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Old 12-06-2014, 12:16 PM   #1
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Lehman 120 Piston slap

Purchased my first trawler a 1974 Grand Banks 32-536 and I am a novice. The previous owner rebuilt the 2714e in 2002. It was seized, 800 hours on it. The shop had a tough time freeing up # 6 cylinder. New pistons .20 over, new oil pump, water pump and one new rod for # 6 . One month later # 6 was knocking, the shop pulled the motor honed cylinders and replaced # 6 cylinder with a new piston .15 over. The block was bored out .20 over. The engine had 55 hours on it since the second rebuild when I took ownership 9/1/2014. I now have 90 hours on it and the # 6 knocks at start up and as engine warms turns to a faint tap. Do any of you have engine knock that fades when engine warms up? Why would they replace a .20 over piston with a .15 over? I sent out an oil sample for analysis and will post results. Thanks for reading, Mike
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Old 12-06-2014, 05:20 PM   #2
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Unbelivelable, 90 views and no comments, whatsamatter folks? This group never has nothing to say.

I don't understand the size difference from the overbore to piston size at all.

Thoughts

how do you know 6 is slapping?

how do pressures of all 6 compare?

What shape are the injectors in?

What shape is the pump in?

What does a REAL GOOD Lehman mech. say?




What say the hive??
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Old 12-06-2014, 05:30 PM   #3
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Unbelivelable, 90 views and no comments, whatsamatter folks?
Not enough knowledge to help, just dropping in to possibly learn something
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Old 12-06-2014, 05:41 PM   #4
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Not enough knowledge to help, just dropping in to possibly learn something

That's fair


We have the knowledge to help this guy out, get in here you guys with knowledge!
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Old 12-06-2014, 06:10 PM   #5
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Scott, The simms pump and all injectors were rebuilt in 2002. I used a automotive stethoscope to isolate the sound. The engine turns over quick and hunts a little at idle. She will run 9 knots at 2200 but i run her between 1600-1750 and get 7 1/2 knots. Temps stay even across block with gun. Back end is 0 to 10 plus degrees and gauges read 170-180 under normal load. I have not done a compression check but engine runs good.
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Old 12-06-2014, 06:14 PM   #6
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Unbelivelable, 90 views and no comments, whatsamatter folks?
While we can offer a lot of guesses and speculation, to my knowledge there are no truly professional diesel mechanics on this forum with real working experience with the FL120 including teardown and buildup. There are a couple on the Grand Banks owners forum but I don't think there are any here.

Rick Boggs came the closest to filling the bill although he no longer participates in this forum. I don't know how much hands-on experience he's had with the FL120, although he once gave me a terrific explanation of how the engine's in-line, jerk injection pump works.

So my answer to the original query would be the same as what I would have assumed the original poster would have thought of--- contact a reputable diesel shop with experience with the FL120.

While it's usually impossible to diagnose an engine problem over the phone, particularly one like this one, another first step would be to contact Bob or Brian Smith at American Diesel. They might at least be able to recommend a good shop with FL120 experience in the poster's area.

But outside of those two things, anything I could offer would just be me talking for the sake of saying something. It wouldn't get the poster any closer to a solution.
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Old 12-06-2014, 06:25 PM   #7
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Greetings,
Given the regular mechanical racket a diesel makes how does one ever hear a knock or a slap? Or a thump or a miss for that matter...

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Old 12-06-2014, 06:27 PM   #8
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Marin, I spoke to Brian Smith and he gave me some good advice. I will call him back after I have the results of the oil sample. He was very helpful.
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Old 12-06-2014, 06:37 PM   #9
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This is at least few inches over my head and probably the reason others haven't responded is for the same reason.

Something that occurs to me is that an engine can make many noises and the cause of each is sometimes easily recognized and others can be mistaken for other problems and much confusion can result.

Piston slap usually takes many years and thousands of hours to materialize.

