pistons of varying weight

[ob]

I have a question to pose about piston weights on partial overhauls.This subject has been debated several times from differing view points as to whether it is feasible to change one piston as opposed to changing all as a balanced set.I have done some thinking on this and have come up with this figure that I hope will be easy to interpret.<br /><br />Say as a hypothetical,a compression test revealed two cylinders that varied 10 psi.One was 100 and the other was 110.The bore was 3" .Assuming it is safe to say that the exertion of the differing pressures could be looked at as weight exertion or imbalance.<br /><br />So if the square inch of the bore was pi x (radius x radius) then the area is 7.065 or say 7.<br /><br />Is it feasible to figure that 1 psi or 16 oz.per square inch divided by 7 square inches = 2.28 oz.of differing weight exertion to crank?If the pressures differed 10 psi then 2.28 oz. x 10 =22.8 oz.<br /><br />Would it be logical to summize that these particular bore cylinders would have an acting weight imbalance of 22.8 oz.?<br /><br />If so,would two pistons that varied in weight of say 3-4 oz.on equal compression cylinders pose less balance problems than cylinders whose pressures varied 10 psi?<br /><br />Or am I looking at this all wrong?Hope this question is understandable to some of the physics gurus.Thanks for any replies.

 

[Dhadley]

Re: pistons of varying weight

Piston weights are measured in grams. There is a tolerance that each particular model goes by. Within this tolerence we may find a "light" and a "heavy" standard piston. This same tolerence applies to a standard and an oversize piston. Remember - even if the pistons weigh EXACTLY the same does not mean the motor is balanced. <br /><br />To balance a motor you deal with rotating mass. The total recopicating weight (including the crank) is what is balanced.<br /><br />There are many classes we race in that do not allow "balancing" or blueprinting of outboards. IE - you cannot add or take away metal from the pistons, rods or crank. Its not unusual for these motors to turn up to 9000 rpm. <br /><br />If you use a well known brand piston today (OE, Wiseco or Vertex) they are all within the given tolerence for total weight. Some of the pistons from overseas do not necessairly follow that guideline. But thats why they are so cheap.<br /><br />As far as compression numbers go, the piston timing edge and ring location has a lot to do with that. Just like port timing. Port location largly determines compression numbers. Thats why the OE's dont give specific numbers for each model - just a 15 lb tolerence from high to low. Race motors with high port timing and wide port areas may not show all that much more static compression than the same motor with stock ports. But they produce a lot more torque. My modified V6 looper doesnt show a lot more compression than my stock looper but the difference is 200 hp vs over 325 hp.<br /><br />Good luck!

 

[jim dozier]

Re: pistons of varying weight

Static compression measured during a compression test is only a measure of the volume change during the piston stroke and the quality of the seal of the piston in the bore. The dynamic pressure during the power stroke (firing)is more than an order of magnitude greater than the static compression. Huge pressures are built up in the firing stroke and are dependent on the mixture, timing, and flow characteristics (i.e. how much air/fuel was crammed in there) of the particular engine/cylinder.<br /><br />The mass (or weight) of the pistons and other reciprocating parts (wrist pin, rod, crank, bearings)is important because Force(Newtons)=mass(gms)X acceleration(m/sec^2). At high rpms the moving parts have huge accelerations and small changes in mass create large changes in force due to the effects of inertia. The effects of inertia are small and probably inconsequential at low rpms but become significant at high rpms.

 

[Waterbugtoo]

