Merc 120 timing gears

[Butt-Ugly]

What is the purpose of the fiber timing gear in these motors? Can it be replaced with an aluminum one? If it had been replaced with an aluminum, what problems might one encounter?

Sorry if they seem like dumb questions, but I am looking at buying a boat with a 120 in it and the owner said it's making a bit of noise under the timing cover since he replaced the fiber one with aluminum and should go away once it gets worn in alittle. I just want to know if this is ok, or if I should runaway from this one.

Thanks!

 

[Don S]

Re: Merc 120 timing gears

I am looking at buying a boat with a 120 in it and the owner said it's making a bit of noise under the timing cover since he replaced the fiber one with aluminum and should go away once it gets worn in alittle.

I would personally avoid a boat that has a noise that the selling owner explains by being the wrong type gears. Big bright red flag.

 

[Butt-Ugly]

Re: Merc 120 timing gears

Hi Don,

Yes, I would normally think the same, but this is one of my fellow firefighters and a friend. I see no reason to doubt him yet. I just don't know enough if this is a regular practice to replace a fiber gear with aluminum, or what results from it besides the ticking.

Can you elaborate on this please?

Thanks again!

 

[Don S]

Re: Merc 120 timing gears

Never buy nor sell to a friend. They could end up not being your friend.
What HE believes is the problem, may not be.

 

[mcleaves]

Re: Merc 120 timing gears

Never buy nor sell to a friend. They could end up not being your friend.
What HE believes is the problem, may not be.

Sage advice

M

 

[Butt-Ugly]

Re: Merc 120 timing gears

Don, have you ever thought of entering into politics? You definately have the knack of responding while avoiding to answer the question asked.

Please don't take that statement the wrong way, it is not a snipe at you, and I am actually laughing my balls off as I type this.

I have come to value your answers to problems posted on this forum by either asking myself or searching through old threads to find the answers I need. Many times I can find the answer without needing to repost an often asked question, but this time I can't find anything in the archives relating to my original query.

 

[djvan]

Re: Merc 120 timing gears

I've replaced my fiber gear with an aluminum set. Don't have any more noise than I ever did. The last few hours on my fiber gear got a little noisy. My concern would be how did the new set get installed? Correct me if I am wrong but if the gear that goes on the crank was pouned on without the other end of the crank being supported the thrust bearing could have been damaged.

DougV>

 

[buggiedad_67]

Re: Merc 120 timing gears

the way i understand it is the fiber coated gear is to help quiet it down, most straight cut metal gears make noise, i have them in my hot rod and they make a ticking nosie much like a lifter and the nosie dont go away with age, however if that is the case a all metal gear in a boat probably will be louder since it sits with you in the boat, i would ask why the gear was changed in the first place , gears sets dont usually jump time like timeing chains do,

 

[Butt-Ugly]

Re: Merc 120 timing gears

I've replaced my fiber gear with an aluminum set. Don't have any more noise than I ever did. The last few hours on my fiber gear got a little noisy. My concern would be how did the new set get installed? Correct me if I am wrong but if the gear that goes on the crank was pouned on without the other end of the crank being supported the thrust bearing could have been damaged.

DougV>

Hi Doug, thanks for your reply. Is the aluminum set available merc parts or are they a crossed-over automotive part? I will need to ask how the gear was installed, and you may have a point there.

the way i understand it is the fiber coated gear is to help quiet it down, most straight cut metal gears make noise, i have them in my hot rod and they make a ticking nosie much like a lifter and the nosie dont go away with age, however if that is the case a all metal gear in a boat probably will be louder since it sits with you in the boat, i would ask why the gear was changed in the first place , gears sets dont usually jump time like timeing chains do,

Thanks Buggiedad, the noise was described just like that, a slight ticking like a lifter, but it goes away at about 2000rpm and higher. I will need to ask why it was replaced. All I know at this point is that much had been done to freshen the motor, timing gears, water pump, manifold/riser, carb, and maybe a few more small items I am forgetting.

Never buy nor sell to a friend. They could end up not being your friend.
What HE believes is the problem, may not be.

Thanks Don, I will see if I can get a mechanic friend to come along with me when I go over to see and hear this motor running. I've heard that tale before about not buying/selling with a friend, but I've also heard not to believe what you read on the internet, and had I followed that advice, I would have not been able to rectify many of the problems with my boat that this forum has helped me with.

I was hoping to get this engine to take the parts from it that I need to get my boat back in the water, but thought maybe it would be easier just to drop in the whole engine. Hopefully I can come to a good decision once I ask the owner more questions with my mechanic friend present, and hearing it running.

 

[djvan]

Re: Merc 120 timing gears

I can't remember for sure if they were gears for a boat or a 60's era chevy II.

My fiber gear just slowly lost it's "bite" and finally wore the teeth off one section of the gear causing the cam not to turn.

Mine sounded like a lifter ticking and I noticed the noise late summer, made a mental note "must check that out when it's time to winterize". Well about four trips out after that and you guessed it, got to row it the last 100 yards to the launch.

Of course into the wind.

If I remember correctly they are not a straight cut gear but have and angle cut. Not sure if these are considered straight cut or not.

No noticeable noise out of them.

By the way, mine is a 140, but I think they are essentailly the same engine block and same gear set.


DougV>

 

[buggiedad_67]

Re: Merc 120 timing gears

I can't remember for sure if they were gears for a boat or a 60's era chevy II.

My fiber gear just slowly lost it's "bite" and finally wore the teeth off one section of the gear causing the cam not to turn.

Mine sounded like a lifter ticking and I noticed the noise late summer, made a mental note "must check that out when it's time to winterize". Well about four trips out after that and you guessed it, got to row it the last 100 yards to the launch.

Of course into the wind.

If I remember correctly they are not a straight cut gear but have and angle cut. Not sure if these are considered straight cut or not.

No noticeable noise out of them.

By the way, mine is a 140, but I think they are essentailly the same engine block and same gear set.


DougV>

you are right about the angle cut its been awhile since i have been into a chevy 4 cyl timeing set, i dont know if they make a straight cut for them or not. the other thing i was thinking is can just the cam gear be bought without the crank gear? if they replaced just the cam gear wear between the two could also make nosie.

 

[djvan]

Re: Merc 120 timing gears

They should be replaced as a set. I don't know if one could be purchased without the other. But maybe someone bought the set and just replaced the cam gear. The crank gear is aluminum originally (or at least mine was).

DougV>

 

[Silvertip]

Re: Merc 120 timing gears

You asked -- so I will answer. Yes -- metal timing gears (actually just the cam gear is not metal originally) can be used to replace the fiber gear. Yes it will be noisy and that's why GM has used the fiber gear since Hector was a pup (that gear train dates back to 1961 and even earlier on the old stovebolt Chevy 6 cylinder). The second part of this discussion deals with durability and potential problems. Will it hurt anything? Who knows -- a stock part has been (supposedly) replaced by a non-standard part. When you do that you have no idea of the quality of the replacement nor its long term affect on the engine. The original fiber gears routinely go 150,000 - 200,000 miles in an auto engine so I see no benefit in a metal gear if noise is the result of the switch and additional durability is questionable.