Need help on piston failure

[prolinews]

Re: Need help on piston failure

it's always fun dropping those suckers all over the shop floor.

 

[Cortes165]

Re: Need help on piston failure

The block has been inspected and cleaned up by the original machinist. All the cylinder bores measure 3.530".<br />I can only give you certain measurements on the piston (because of damage). The following are spots that have no visible damage.<br />Above top ring --> side/side 3.509 and front to back is 3.506<br />Between the rings--> side/side 3.511<br />Just above wrist pin, side/side is 3.514<br />Can't get an accurate measurement past that point.<br />And I don't have a new one to compare to.<br /><br />Type of piston.....Part # 393328 and 030 is stamped on the top.<br />I have better and more pics and they will be set up tonight if that helps.

 

[Dhadley]

Re: Need help on piston failure

The loose bearings are called "full comliment". You can use that style or the caged bearing. At the rpm youre going to turn it, it doesnt matter.<br /><br />Since the pistons were damaged we assume there was some damage to the cylinder walls. Especially since you said

The block has been inspected and cleaned up by the original machinist.

So since he cleaned it up, and we further assume that means he honed it to clean (take material out) it so it could be measured, and it still measures only 3.530, we can be assured that it was too tight to start with. Actually its still on the tight side. It should be at a finished bore size of 3.5305" minimum. It almost sounds like an automotive shop did the boring.<br /><br />Thats a number for an OMC/BRP piston. The piston and ring part number is 396588. Told ya it wasnt a Wiseco.

 

[captweed]

Re: Need help on piston failure

the best way to avoid having to worry about a VRO or other make oil injection system is DON"T USE THEM! Some just plug the oil inlet but I have been told the VRO can still fail on the fuel end.Also was told VRO fails on the rich oil side.You can buy a mixed fuel only OMC pump.Mercs are notorious for failing. My motto: it's not a question of if it's going to fail,it's when.PS; Big thumbs up to the guys at Mar Fab,wonderful group.

 

[Cortes165]

Re: Need help on piston failure

Here are a few more pics of the piston.<br />.<br /> More piston pics <br />.<br />Also one of the big questions was "what was the original failure?"<br />The person (the original owner since '91) who had the engine rebuilt was told that it was a lubrication issue. Rings had scored the cylinders. Basically he told the shop "whatever it was, just fix it". He was just going by what the shop told him. He said leading up to that, the engine was just running crappy, had no power and finally just quit. <br />Now the shop that rebuilt the engine, told me that a ring broke and scored the cylinder.<br />***OK, you take your pick. How can I get a real answer.<br />I am leaning one way since the guy I purchased the boat from is helping me pay for the damage. <br /><br />The clearance sounds like a good theory. I need a measurement off a new piston. How much clearance do you run on these??<br />The cold seizure comments, I can only say the engine was "always" warmed up before giving her rpm.<br />I read above a comment of the piston warming up but not the cylinder because faulty thermostats. I can only say that they are the originals, and were not changed when the engine was rebuilt.

 

[rodbolt]

Re: Need help on piston failure

helo<br /> from the looks of the piston there just was not enough clearence. also looks like boring/honing debris was in the cyl at reassembly. if you reuse the wrist pin bearings use vasoline to hold them in the rods and slip the washer on and slide the pin through. if the rod has an oil hole it goes up. if you buy new BRPwristpin bearings use the beeswax the come shipped in and just stick them all in the rod. DO NOT use a magnet to pick up loose needles. can cause a major headache. its also possible that the shop broke the ring on installation. any shop that will rebuild any motor,especially a cross flow v4,without replacing the t-stats and water pump and usually the VRO/OMS pump on a jonny is a patchem up place and should not be used. the block assy is only as good as the rest of the sub assemblies. looking at the piston pics the piston is scored around the entire circumfernce. normally when a ring preaks up it hangs in a port and causes dots on the piston top and head. carefully check the exhaust port webbing for hairline cracks. I have actaully seen machine shops bore the cyl and miss a cracked sleeve. cross flow jonny's like to crack the webs between the exhaust ports if overheated. sometimes they are tiny and hard to see. however when it heats up it shifts and hangs rings

 

[Dhadley]

Re: Need help on piston failure

Cortes you do not need a piston measurement. Your machinist needs to hold to a specific finished bore size. Thats the cylinder measured after it cools to room temp after the final hone. If youre using an automotive shop I strongly suggest we get you hooked up with a good marine machinist like Mar Fab.<br /><br />It sounds like the original failure was a broken ring from coking. If the rings broke on the starboard cylinder(s) first its probably a safe bet the top rpm was in the low 5000 range.<br /><br />These pistons look like a size issue. Especially since you say the cylinders still measure tight even after they've been re-honed to clean them up.

