Are boring bar witness marks in cylinder cosmetic?

[SunDaze]

I have a question about boring bar witness marks inside an engine cylinder and am looking for some input from persons who have actually machined blocks with boring bars as well as anyone with experience that might help answer the question.

I had a 2006 3.0L V6 Optimax (200 HP) block bored and honed .015” over as part of the rebuild process. This was done by a professional shop. I did the final cleaning of the machined block. I saw a nice cross hatched pattern on the cylinder walls left by the ball hone the professional shop had used after boring. All seemed good.

I reused the crank, connecting rods, heads and sealing rings but put in all new pistons, wrist pins, rings, bearings and seals. During assembly I noted the ring gaps were about .019” to .022” instead of the Mercury specified .010” to .018”. Because the pistons were sourced by the engine shop at Pro Marine I called the shop to confirm these end gaps were OK. If these were stock Mercury Marine pistons and rings I would not have done the assembly but because they were Pro Marine I did not know if they should have the identical ring gap spec of the stock Mercury pistons. My engine shop said to proceed with the build and that these gaps were good. I finished the build.

As part of the rebuild I had all fuel and air injectors cleaned/flow checked at Brucato. That lead to replacing 3 of the old air injectors and 1 of the old fuel injectors as part of the rebuild. The 3 useable air injectors flowed about 915 CC/min on average after cleaning. I also put in a complete set of brand new stock reed valves because one of the original petals had about 10% of it chipped off (2170 hours of use...). Finally, I put in a brand new electric oil pump although the old one seemed fine.

The engine break-in was done on Lake Ontario over 2 weekends with seawater temps between 50 and 60 degrees. The engine survived break-in and ran great during break-in. Not a hint of trouble. The boat is a twin screw 26 foot Boston Whaler Conquest. The rebuilt motor was on the port side. I simultaneously was breaking in a new long block on the starboard side. I never took it over 4500 RPM during the break-in or tried to trim her out. I followed the Mercury manual. For those who might not know, the manual says that during the 120 minute break-in, when over 2500 RPM (and some amount of minimum but undefined vacuum load) a timer runs and 2X oiling is in effect. Below 2500 RPM it is normal oiling and the timer is not accruing time.

A few minutes after break-in had timed out while cruising at about 4000 RPM I brought the throttles gradually full forward and trimmed the boat out for maximum speed to check wide open throttle RPM. Both engines turned about 5350 to 5400 RPM (spec is 5000 to 5600). I held it there for about 8 seconds as I checked water temperatures. All seemed fine. I then began gently backing off the throttles towards 3/4 throttle position. Somewhere around 4500 RPM the rebuilt engine dropped out. I did restart it briefly 10 minutes later for docking maneuvers. A dockside compression check showed no compression on cylinder #2. All other cylinders showed about 105 to 115 psi. A teardown showed the #2 piston to be badly scuffed over a large majority of it’s circumference and length. The rings were still in place but badly scored. The cylinder was badly scored with a lot of residual aluminum on it, too. The #4 piston also showed significant scuffing from the bottom of the wrist pin bosses all the way to the bottom of the piston skirt. The other 4 pistons/cylinders looked good. Here are pictures:


I was concerned about the tiny aluminum particles possibly having damaged the brand new air injector that happened to be on cylinder #2 so I sent it to Brucato. I did not send the fuel injector because I had no concerns about it having been damaged by the event. The air injector flowed good at idle but just a hair out of spec, LEAN at wide open throttle after it had warmed up. 896 CC/min. That is about 2% below the average of the three original good ones. Previously Brucato flowed a set of six air injectors for me and saw #s like 903 minimum to 930 maximum. So this one being 896 seems to fall just below the acceptable minimum of about 900. Part of this lesson is to not trust brand new injectors. Have them checked prior to use. But I question if this slight bit (2%) of lean tendency on a stock motor is enough by and of itself to generate sufficient heat to cause this failure. Maybe it was another bit of weight on the camel’s back, though.

During the cleaning of the failed block after strip down I noticed something odd in the 4 “good” cylinders (good meaning there was no scuffing on the pistons or scratches in the cylinder walls and compression was in spec). There was a series of very evenly spaced circumferential rings the entire length of the cylinders. At first I thought it might have been caused by the rings somehow but the marks extended the entire length of the cylinder, well past where the rings can travel. My next thoughts were that these were witness marks from the boring process. Although a bit more difficult to see because of the scratches in the #2 and #4 cylinders, they also had the circumferential marks. Here is a picture showing the witness marks from the boring process:


I called the shop which bored and honed the block to discuss the fail and marks in the cylinders. I sent them pictures to review. They confirmed these were indeed boring witness marks. They said a profilometer check would show less than .0002” worst case and probably a lot less than that. The ball hone’s cross hatch pattern had obscured these boring bar witness marks during my initial inspection of the freshly machined block. But now after the 2 hour break-in they were quite visually evident as can be seen in the picture. They can not be detected by touching or finger nail so I believe they are less than .002” and believe the engine shop saying these are very tiny deformations. I have rebuilt a few ( 4 stroke car) engines before and never seen this but am not an engine machinist or expert. I envisioned that the honing process, while not having to be overly deep, would usually remove a sufficient amount of material to totally remove the boring bar witness marks. My engine shop told me these marks are cosmetic and not the cause of the fail. The said it was likely a lack of sufficient lubrication or a lean condition or both. They also said trimming her out for a max RPM check with only 2 hours run time on her, even for 8 or 10 seconds, was tempting fate. They suggested 10 hours of run time is a much safer break-in time prior to checking for maximum WOT RPM. The Mercury manual says that after the 1st hour of break-in short WOT burst are OK. So I figured that after the complete 2 hour period it should not be a big risk to push her for 10 seconds to check WOT RPM.

I did a crude check of the relationship between piston size and cylinder size by using plastic Artus shim stock. The bore is about .0055” bigger than the piston. That clearance, I believe, is right on the Mercury spec nominal. I do not have capability to check roundness or taper of the cylinder bore.

I am looking for the root cause of the damage seen on #2 and #4. I am thinking that there may be two different mechanisms between #2 and #4 since the symptoms look a lot different. Perhaps I caused a burr at the #4 piston bosses while assembling it and the #2 scuffing is a totally unrelated issue. Perhaps it is as simple as an improper cleaning by me of the machined block.

While working on root cause of the failed rebuild I bought a brand new factory long block to replace the failed rebuild. I swapped over all components (except spark plugs and the #2 air injector which went down to Brucato for verification) from the failed rebuild such as fuel rails etc to the new long block. I put yet another brand new air injector on cylinder #2 for this new long block while Brucato looked over the one with a mere 2 hours use on it from the failed block. The new long block survived break-in and is running great although I have refrained from pushing it to WOT until I have a better feeling about what caused the rebuilt motor to fail. The new long block’s compression was checked after break-in and looks great. Spark plugs all look good. As an afterthought I sent the Fuel injector from #2 down to Brucato and put a known good one onto the new longblock. The #2 fuel injector from the failed rebuild flowed perfectly. As extra insurance I have started a 2nd break-in on both long blocks.

I also checked relative oil flow between port and starboard units. For the 1st break-in I filled both oil tanks right to the top, put both engines into break-in mode, and then confirmed after the 2 hour break-in that both took the same amount of oil to top off (about 1 gallon each). That seems to map out right to about 25 to 1 oiling ratio based on using about 25 gallons of fuel on each motor during break-in.

I guess my primary question is a confirmation that if the boring bar marks are indeed less than .0002” peak to valley that the boring bar marks are cosmetic and sometimes an inherent phenomenon of a proper bore & hone job if a cylinder is checked a couple of hours into it’s new life. My 2nd question is about the 2% lean air injector. Is that really a big enough issue on a stock motor to cause this failure or is it more likely that I am missing other factors that helped kill this engine?

Thanks for your input.

Rocket

[baja200merk]

Were the water deflectors reinstalled in the block?

It looks like a cooling or oiling issue my 04 opti is just out of break in I followed the factory 2 hour breakin as you did and it has pro marine Pistons in it as well. It has been on the rev limit a few times by now with no issues.

my boring bar does not leave those marks in the bore but .0055 seems tight to me.

[TEXAS20225]

HMMMMM? i get around 25-30 blocks machine every year some up to .030 over size and ive never ever got one with those marks on the sleeve also a ball hone is never used on my blocks i get a Sunnen 8 stone ridged hone job ,cylinder is perfectly round and has a machined cross hatching not some one moving a hone up and down in the hole ,then each hole get the ports hand champered, my first question was the hole bad before?, because this is exactly what happens when sleeve is not touching sleeve holder all way around the heat will NOT transfer when motor warms up heat collects in that hole and the sleeve expands in and starts material to start being removed from piston and deposited on the sleeve wall . my first 3.0 the sleeves had spun in the sleeve holder and i put 6 new over size sleeves in .

[Dave Strong]

That is some ugly azz machine work.

Dave

[Onetime]

Step out on a limb here.
Looks like cylinders were bored to finish size then hit with a ball hone.
My guess the machine shop didn't have proper honing equipment for honing cylinders with ports in them.
A ball hone is only meant to address the plateau of the cross hatch from proper honing. Personally I don't like them and never use them.
Properly done cylinder is bored undersized and then finish honed to size. This process removes all the machine marks from the boring and produces the proper finish with a crosshatch pattern for the rings.
Was the shop an outboard machine shop or an automotive machine shop?

