Intermittent "knock" in 1969 mercruiser 160 inline six

Silvertip

Supreme Mariner
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Sep 22, 2003
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28,771
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

Some of the mid - late 60's in-line six cylinder engines developed a knock that was due to one of three things. Piston slap, excessive wrist pin clearance, or a collapsed or broken skirt on a piston. If the skirt issue is the problem, this causes the piston to rock slightly in the bore. The only fix for any of these is replacement with high-limit pistons or a rebuild. High limit pistons are those hand selected stock pistons that are at the very top of the diameter specification. If the engine has more than just normal wear this may not be an option.
 

rpeter46

Seaman Apprentice
Joined
Apr 14, 2007
Messages
41
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

Started again with the new pump reinstalled. Pulled it off to make sure it was on correctly. Motor knocked for first 15 seconds-then quieted right down. What sound remained could maybe be attributed to lifter noise. I tried the 'mechanics stethoscope' and couldn't pinpoint anything. Did same while pulling one plug wire at a time. I still hear a light 'knocking', but cannot pinpoint anything. Gonna change oil and take out on the river asap.
Bravo- I know that there is some trick to running the engine without the valve cover on to observe what is going on and adjust valve lash. How do you do it? I live in St. Clair, MI.
Silvertip- what causes piston skirt collapse? Just bad product? How do I diagnose the slap, or broken skirt without disassembly? Anything that would give it away?
 

Silvertip

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28,771
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

Pulling plug wires one at a time while carefully listening to any change in the sound will identify the cylinder(s) causing the issue. Wrist pin clearance issues or a broken or collapsed skirt will usually be limited to one cylinder. It can happen to all cylinders but it would be quite rare. What causes it is simply the old addage that "stuff happens". Some engines are the unfortunate victim of assembly with components that are all at opposite ends of the manufacturing tolerances. A piston for example that is at the lower end of the diameter spec goes into a cylinder that is at the high end of the bore specification. While the engine may sound perfectly normal for a long time, any wear begins to make the issue appear. Broken skirts happen -- it just does. All of those noises are limited to the block area of the engine and a trained ear can pinpoint it. Lifter noise is manifested at the top of the engine in the valve cover area. Running the engine with the valve cover removed is very messy since oil gets sprayed everywhere unless aluminum foil is wrapped over the pushrod end of each rocker arm to deflect the oil spurts. Even then, unless adjustment is performed, you may still not detect a loose lifter. So the process is to adjust the lifters rather than trying to just identify a loose one. It also takes care of any that may have been "tight".
 

Bravo

Petty Officer 3rd Class
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May 28, 2010
Messages
81
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

Started again with the new pump reinstalled. Pulled it off to make sure it was on correctly. Motor knocked for first 15 seconds-then quieted right down. What sound remained could maybe be attributed to lifter noise. I tried the 'mechanics stethoscope' and couldn't pinpoint anything. Did same while pulling one plug wire at a time. I still hear a light 'knocking', but cannot pinpoint anything. Gonna change oil and take out on the river asap.
Bravo- I know that there is some trick to running the engine without the valve cover on to observe what is going on and adjust valve lash. How do you do it? I live in St. Clair, MI.
Silvertip- what causes piston skirt collapse? Just bad product? How do I diagnose the slap, or broken skirt without disassembly? Anything that would give it away?

Oil deflectors can be bought at almost any parts store for about $10-$20 ish. They snap on top of the rocker, and deflect the spraying oil back down onto the head. Very handy... as for adjusting your valves, do one cylinder at a time. Start with the first piston on the compression stroke. (Make sure both valves are closed all the way) . You loosed both valves until the pushrod has slack in the upward and downward area. Slowly start to tighten the rocker nut until all the slack is out. Again, make sure you go up and down, NOT twisting left and right. Next you go usually one extra turn, but each motor is different, and I don't have my manual with me right now. Hopefully someone else can chime in with that info. I not I'll look it up when I get home. Repeat that for all valves and you should not have any more noise from lifters or valves. Hope that makes sense...
 

rpeter46

Seaman Apprentice
Joined
Apr 14, 2007
Messages
41
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

Running out of time for this year's boating season, so I took it to the local Mercruiser mechanic. He ran it on the hose for 45 minutes and couldn't reproduce it.:confused: He suspected either the gimbal bearing or the u-joints. We agreed to do the sterndrive maintenance we usually do in the spring(impeller, engine alignment, replace fluids) so he could pull the outdrive off and do an inspection. Nothing.
He did adjust the shift mechanism and moved the timing back to factory spec-he said it was way advanced. Guess I better take it out this weekend and see if it's reproducible. If it is, I'm going STRAIGHT to the dealer!
 

cpt ron jc

Petty Officer 2nd Class
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Aug 25, 2010
Messages
116
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

sounds like rebuild time or repower,if its that old,mine is a 1990,at high rpms it raps and knoks,its just age,fuel pump is not the problem,do a compressure test .
 

191Seanymphstriper

Petty Officer 1st Class
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Aug 23, 2010
Messages
265
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

Listen! Its a old motor! Put some lucas oil stabolizer in it and enjoy the rest of the season. Then over the winter buy a 4.7 and be done with it. don't waste your time and money dumping dime into a 1969 motor. Just save your money and repower! Or do a complete rebuild. It may be your gimble bearing. I had a 5,7 mercrusier in a sea ox and the gimble bearing went and it was a intermediate knock. Listen try the lucus in it. If it quits down then the motor is just old and the lucas helped seal worn open gaps and uped the presure. And run her till shes broken. Don't fix it if its not broke. Just enjoy it! When she brakes then fix her....
 

rpeter46

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Apr 14, 2007
Messages
41
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

191seanymphstriper- not exactly what I wanted to hear. Have to tell you that I took your advice about the Lucas, though. It seems to have done the job. I don't know exactly what changed, but I haven't heard the knock again after 6 runs since changing oil and using the Lucas. Thanks. I STILL would like to know what/why. I like to make things run in top condition, not 'just run it' till it breaks. However, you are probably right. Thanks.
 

mohead1

Petty Officer 2nd Class
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Jun 18, 2011
Messages
134
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

Wrist pin knock will come and go with rpm. Usually low at idle but increases with above idle and will chatter like its on and off until the pin gets really out there then constant knocking. Or a rod bearing/main bearing. Thats why you need to use a sounding device (stethascope type thing) to identify where on the engine the noise is coming from. If its a wrist pin, figure on a rebuild, cause that noise is it hitting the side of the cylinder wall......sorry
 

rpeter46

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Joined
Apr 14, 2007
Messages
41
Re: Intermittent "knock" in 1969 mercruiser 160 inline six

Realized that I did not follow-up with any progress reports.

Took 191SeaNymphStriper's advice:
"Listen! Its a old motor! Put some lucas oil stabolizer in it and enjoy the rest of the season. Then over the winter buy a 4.7 and be done with it. don't waste your time and money dumping dime into a 1969 motor. Just save your money and repower! Or do a complete rebuild... ...Listen try the lucus in it. If it quits down then the motor is just old and the lucas helped seal worn open gaps and uped the presure. And run her till shes broken. Don't fix it if its not broke. Just enjoy it! When she brakes then fix her...."

Put new oil and Lucas Stabilizer in it. No more knocking-AT ALL. Yes, just an old engine. Thanks so much for the advice! I HATE the prescriptions of "snake oils", but this really was the right answer(at least temporarily) for my problem. I know that a major tear-down is on the horizon, but its a little further away for now... Been boating all summer, and the family thanks you!

Paul
 
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