 

[Cortes165]

Re: Need help on piston failure

Rodbolt says "looks like honing debris left in cyl when reassembled".........how exactly can you see that??<br />Waterpump was replaced but I agree with the thermostats. Why not change a $10 part.<br />The ring breaking was supposidly the cause of the first failure before this rebuild. This time the rings on all 4 look ok. I checked with the webbing with the old eyeball and seems ok. I will keep a note on that for further checking once it's perfectly clean.<br /><br />Dhadley.....i asked about the measurement because I have had other engines built. On them, the cyl was sized accordingly to match a specific piston. Outboards are obviusly different.<br />And yes this was done at an automotive shop. Why the difference? <br />Coking???? what is that?<br /><br />OK, this is what I'm thinking.<br />If this is a size issue, I know what the block measures. If this is an OMC piston .030 over, all I need to do is get the reccomended size for the overbore. Armed with this info, I should be able to approach the rebuild shop and show them my findings. Of course with all the other little details, (like thermostats, etc).<br /><br />Also if the shop decides to help me out and rebuild this thing, that's great for the wallet. But from you guys have said, this just looks like a poor rebuild. Why would I want them to build my engine again if that's the workmanship I can expect.<br /><br />And finally, I really appreciate all the help. You guys have given me great info.

 

[walleyehed]

Re: Need help on piston failure

Rodbolt says "looks like honing debris left in cyl when reassembled".........how exactly can you see that??<br />

Consistancy in the size and spacing of the verticle scores...this is consistant with glass beads, fines that were not cleaned out properly after finish hone, etc...<br />Here's a thought for ya on piston size...If the shop doing the machine work asks for the piston to insure a proper fit, find another shop, period. These pistons are made to fit over-bore sizes as stated,"OVER BORE SIZE", ie, .020, .030, etc...this is the size of the BORE OVER STANDARD. simply put, if we know our std bore was 3.500 and will require "20 over" pistons, the block is bored to 3.5200-3.5205.<br />The shop has NO WAY to tell what the expansion rate of the piston is, and has no reason to "Fit" the piston to the cylinder unless he is not able to properly measure a bore, in which case he is just "being consistant", whether tight or loose, he does not know...only the piston manufacturer knows, and that number changes with bore size, so all they want you to do is bore to whatever size over std is required.<br />Sorry for the ranting....this subject of "fitting pistons" just kills me..

 

[Dhadley]

Re: Need help on piston failure

OK, it was an automotive shop. This is not unusual for outboard blocks bored at a machine shop un-accostumed to marineland. Nothing unusual. Are you saying they assembled the motor too?<br /><br />Coking is a sticky carbon-like substance that builds up in the combustion chamber and on top of the piston. When it gets into the ring grooves it will eventually cause the ring to stick in the groove. It usually starts in the starboard cylinders first. As the ring sticks in the piston, it happens on the backside first, the gap end is left sticking out. As more of the ring sticks the piston loses support and starts to rock excessively. Eventually the piston rocks far enough that the gap end shears off. Broken rings.<br /><br />Coking has a lot to do with fuel and set up. Motors set up in the low 5000 range coke up quicker than those set up around 5600. Motors set up near 6000 rpm may never coke up.<br /><br />Who you use for machine work is up to you. If you dont want to send it to Mar Fab in the states, try Outboard Rebore in Canada.

 

[Cortes165]

Re: Need help on piston failure

The automotive shop only did the boring. He is one of the few that bore outboards and has for 25 years. When I was there a couple weeks back, they had several sitting on the floor waiting to be worked on. <br />The cleaning of the block issue, I could see that as a fault of the engine builder. From what has happened, it wouldn't surprise me that they don't clean the block. And if that was the case, wouldn't that show up earlier instead of running for 20 hours.<br />I can see the coking happening, especially since outbaords run a lot lower rpm.

 

[Dhadley]

Re: Need help on piston failure

I understand what youre saying that it should have shown up sooner if it was an issue with cleaning but you never know. At any rate the double oil was apparently what was keeping it alive. The info youve posted indicates that and that is was too tight.<br /><br />

I can see the coking happening, especially since outbaords run a lot lower rpm.