Re: pistons of varying weight

ob,<br /><br />"You're barking up the wrong tree", so to speak. Piston weights on a stock motor cranking stock RPM's is not an issue that requires any serious reckoning.<br /><br />The debate on replacing one piston during an overhaul should be centered on the fact of reusing old pistons in the remaining cylinders.<br /><br />Here are the critical issues:<br /><br />Factory pistons are cast. Although handling stock RPM range with no problem at a young age, cast aluminum will stress crack fairly easily over a period of time. Therefore, its only a matter of time before something is gonna give. Do you take that chance with an item that is not much more than pot metal?<br /><br />Outboard pistons have a locating pin for the rings. The rings do not rotate. This can cause a tiny groove or grooves to form at any given location of the cylinder wall. Obviously if old pistons were reused, at least new rings would be installed, correct? So rings sort of disappear from question, right? Old ones do, but what about the new ones? Many ring grooves in pistons are tapered, can distort from heat and wear, and have damage from carbon buildup. Do you trust and feel confident about a new set of rings in those old worn and distorted grooves?<br /><br />What is the shape of the cylinders where old pistons will be reused? Are those grooves or scratches from the old rings present? Are they egged? Tapered? Pretty much polished out to a real slick surface? Any of those conditions warrant a minimum of honing, and most likely boring. If they are a minimum of honed, clearance automatically becomes an issue.<br /><br />Outboard 2 strokes possess an extremely critical need for proper piston to cylinder wall clearance. To little and it will be guaranteed to sieze, too much and it will cause piston slap (just as bad as siezing), excessive blowby conditions, and suffer in performance and efficiency. Egged and tapered cylinder conditions speak for themselves in results of operation.<br /><br />Many people cut corners and ignore these very things simply to save a buck. Do they save anything when a few hours down the waves, the powerhead explodes in a million pieces? More power to em if they get away with it. I feel however, that the so called experts on this and other boards, should stop providing the answer of "yes you can" to the question of "can I just replace just one piston in an overhaul". I cant imagine the "experts" rebuilding their own powerhead in that fashion knowing the preceding described conditions can be present, can you?

 

[ob]

Re: pistons of varying weight

Thanks for the feedback.Lots of good info to consider and I think I have my answer.Waterbug2,I was not aware that two stroke outboards rings were fixed and were not able to rotate in ring grooves.Seems this alone would be an item that bares close attention if considering a re-ring of the remaining pistons in a one piston change out scenario if the remaining were simply re-ringed and the cylinders not honed.<br />I hope the input continues as this discussion will be a valuable link for someone posing the question : "can I just change out the one piston."

 

[Hooty]

Re: pistons of varying weight

Following Waterbugtoo's reasoning to it's completion, if you burn a hole in a 200hr. piston due to a rat tu*d in a main jet, be sure and rebuild the l/u because it may need it too.<br /><br />c/6<br /><br />Hooty

 

[Hooty]

Re: pistons of varying weight

My last posting was uncalled for. Unfortunately, I can't erase it.<br /><br />c/6<br /><br />Hooty

 

[ODDD1]

Re: pistons of varying weight

Hooty, I was thinkin the same thing...the questions of whats a good part? whats a bad part? how long should a piston last? under what conditions? are all vaild ones, and we obviuosly have different ideas of the correct answers....to me, when you take down an engine with a cylinder failed, you have to go oversize on that cyl, or sleeve it back to standard....but the other undamaged cylinders, if inside of specs for taper, out of round and actual size..they are left with a deglazing hone, not removing a measurable amount of material, but surface finishing....same for pistons, after measurement for size, skirt taper, out of round and a visiual inspection for wear, they are cleaned and the good ones tossed back in with fresh rings....<br /> to put it shortly, if a cyl/piston meet factory specs for wear/damage, why screw with it?<br /> I have gotten heat on this issue in the past,part of which I think is this...an unfailed cylinder is NOT undamaged...with out the proper, careful measurement of the related parts, you cannot make sound judgement of good vs. bad...aside from that, my experience has been with mercury product,these are the posts I limit myself to....I cannot tell you about Johnnyrude stuff, or jap stuff[unless merc made it]...I do not know their strengths and weaknesses.

 

[Hooty]

Re: pistons of varying weight

ODDD1,<br />Roger your reasoning. OEM shop manuals give specs for wear and even tell you to be sure and put the old pistons, rods and bearings back in the same ole hole they came out of. <br /><br />c/6<br /><br />Hooty

 

[dick]

Re: pistons of varying weight

Thought you guys might want to know this .I weighed 6 pistons today.I used the postage scale so you all know.All the pistons are OEM and all port side .3 are std. part #439867 and 3 are .030" over part # 439527.They are for Ficht 1997+ 150-175 hp.<br />All the std. pistons weighed 480 grams.<br />All the .030 pistons weighed 455 grams.<br />Yes the oversized weighed less , go figure.

 

[clanton]

Re: pistons of varying weight

I think a lot of the parts replacements depend on who is building the engine, and who is going to warranty the rebuild, and who cares about the quality of work thay do. If I paid 3500.00 for a rebuildt powerhead I would want 6 new pistons/ rings. If I was doing engine for self, I would not replace any part in specs except for the rings and piston pins circlips. I also dont like rebuildt powers heads 2 stroke or 4 stroke in board engines, because the builders never want to pay labor if you have a problem with the engine, always offer to replace engine, or parts. I dont eat labor for someone else screw ups.