In this case the engine has been worked on by several professionals (assumeably) and the fixes have not fixed. That leads me to suspect that piston slap may not be the problem. But the mecs not only think it's piston slap but on #6 cyl.

I may be just a bird barking in the tree tops here trying to make up for lack of response but it seems to me that piston slap is caused be excessive wear between the piston and the cylinder. The cyl being iron and the piston being aluminum the wear would seem to be occuring to the piston and that wear should be visible and better yet very measurable by any competent mechanic or machinist. The only thing I can think of that would cause extreme wear on the piston in a short period of time would be installing a piston too large for the cylinder bore. In that case your oil should have lots of aluminum from the piston in it.

Another off the wall thought is that the ring gap could be too tight causing severe wear of the rings and perhaps to probably stuck rings permitting lots of blow-by and heat and expansion of the piston skirt and thus wear and hence piston slap.

Just thinking out loud text wise. In a think tank thoughts from one person can lead lead to trails that can be followed by persons w different knowledge and sometimes all the dots of a problem can be connected by a group. Innovation is said to be frequently a product of a group. And IMO innovation and problem solving are similar. But w innovation frequently you don't know the problem.

Marin I read your post and disagree that FL120 experience is necessary and could even put the problem solver at a disadvantage.
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Old 12-06-2014, 10:27 PM   #10
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Marin I read your post and disagree that FL120 experience is necessary and could even put the problem solver at a disadvantage.
That's an interesting take. So you are saying I should take my Ford pickup to the Audi shop for troubleshooting and matainance, and my Range Rover I should take to the Fiat shop?

The one thing I would ask the original poster is are you sure you're not hearing a valve or valves that need adjustment? The FL120 has mechanical valve lifters (as you probably know) so the clearance between the top of the valve stem and the rocker arm needs adjustment every couple thousand hours or so. If the gap is too wide, there will be a clack every time the rocker arm comes down on the valve stem. This is most noticeable at idle rpm.
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Old 12-06-2014, 11:49 PM   #11
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Busterbrown

You may want to post same information and question on boatdiesel. Number 6 piston you say, there might be a bit more to this ------.
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Old 12-07-2014, 12:59 AM   #12
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Marin I have had the valve cover off while engine was running and I think the knocking sound that fades away when engine gets up to temp is piston slap. I like to fix things myself but at some point I will pay an expert once I give up.
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Old 12-07-2014, 01:49 AM   #13
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Can I offer the non mechanic thoughts of a then amateur motor sport competitor doing his own maintenance many years ago.
I believe No.6 is often the site of sludging in the coolant passages.
It can`t be coincidence no.6 was a problem with the seizing, required revision later, and is the focus of piston slap now. You can use a kind of stethoscope to listen at different points, but with all the clatter, not easy.
Wonder if 6 had to be bored out more than the others.
Surely piston size should match the rebore, seems it did initially. Why a smaller diameter piston in no.6 when the rework was done? Isn`t that a recipe for piston slap as the rings wear in.
Brian Smith is indeed your friend.
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Old 12-07-2014, 02:04 AM   #14
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Scott, The simms pump and all injectors were rebuilt in 2002. I used a automotive stethoscope to isolate the sound. The engine turns over quick and hunts a little at idle. She will run 9 knots at 2200 but i run her between 1600-1750 and get 7 1/2 knots. Temps stay even across block with gun. Back end is 0 to 10 plus degrees and gauges read 170-180 under normal load. I have not done a compression check but engine runs good.
As a quack, and not a diesel mechanic, but who owns a FL120 of similar vintage, which still goes well, burns negligible oil, and runs like a sewing machine, but does clack a bit until warm...I'd be tempted to say, chuck the bloody stethoscope away - they hear way to much. Of course that's just my view. Some others on here might want you to strip her down. But really, if the compressions are within coo-ee, (Aussie for near enough), and she's not bellowing smoke, and using a lot of oil, why worry - be happy…
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Old 12-07-2014, 03:31 AM   #15
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I would question how well the block was cleaned during the first rebuild.
I can think of several things that would cause #6 to seize that soon after a rebuild.
1. a fuel problem- amazes me how few people check the injection pump for flow at time of rebuild and also test the injectors for opening pressure and spray pattern. You say this was done - well I would be tempted to do it again. Pull the pump get it on a flow bench and make sure it is delivering the specified c.c. per thousand strokes. Test #6 injector. The noise you hear could be a fuel knock.

