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Old 05-22-2018, 12:18 AM   #21
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So the faulty pushrods are different than the others? That may explain the problem. Pushrods that are the wrong length will affect rocker geometry.
Do your bad pushrods have oil holes in them and good pushrods do not, or vice versa?
When you tested the engine after installing the new pushrods it ran smooth. Why do you think a valve stem is bent?
I'm undecided on the nicked stud and why the rocker wanders to it. I'm thinking the nick occurred when the rod bent and the rocker came off of the valve. The nick seems to low to affect rocker operation. Do you see metal shavings at base of the nicked stud? How about corresponding damage to the rocker?
I'll repeat my borescope suggestion. If piston head looks good then it may not have contacted the valve.
I only have my own experience here, and that is that a pushrod can be damaged without damage to any other component.
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Old 05-22-2018, 12:26 AM   #22
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Jrsick I suggested the valve stem may be bent as you have stated is may only be a bent pushrod. The only way to know is to remove the valve and inspect it. I only suggest this because if itís damaged it could break and drop the valve and I have seen the results of this and they arenít pretty.

Just looking out for the OP best interest.
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Old 05-22-2018, 01:17 AM   #23
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Since you have the engine running perform a compression test on that cylinder, if the valve is bent it would not seat properly and the compression would be low. Then you would just need to decide what to do about the stud.
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Old 05-23-2018, 05:33 AM   #24
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jrsick, I saw the rocker arm slide to the side and right into that groove in the stud during engine run. I'm taking it to machine shop hopefully this week to replace the stud.

These 2 different push rods that were bent have identical length and oil holes in them (clean too) but just different endings than the remaining 6. Now all 8 are identical. I saw no metal shavings, but beside the nick in the stud, one push rod filed a groove on the side of the opening in the head. So they both slide off to the side.

Mike, good idea about the compression test. Once I've replaced all parts, I'll realign everything and test it out. I don't know if the valve is bent, compression test might tell it. All I know is that the top of the stem where it touches the rocker arm is slightly worned out. That's the part that worries me because once everything is new, it will either shave off naturally and flatten with the rocker arm or it will force the new rocker arm to slide off, bent the push rod again and I'm back to square one. Time will tell.
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Old 05-23-2018, 11:36 AM   #25
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Take the new stud with you to the machine shop. They may need to recess the boss to ensure proper stud height. Are you getting a threaded stud? If so, look at the threads below the flange (part you put wrench on), you'll notice that the threads end about 1/8 inch prior to flange. The machine shop will need to chamfer the top of the new bolt hole so the flange can sit flush to boss and be torqued.
On your worn valve stem, this was probably caused by incorrect pushrod length. Setting proper lash and gap should take care of this.
Since you are removing the head, also check the manifold to ensure you are not getting water into your cylinder. I don't think you have ruled this possibility out. Look for video on how to perform manifold leak test.
Post some pics of piston heads, valves, and manifold if possible.
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Old 06-02-2018, 02:16 PM   #26
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Default Fixed head

I've got the head back from the machine shop. They've welded it and redid the hole and put a new stud. They also filed the ends of the valve stems. Apparently the valve was perfectly fine so they haven't replaced it.

Out of precaution I'm replacing the rocker arms on that cylinder as it appears that one of them is a bit worned out on one side. They're cheap too but buy them at car parts store, they're only $7/kit vs marine store $18 for the exact same part.

I've added a pic of the rebuilt head. If you curious, it was $200. They took the valves out, cleaned it, welded the hole & replaced the stud.
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Old 06-02-2018, 03:09 PM   #27
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Default The cause

From talking to different mechanics and the Mercruiser manual it appears that these can cause bent push rods:

1. Incorrect valve adjustment
The rocker arms must be tighten slowly and properly to allow lifters to adjust themself, preventing possibility of the valve head contacting the top of piston which can result in internal damage or bent push rods (from service manual)

2. Over revving the engine
That's what the mechanics think could cause that. My engine runs at 5700 rpm with 4 people onboard at max speed. That's too high, 4800 rpm should be the max. I don't know yet if it's the faulty rpt meter (marine shop mechanic said they fail often) or if I have the wrong propeller on it (14.5 x 19). I just bought a digital timing light with rpt readout to determine that once I've put the engine back together. If it's the propeller, I need to buy one with higher pitch (21?)

Additionally, in my case these could be the contributing factors as well:

3. Different style push rods were used on both valves on Cylinder #3 than the rest
Those that are bent have straight ends. The other 6 which were not bent have a little ball at the ends (see pic)

4. Engine timing might not have been right
Unfortunately I have not measured it before I started taking the engine apart.

5. Pressure on Cylinder #3 was too high
Looking through the paperwork from previous owner, on the last check 9 months ago the pressure on Cyl #3 was 205 psi but Cyl #1 was 155, Cyl #2 165 & Cyl #4 155. They should be ~150 psi and within +/- 15 psi of each other. What could possibly cause that?

There was no water in cylinders, oil was clean when I took the head off.
Any other thoughts on the cause or what to watch out for?
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Old 06-02-2018, 03:50 PM   #28
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Thanks for the update.

Typically over adjusted valves results in low compression as the valves donít fully seat. High compression is typically either the reading was taken when the engine was cold or water in the cylinder.

As you said 150 is the compression for a new engine and they should all be within 10% of each other.