 

[Hooty]

Re: pistons of varying weight

Now that is weird !<br />Does that mean the stb. bank is gonna run faster than the port bank? <br /><br />c/6<br /><br />Hooty

 

[dpminc]

Re: pistons of varying weight

I have commented on this in another thread,as well. I would not rebuild one hole on a customers rig, or any sold as "remanned". However, if I was doing one for myself, I might, if it was on a cheap boat that only saw small lakes, and the only thing in danger was the family picnic. If It was on my great lakes boat, or an ocean boat, I would do it right, as mine,or my friends/family life might be at stake! <br />The manufactures say, in their manuals, this practice is OK. They are talking about failures in the warranty period where hours are very low,and the failure of one piston was prob' assembly related.They dont want to pay for 5 pistons that have low hours on them.<br /><br />In defence of rebuilders, most try very hard to produce a quality product. Some mistakes happen, and the best ones quickly respond to these claims. However, as a rebuilder, you have no idea who is installing the product, if they have solved whatever caused the failure in the first place, no control over break-in, no control over warm-up(we all seen the "off-the-trailer,on-the-gas" guys, no control over debris clogged water intakes, no control over improperly used jackplates, etc,etc,etc. This is why they try not to cover R&R labor. The installer is making money (or saving it) and bears some responsibility to properly affect the repair.

 

[Forktail]

Re: pistons of varying weight

Here's a few things to think about...<br /><br />The stock pistons will be cast. A replacement Wiseco will be forged. Each has quite different coefficients of heat expansion. Each requires different piston-to-cylinder wall clearance. Each will require different rings. Each will react different during cold siezure conditions or high heat conditions.<br /><br />Boring a cylinder increases the amount of air/fuel mixture within that cylinder. The added air/fuel mixture causes more heat in the cylinder when it combusts. This puts more heat on the replaced piston, which is already lighter in material in comparison to it's original size (assuming the replacement is equal to the original in weight). Also removing metal from the cylinder walls increase heat transfer from that cylinder to the entire engine rather than out the exhaust. Unless the cylinder has it's own carburetor which can be jetted properly (unlike shared carbs), it will run leaner than the others. This is more heat. Furthermore, boring usually widens out the transfer ports, which in affect changes timing and fuel scavaging. Agian, possibly more heat.<br /><br />Finally there is a compression change. Compression is directly related to the area above the piston when it makes it's power stroke. This is why compression can be changed vastly just by piston or head design. Of course boring increases that area. Small changes in compression have great affects on engine stress.<br /><br />Balancing has to do with kinematics and inertia forces (F=MRw^2). Any change in mass, radius of rotation or angular speed can have an affect on balance. And the slightest out-of-balance engine rotating at higher speeds can cause problems, sometimes only from vibration or critical stress points.<br /><br />I guess it's a matter of what standards you have when you rebuild and how you plan on using that outboard after.

 

[clanton]

Re: pistons of varying weight

Hooty, I think the different weights on the piston is a quality control problem and geting pistons from different venders. There was a change in headbolts on the 60 degree V4/6s, which required different torque specs. It is possible to find some engines with the new style bolt on one bank of cylinders, and the old style bolt on the other bank of cylinders. There was also something about different bolt size on the rods of the 90 degree looper engines, seems like rod builder, just decided to change bolt size. DHadley or Seahorse may know about the rods, I find no service bulletin for this.

 

[Dhadley]

Re: pistons of varying weight

The larger rod bolts seem to have evolved with the 3.3 L blocks.<br /><br />Another FYI - Vertex pistons are cast like the OE's. Wiseco's are the only forged ones.<br /><br />Aint this fun?

 

[Forktail]

Re: pistons of varying weight

Thanks Dhadley. I forgot that.

 

[Dhadley]

Re: pistons of varying weight

And if ya wanna get trick stuff, Wiseco will make custom pistons for you too. <br /><br />Now we're cookin!

 

[dmessy]

Re: pistons of varying weight

Question for Richard,fka****...did you weigh the pistons with wrist pins/circlips/rings? The whole setup is the reciprocating mass. Just wondering....

 

[dick]

Re: pistons of varying weight

Just pistons , no wrist pins , clips or rings.And just to let you all know ,starboard std. and .030"<br />are also the same weight as the port piston weights(checked today)<br />Just a thought , do any of you think the lighter weight of the .030 over pistons is to compensate for the increased displacement of that cylinder ? <br />Where's a engineer when you need one ?