A lot lower rpm than what? Dont get confused on the rpm issue. Setting that motor up at the lower rpm's increases combustion temps which promote coking. Thats what we call "lugging" the motor. Its working really hard. Too hard. Its not supposed to be set up that way. Setting it up correctly helps keep the combustion temps in line with design parameters.

 

[rodbolt]

Re: Need help on piston failure

the cleaning issue is something I see a lot of in both auto rebuilds and outboard. cleanleness on reassembly aint next to godliness its absolutely mandatory. that same cast iron from boring and the abrasive debris from honeing must be all gone before reassemby. if the intake and exhaust covers are not removed it CANNOT be cleaned. on my motors I use hot soapy water and a siphon type blow gun and a bristle bottle brush. I will scrub it twice in the sink rinsing well after each scrubbing. probably 2/3 the time spent on a rebuild is spent cleaning parts,blocks and gasket surfaces. most shoips delegate that to the minimum wage hands and its just not done properly. most hands that clean parts for me usually do it 3 or 4 times berfore I willaccept it. if I can wipe the surface of the gasket or the bore with a white rag and get any gray on it it goes back or I do it.<br /> and like Dhadley says, just cause it will turn 6k does not mean ya have to run it 6k. but if all it will turn is 5 k then the motor is lugging at all speeds.

 

[reeldutch]

Re: Need help on piston failure

it all makes sence to me and i thougt for shure it was the VRO failing

 

[Cortes165]

Re: Need help on piston failure

I was comparing to a 2 stroke sled. They run around 8000 rpm. Engines are ususally quite clean inside.<br /><br />In regards to the debris, I can't really say what they did. After removing all the covers on the block, I can see where debris can hide.<br /><br />This weekend I am meeting with the builder and hopefully this will be resolved.<br /><br />thanks again

 

[Cortes165]

Re: Need help on piston failure

Well let's see. I have double and triple checked all the cylinder bore sizes. I have even checked the tools to make sure they are calibrated.<br />End result is that all cylinders are 3.529 to 3.5295".<br />The cyl's have seen a quick hone by the machine shop to clean them up from piston gunk. Even a quick hone must take off .001"????<br />In an earlier post, Dhadley said the finished bore should be 3.5305". If that is true, these bores were too tight from the beginning.<br /><br />So I went to the boat shop that did the repair with all this info. Basically they told me I was full of crap. Quote "With a few thou tighter, the engine would run way better. With our years of experience, we know what we're doing". <br />I won't tell you what i did next, but the cops never came so.......<br /><br />This is enough to drive a guy crazy. I might paddle my boat out in the middle of the lake and let it sink. (well maybe not that bad).<br /><br />So is this why my motor died??? Does that extra .001 or 2 make a big difference.???<br />Did the double oil keep it alive????<br />What bore should I end up with to install new pistons?????

 

[R.Johnson]

Re: Need help on piston failure

This whole discussion has only added to the confusion. The cylinder bore must be straight, "no taper". It must be slightly under-cut at the bottom of the bore. ports lightly chamfered. Let's forget about holding tolerance to .0005. You need temperature, and humidity control to do that. You can use a dial bore guage, a telescoping guage, or an inside micrometer to measure a bore diameter. They all depend on the skill, and experience of the operator, and they must be checked again'st a known standard. There is no "black magic" to rebuilding an engine. I don't know what the manufacturing tolerance is on bore size, I would guess + or - .001, after all this is a mass produced part. I doudt if the man producing these parts takes them to bed with him. This engine must have proper fuel flow, ignition timing, cooling, Etc. Now the owner is so "spooked" it will never be right.

 

[reeldutch]

Re: Need help on piston failure

im not confused.<br />if the cylinders are not bored within the propper spec for that piston wich Wiseco provide than it aint confusing to me.<br /><br />reeldutch.

 

[Dhadley]

Re: Need help on piston failure

It was bored tight. The measurements reflect that. Obviously it does make a difference. <br /><br />We have built race motors (stock powerheads) on the tight side and have never, never, never seen an improvement. No rpm gain, no speed gain, no time gain over a measured course. <br /><br />What your seeing and hearing is a typical response from an automotive shop. Nothing new.<br /><br />3.5295 is too tight so we can only imagine how tight it was before it was cleaned up. <br /><br />If you are going to pay him to do it again I would get something in writing that says he'll foot the bill if it happens again.

 

[walleyehed]

Re: Need help on piston failure

Here's the kicker to me...Right on the P/N label on the piston box AND the instructions for that piston, it says: For bore size 3.530-3.531.<br />What part of that is not understood?????<br />Why does the automotive shop know better than the manufacture???