Keep us posted on the progress.
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Old 06-02-2018, 04:09 PM   #29
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you did a great job with this problem, and there is plenty of summer ahead.
That high compression in #3 is a likely the cause. High compression will result in pre-ignition. Cause is probably carbon buildup on valves. YOu should be good now.
Regarding prop pitch, you would at least want a 21p, and you could even go with 4 bladed prop. But check WOT again before going up in pitch.
That's assuming your rpm is reading correctly. 5700 rpm with 4 people in boat and 3L engine is extraordinary. I'm not sure if the supercharged compression in one cylinder is enough to cause this.
After you get it back together, I recommend doing a compression test, and when that checks out try your WOT (not to exceed 4800 rpm). 19P is about right for a 3.0L.
Do you know what your drive ratio is? I'm assuming 1.98.
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Old 06-04-2018, 02:06 PM   #30
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Quote:
Originally Posted by Maxum1800MX View Post

2. Over revving the engine
That's what the mechanics think could cause that. My engine runs at 5700 rpm with 4 people onboard at max speed. That's too high, 4800 rpm should be the max. I don't know yet if it's the faulty rpt meter (marine shop mechanic said they fail often) or if I have the wrong propeller on it (14.5 x 19). I just bought a digital timing light with rpt readout to determine that once I've put the engine back together. If it's the propeller, I need to buy one with higher pitch (21?)
This is my frustration with 'some' previous owners (PO's). They insist on 'going off script' for a myriad of reasons. Commonly they are attempting to jump up on plane with an overloaded boat. But then they never mention or revert the change when selling it. IT sounds like someone under pitched the prop for whatever reason.
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Old 06-19-2018, 01:13 PM   #31
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Thumbs up She is finally running

I've finally managed to put it all back together and tune it up properly. It wasn't without a hitch - first the machine shop damaged the thread on new rocker arm stud, then I've overtightened one of the push rod valve cover screws and broke it (it was leaking oil at the corner so I kept tightening it up, mistake). I also found after cleaning the cylinders that cylinder #3 where I had the issues with the bent push rods, was hit by a valve - huge nick on the cylinder. But this must've happened before I bought it because when machine shop took out all valves for head cleaning/welding prep, they were all straight. I didn't replace the cylinder, I don't think it should affect anything.

That push rod valve cover is very difficult to put in, I had to remove the whole distributor to do it and seal it properly with liquid gasket. After that, I've adjusted the timing (it was a bit off, not much) and checked the rpm with a tachometer. At 1000 rpm on the dash, it was reading 700 rpm. Then on the plane at 4000 rpm on the dash it was reading 3450 rpm. So my dashboard tachometer is off by about 550-600 rpm which is great to know. Since it was showing 5700 rpm at max speed (loaded) it was actually running at about 5000 rpm. So a prop with higher pitch like 21 should work great as it would lower my rpm by about 600 thus always staying within the recommended range of 4400-4800 max rpm. Added that to my Christmas list

Compression test shows 120-135 psi on all cylinders, so that's good for a used engine. I'm wondering if improperly tightened rocker arms plus the 2 that were different style originally were the main cause of this failure? I would love to test out this theory someday by over tightening the rocker arms to see if the compression goes up?

I've found no water in the engine, all clean.

Took the boat out twice this weekend for good, long drives with the family and she runs like new!

Thank you all for all the insights and help. Appreciate it very much
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Old 06-19-2018, 02:35 PM   #32
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Thanks for the update, and congratulations on tackling this problem.
For your tach, have you tried cleaning the connections on both the back of the tach and the wiring harness?
Do you know if you have the original alternator, or if it has been replaced?
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Old 06-19-2018, 04:04 PM   #33
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Glad to hear she's up and running.

Two comments:

1. The valve hit the piston not the cylinder. The piston travels up and down in the cylinder.

2. Overtightening the valves will decrease compression as this will compress the valve spring and slightly open the valve.
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Old 06-19-2018, 04:50 PM   #34
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Ah, that's great to know Mike about the valves. Originally that cylinder had 205 psi, now after proper alignment it's at 135 psi. Which makes perfect sense, since the rocker arm wasn't hitting the valve stem but the valve spring shield instead. Sorry, still getting used to the terminology, I'm not a mechanic
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Old 06-19-2018, 04:54 PM   #35
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Good questions jrsick. I don't have any receipts for the alternator but it looks original. Distributor looks like it's been replaced though. I'll definitely clean all contacts, most of them has that green/grey precipitate on them. What's the easiest way to clean it? Dip them in CLR?
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Old 06-20-2018, 11:45 AM   #36
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A small wire brush or sandpaper is good way to clean distributor contacts.
The situation I'm thinking of was a distributor that wasn't sealed properly. Distributor cap was either cracked or not seated securely. Moisture built up inside of the cap after an hour or so of running.
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Old 06-20-2018, 09:57 PM   #37
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When I disassembled the distributor cap I've cleaned up the contacts with sandpaper. I'm thinking about the wire connectors, if they are corroded it would increase the resistance thus show incorrect reading. Not sure what else can cause tachometer to be off?

there is a little switch on the back but its setup properly to 4 cyl engine. No other way to tune it up, manual says to replace it if not correct
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Old 06-20-2018, 10:12 PM   #38
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Sometimes the tach will be bouncy and rotating the cylinder select back and forth will correct this.

The tach gets its signal from the negative terminal on the coil which is connect to the sensor in the distributor. The spark plug wires will have no impact.
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