While looking online at diagrams of my thermostat housing, I noticed I'm missing the 2 plastic balls and spring that restrict flow to the exh manifolds. I remember reading posts abt this many years ago, but can't remember why the factory installed them and if they are really needed.

From in and around 1993 Mercruiser V6 AND V8 THERMOSTAT HOUSINGS:
Changes to Thermostat Housing on Raw Water Cooled Engines
Production has been using a thermostat housing that doesn’t use the nylon check balls. This new housing has internal changes to it so the nylon check balls aren’t needed any longer. DO NOT install the check balls in the new housing.
The new housing, P/N 16413A9, can be used as a direct replacement for the older one with check balls EXCEPT on certain model. The reason for this is that the by-pass hole inside the housing is sized for a certain minimum seawater pump output and some older models don’t meet this requirement. If the new housing is put on these older models, the engine will overheat at idle or low engine RPM.
P/N 16413A3 Thermostat Housing WITH Check Balls. Must be used on all V6 or V8 engines that use an R, MR or Alpha One stern drives with the smaller water pump impeller.
P/N 16413A9 Thermostat Housing WITHOUT Check Balls. Can be used on all V6 or V8 engines that use Alpha One Gen II stern drive with the “floppy vane” water pump impeller. Also, it can be used on any V8 engine that uses a belt driven seawater pickup pump which includes all Bravo and inboard engines.

Humm, I have the P/N 16413A3 thermostat Housing WITH Check Balls according to https://www.mercruiserparts.com/bam/.../31808/961/160
But I also have Bravo drives and belt driven raw water pumps from the factory. I haven't had any overheating issues in the 6 years I've owned the boat so it doesn't sound like I need them.
Thx lakematdude!

Do you believe the housing to be original to the engine?....Yes using your engine year is going to show the original design even though they made some changes over the years....Sometimes you may see a few part numbers listed to help cover some changes, etc..

In 1999/2000 they went to Merc part number 90736A24 that re-deals with the check valves in a slightly different location, https://www.mercruiserparts.com/9073...-kt-thermostat

• 16413A3 service bulletin

Per the s/b, mine are the P/N 16413A 9's.
Starting to look like the nylon balls were for Alpha drives.
Thx again!