Briggs09hp

I have an 89 fountain lightning. Twin Mercruiser 420 model 454 engines. I replaced the thermostat in my starboard engine yesterday and I am confused. When i took the thermostat housing apart. I found that there was a brass spacer. My thermostat was ON TOP of that brass spacer. According to the diagram I found the thermostat is supposed to be BELOW the brass spacer.

https://www.mercruiserparts.com/Show...using%28420%29

Also, there is a rectangular notch in the thermostat housing and upper gasket. The gasket i removed had an open notch. But the gasket I replaced had the notch blocked off.

What is this notch for? What would the effect be of having an open vs blocked off gasket? Also, what would the effect be of having the thermostat above vs below the brass spacer.

Also On the top of the housing is a brass T with two spring loaded balls. What are the purpose of these balls?

Here is a link to a picture of the thermostat housing I have:

http://i56.tinypic.com/28tdkq8.jpg

Thanks,

Matt
Buffalo NY

Tinkerer

The spring loaded balls create back pressure to make sure the engine fills with water instead of just running out of the exhaust. And it helps balance the water flow to each exhaust side.

MichiMike

The diagram is wrong...... stat on top, cork gasket bottom of stat.just did mine same gasket issue with the hole. put the solid gasket on ,don't worry just did a 50 mile or so run out on erie, mines same way.

BUP

iTrader: (9)

I think the change is because of a newer t stat housing / design. If some one could pull up a drawing or service manual for the Mercruiser 420 during that timeframe because here is how remember it.

you had in order from the bottom up description as how I remembered it.

t-stat housing
then inside of the housing the brass sleeve with the turned in edge facing up.
then cork gasket
then 140 degree t-stat
gasket (tab forward on the gasket)
then a SS plate
then gasket (tab forward on the gasket)
then top cover for the housing. Top cover should have to hose necks on it. NOT again NOT the necks for the check balls & rod.

This is the set up from many years ago and I described it from the bottom up.

THE check balls are opened by water pressure to help cool the risers at higher rpms.. At low and idle rpms / speeds the check balls are closed against the necks in the T - Fitting to help and direct more water flow to the engine to keep it cool.

They have spring tension on them so make sure they are moving and closing correctly.