[powerabout]

ball hone is for rounding the edges of the ports not honing the cylinders in lieu of a rigid hone
and
the $64k question....what went wrong in the first place or was it just a rebuild for worn engine?

[SunDaze]

Thanks for your input. Yes, the rubber water deflectors were in the cylinder block. The oil in the 3 gallon containers is always Mercury DFI oil. I have owned the engine since it came from the factory 9 years ago.

[tlwjkw]

don't use tha powered Sunnen for cylinder finish they aren't a complete machine shop.....

[SunDaze]

The 9 year old engine which I have owned and operated since coming from the factory started acting up late in 2014 with a rough idle and an occasional stalling issue during docking maneuvers. I eventually tracked that back, at least partially, to out of spec compression. One of the reed valves on cylinder #4 also had a small chip out of it. Brucato found a lot of the injectors flowing rich but 11 of the 12 (6 air and 6 fuel) were cleared by him after cleaning and reflow for use. The compression check just prior to tear down of the 9 year old original engine was as follows:
2-87 1-95
4-95 3-85
6-62 5-68

Upon tear down of the original 9 year old engine there was nothing odd seen like spun cylinder sleeves. As to #2 piston/cylinder specifically there was some moderate localized scoring at the bottom of it's skirt and cylinder. The low compression on the bottom two cylinders was caused by uneven ring wear. This specific block design feeds the cooling water to the cylinders in on the lowest cylinder then forces it up and around by use of rubber water dams between the cylinders. So the bottom cylinders are subject to the coldest water on one side where the water comes in yet also subject to the warmest water on their opposite side just prior to a junction where the water can either flow into the cylinder head proper or else out into the mid section's poppet valve. I will start another separate thread on that with pictures in the near future as I believe it to be an intrinsic issue which caused many problems for a lot of people. But I don't think it had anything to do with this specific rebuilt block's premature demise.

When I talked to the engine shop they said they only used boring bar and then ball hone. No conventional stone hones were used in the process. I think the general flavor of the feedback so far is on track – the boring came too close to the finished diameter and the ball hone did not remove enough material to completely remove all traces of the boring. That said, is that enough to have been the root cause of this failure? 4 of the cylinders and pistons have no scuffing at all. Also, I am still amazed that I missed these boring bar marks upon receipt of the machined block. If I did not have a reason to tear down the rebuild 2 hours into it’s new life I would have never seen these marks.

Thanks again for everyone’s input.

[tlwjkw]

When I talked to the engine shop they said they only used boring bar and then ball hone. No conventional stone hones were used in the process. I think the general flavor of the feedback so far is on track – the boring came too close to the finished diameter and the ball hone did not remove enough material to completely remove all traces of the boring. That said, is that enough to have been the root cause of this failure? 4 of the cylinders and pistons have no scuffing at all. Also, I am still amazed that I missed these boring bar marks upon receipt of the machined block. If I did not have a reason to tear down the rebuild 2 hours into it’s new life I would have never seen these marks.

Thanks again for everyone’s input.

...............

[baja200merk]

For proper crosshatch use a ball hone then rigid hone then Ultra finish plateau hone. I sleeve, bore and build 30 engines a year this way with no issues.

[Slimm]

I do not consider myself a true machinist because I have only about 5 years time doing that work. I do know from that experience that you DO NOT bore a block 'close' to finish then toss a ball brush in there. Those ball brush (dingleberry brush around here) are for surface plateau finishing and will never remove enough material to smooth the rough finish that boring a cylinder leaves, that is what a stone hone is designed to do.

As far as the witness marks go, I have seen them before but I lack the years of experience to say with any confidence yes or no in regards to if they are good/bad/problematic.


Slimm

[[email protected]]

You did not see the marks as they were not visible during assembly,
not only is he incapable of honing to industry standard, but I question his boring techniques as well.

a ball hone is only used to create a surface for the rings to seat to such as a rering and has no accuracy to finish a boring procedure.

in the future make clearance measurement during assembly or require a blue print from you machine shop.

[thornl01]

That is some sorry ass machine work there.

What cross hatch ?

[TEXAS20225]

whats cross hatch????? you may be on the wrong postcross hatch is the finished surface on the cylinders they are diagonal hatch hone marks going both ways in the hole its in every mercury block ive ever seen unless its was hatchet jack machine shop , Jim ruck used a high platue brush on all my Nic blocks after he machined a cross hatch job in it it gave a ultra finish in them, the machines with all hone jobs is a BIG AZZ drill mota it is however not jumped up and down a few strkes and they ya go deal its is ridgid and hole is perfectly round . slim daddy is correct hole should be slightly smaller and finish out with the hone James perry has done my blocks for years he does not own a ball hone i dont think . i ve used a lot of shops in the last century (well almost seems like) most of them were BS artist . Tommy ball hone