS with 8v92's has cracked both heads on the starboard engine in the last 2 years. Today, I take the boat from my dock to the travel lift for winter haulout and find the port engine coolant bottle full and foaming. The engine was warmed up and never more than idle.

I'll have to figure which head but thinking of just replacing both over the winter because I have more time and can pull the floor to make it a bit easier.

Is there anything I should check while I'm doing this that might help explain it?

The only common thread is that the previous owner replaced the heads. The mechanics around here say he wasn't the mechanic he thought he was so maybe he used poor heads or faulty installation? I don't see how, I've owned the boat for 3 years so I think faults would have shown up before.

The only other information i have is that the previous owner did an engine flush prior to me buying it and i did fint lots of crap in the overflow bottles that I had to clean out repeatedly for a year.

Thanks, I'll definitely check that.

I’ve cracked both heads on the other engine and this was the symptom. The engines did not overheat, they got to running temp of about 150.

I will definitely check before I tear it down. The check is pretty easy. Pull the thermostats off and turn it over. Bubbles mean cracked head. My 2 stroke DD expert says foaming is likely cracked head.

The cracked heads aren't too expensive, but the outboard one is anything but simple!

My buddy used to run service for Harper Detroit and still has all the old connections, so I get the best parts - even when I don't know what the best is (which is just about always). I think poor torque down and/or poor replacement heads may be the cause.

I just replaced the outboard stbd head in August! That job sucked. This one will be better. We have all winter, and I can just move the couches and pull the floor up giving me much easier access.

It's been between valves both times so far, yes.
My plan, after validating which head it is just to be sure, is to replace both over the winter. I'll keep the undamaged one as a spare - if there is an undamaged one.
Head bolts are all good, and have been torqued properly after I've replaced them - I can't speak to the work prior but nothing stood out. We pulled the heat exchanger off the other engine and had it looked at - all good.

I think after the PO did that flushing, there's a ton of crap floating around and wonder if occasionally the crap just blocks things. I need to clear it out but not sure the absolute best method. I'm looking for one I can do over the winter inside the (heated) building the boat is in. I don't have water, so will have to bring buckets in with me (which I can and will do). Any advice here would be really appreciated. After the first head replacement, we emptied the overflow bottle and cleaned it out at elast 6-8 times after each run and there was crap in it. We did both. I'm seeing more crap now in the port (cracked) engine.

As to overheating, I don't think I'm overheating but I'll add this bit of information in hopes it means something to somebody smarter than me...
On my starboard engine, we installed a murphy gauge to one of the thermostat ports, and the regulat thermostat that displays on the bridge to the other. They read differently, almost 20 degrees off. I've switched them around and the difference is always about the same. I'm assuming the murphy is accurate and the 30 year old wiring to the bridge might be suspect for the others. I'm going to rewire those this winter also. I'm also considering setting up a N2K gateway and throwing a few different temp sensors to it so I can monitor different areas better and have alarms so I can be notified if necessary. I like the idea of the N2k solution, but not the cost.
The engines run, according to the bridge displays, around 185-190 when on plane. the port engine always runs a bit cooler (>5 degrees cooler).
Would multiple temp senders be beneficial, or just more distraction?
I don't think it's ever gone over 200. It has creeped up a few times past 190, and I simply back off the throttle and it cools down right away and I'm up and running a moment later. This is what makes my amateur mind think there may be some blockage floating around in there and backing off lets it move, unblocks it.

I'd been using Powercool Plus but now am using Powercool (non-plus). It's lighter coloured pink instead of darker orange. Nobody can say there's a difference or which is better. I just figured I'd pass this info along. Hopefully something stands out to somebody.

I love this boat. I love these engines. I spend a day or two each week during the winter waxing, varnishing, doing repairs, updates/upgrades. I know it's old, and Ocean isn't the best name out there, but unless I really win the lottery, I haven't seen a boat I'd rather have. I cracked the head the second day of my summer vacation, just kept to hull speed but stayed out for 5 weeks, traveling each day (and returning the overflow to the engine each morning before leaving). I'd just like to know why this is happening!