Re: 70hp thermostat compatibilities?

[Don]

Well, this is a most confusing topic, even with the directions right in front of you for sure. The first thing we need to know is if this engine still has its original powerhead… The reason I ask is because if the powerhead was replaced, it may have the later style head that uses the thermostat set up with all the cups/diaphragms, etc. Someone may have mistakenly installed older thermostat components into a newer head, if that is even possible.
The original set up used a vernatherm encapsulated in a white plastic housing that could be unthreaded so the vernatherm can be replaced. This was combined with a hard rubber thermostat seat that was retained with a SS retainer that was press fit into the head. The problem with this set up was in salt water. Salt could collect between the head and rubber thermostat seat, that would deform/compress the seat. The thermostat had long flutes that helped align it into the seat. When the seat got deformed/distorted, it would jam on the long flutes which kept the thermostat from "blowing" off the seat at high RPMs. The "fix" for this issue was a redesigned thermostat assy, black in color, and a redesigned thermostat seat. The black thermostat did not have the long "flutes" that could get jammed in the thermostat seat. The replacement seat was a very hard rubber/plastic that was driven into the head without the SS retainer. The problem with this set up was that the seats were often distorted when driven into the head, they had a tendency to pop out also. The newer style black thermostat was more likely not to seat properly due to the lack of alignment flutes. This situation resulted in cold low speed running, the opposite condition to what happened with the original set up.
I usually ended up just using the original seat set up with either of the thermostat set ups. The problem was trying to remove the SS retainer, near impossible without a big mess….Kind of like trying to removed gearcase seals. So, I usually just fished the rubber seat out of its pocket without removing the SS retainer. I then got inside and cleaned the salt build up out of the head before replacing the original style rubber seat into/behind the SS retainer.
The next issue was changes to the water pump design. Many of these changes were brought about in an effort to deal with powerhead failures on these engines. This is why the water pump kit is so complicated/confusing. These kits include so many different pieces to deal with all the various designs/updates. I only installed a few of these kits, and was confused, and felt the directions were not complete enough to deal with all the various combinations of pieces parts that were used during the years. Some of the kits used a three blade impeller, which I was totally against. One of the other changes that was made during the years was the location and design of the overboard indicator hose. Early engines used a smaller ID elbow mounted on the side of the exhaust manifold. This setup had a restrictor plastic nipple that snapped into the lower pan, just like most of the other OMCs of this era. Later models used an larger diameter overboard hose with a larger ID elbow that was mounted on top of the exhaust manifold. The hose went through the pan with no plastic restrictor, it just slid through a rubber grommet through the pan. This change was made in an effort to remove air pockets from the manifold and block which may have been causing hot spots. The problem here was mismatching systems and water pumps/thermostat set ups. The original style overboard indicator created more of a restriction and higher water pressure at low speeds. So, using the older indicator components with a newer style pumps/thermostats might create a high water pressure situation that blows the thermostat off its seat at low speeds, cold running. Using the newer style overboard indicator pieces on older thermostat/pump set ups might create a low water pressure situation at high speeds, or hot running at higher RPMs.
So, one of the hardest things to do is to figure out if your engine is running cold at low speeds due to a high water pressure situation, or a mismatched/improperly installed thermostat/overboard indicator.
To be honest, you really need to be able to identify the various versions of these components just by looking at them…This is near impossible.
I wish I could provide pictures, worth so much more than the volumes I have written here…
Hopefully Dan Anderson will chime in…..D