Which oil filler cap to use on Mercruiser inline 6 165

[deerhuntre]

I found a crack in the rocker cover and purchased a replacement. The existing (cracked) cover had a non-vented oil filler cap and a grommet in the center top with a thick vent hose to the flame trap. The new-to-me rocker cover has a very thin fitting out of the top-side, maybe 1/4" diameter, much narrower than the original. Additionally, the new-to-me cover came with a vented oil filler cap.

Will I need to use the vented cap with this replacement cover since there is a narrower vent to the flame trap or is it better to stick with the non-vented cap?

Help educate me on why and what is going on with these! Thank you!!

 

[ziggy]

Re: Which oil filler cap to use on Mercruiser inline 6 165

i don't think i can answer why.
but i can say that older 165's had the vented cap, along with a small dia. hole in the valve cover that lead to a port just below the top of the venturi on a 2gc. my '72 165 was this way.
my '75 has the non vented cap w/the larger dia. hose that runs to the fa. i suppose the '75 was new technology at the time. better than the old vented cap.
but both were used. imho. i think if ya got the vented cap on the new cover. i suppose i'd go that way along with running the small dia. hose to the top of the carb.

after thinking a bit. i'll stick my neck out and say i think this crank case ventilation is product of emission control. think down draft tube on a car. the fumes originally would just vent to the air. next the mfg.s routed them back to the engine to hopfully be reburned...

take everything i said w/ a grain of salt. only thing i'm sure of is that mercruiser used both the vented(early version) and the crankcase vent from the valve cover to the fa. (2nd version)....

imho. i'd use what the new valve cover came with. it was designed to work that way (w/vented cap).

 

[deerhuntre]

Re: Which oil filler cap to use on Mercruiser inline 6 165

Yeah, that does make sense and thank you for the info.