Help mercruiser 120 engine valve nuts

[floridaboater1]

I have a 1975 mercruiser 120 inline 4cyl 2.5l. When i bought the boat the valve studs were pushing up through the head. I replaced the head and rebuilt the lower end. Now my problem is the nuts the hold the rocker arms on are vibrating loose. So i just replaced them all with poly locks. I took the boat out the engine ran good then i heard a taping noise in the valve train so i took off the valve cover to discover that one of the poly locks had broken in half??? What would be causing the rocker arm nuts to be coming loose and what could make the poly locks break in half????

 

[Don S]

Re: Help mercruiser 120 engine valve nuts

They are supposed to be lock nuts, if they vibrate loose, replace them with the proper type nuts. There is no option.
I would also think there is something else wrong that you keep pulling studs and breaking nuts.
How sure of yourself are you that they are adjusted to Mercruiser specs. Does your engine run nice and smooth when warmed up, and idle nicely at 650 rpm in gear?

 

[floridaboater1]

Re: Help mercruiser 120 engine valve nuts

Yes it runs smooth and at 650 to 700 rpm in gear. now on the adjustment everytime i do it by the book it runs like **** if i snug them up and run the engine at idle and slowley tigthen the nuts untill the is no more taping then it runs great untill now when the lock nut broke in half

 

[floridaboater1]

Re: Help mercruiser 120 engine valve nuts

also my studs are not coming out or loose now... the only problem is the luck nut breaking

 

[Fordiesel69]

Re: Help mercruiser 120 engine valve nuts

Use the proper nuts not poly locks. The reson they come loose if you have the valves set too loose. You need to center the lifter plungers in their bores to keep the proper pressure on the push rods & arms. When the tap stops, you need to go the extra 3/4 turn very very slowly so it will bleed as you tighten.