2. Heat- it is not uncommon for different ends to be hotter than the other end. I wonder how well the block was cleaned out at rebuild time. I would think that any shop doing a rebuild would clean the block in a tank type cleaner or in a modern spray wash unit. Now the down side to modern enviro friendly units (in my view) is that you may not be able to clean some types of scale deposits out of a block that the old soak tank with nasty caustic solution did a great job on.
However, it is possible to have a localized overheating event.

Now you say "they" honed the cylinder out after the last failure. By Hand? by machine? was the engine out of the boat?
Honing - to me - means removing the engine block and mounting said block in a honing machine - These machines make round straight holes within a few ten thousandths of an inch. If it was hand honed that could be your problem too.

Lots of questions few answers. Did anyone ever work out the cause of the first failure?

Now in term of the 15 thou piston in the 20 thou hole- that is usually done if the engine has a history of heat related seizures in a particular cylinder. As a for instance- it is common practice with the 6.5 GMC diesel that is used in Hummers to make the last cylinder on the passenger side slightly larger than any of the others. These engines have a well known problem with cooling that cylinder because of the engine design. I think 5 thou might be a little high but probably not too out of line. I will look for the article I read about the GM overhauls.
However, your shops solution is likely based on that type of thinking. And they may not be wrong. you could be having a heat/ cooling related problem. I don't work on enough of these engines to know if there are any of those sorts of problems in this engine family.
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Old 12-07-2014, 10:57 AM   #16
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Bryan, I dont know how the block was honed and I dont think the previous owner put enough hours on the engine to make sure everything was worked out. From what I have read the #6 tends to run hotter, it is the furthest away from the coolant pump. I called the shop that did the rebuilds and asked why a smaller piston was used and his reply was it was 14 years ago I was not working here so I cant tell you why they did that.
Thank you for explaining why a smaller piston might be used I can see the logic.
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Old 12-08-2014, 07:52 AM   #17
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Water cools better than anti freez , so you might consider a 35% rather than the more common 50% mix , if the local temperature does not require 50/50.

Same anti corrosive protection , just a bit better cooling.

Piston slap might be annoying , but if it goes away warm , , cure it on the next rebuild.
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Old 12-08-2014, 01:45 PM   #18
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Just my 2 cents, and we talked about this and this is what I was thinking, why not do a cooling system flush? It wouldn't hurt and might loosen up any sludge that stuck on the cylinder wall or area.

Then look at the fuel system. But, you say the sound lessens as it warms so I'm not convinced that it's fuel. If it smooths out idling as it warms, and sound decreases, it might not be a huge problem. But I'm guessing that.
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Old 12-08-2014, 08:17 PM   #19
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Marin,
I wouldn't take your Ford to the Audi shop or the Range Rover to the Fiat shop ... that would be stupid. But for a quirky problem that other professionals haven't been able to find I'd consider taking the problem vehicle to a different brand but similar vehicle. Your RR to a Jeep shop and your Ford to a Chevy shop. The mechanics of other but very similar machines may pay attention to things that complacency would normally lead them to haze over or just not notice.
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Old 12-08-2014, 08:53 PM   #20
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I think that should be about sending it to a shop that does nothing but engine rebuilds. If the automatic transmission in your car or SUV dies I would suggest that a shop that only builds transmissions might be a better choice than the dealer who might build one once in awhile ( or maybe never now days- exchange being the norm) In this case a shop that does extensive engine rebuilding and has the MODERN equipment to do the job might be a better choice.
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