is piston wear normal vs. bore wear?

[captweed]

tearing down an engine for rebuild we found the bores to be only .001 worn over spec(3.00"),but the pistons gave a measurment that put cylinder/bore clearance way out by several thous.Is it normal for the piston to wear vs.bore?No signs of scuffing,etc at all.I need to get the exact measure of a new piston to ensure a good fit.How can I get this measurement before buying pistons? Crank broke on 70 hp.thanks

 

[rodbolt]

Re: is piston wear normal vs. bore wear?

hello<br /> most jonnyrude stuff is manufactured to the tolerance. no need to fit the pistons to the bore. its a good idea to check it at installation as the pistons are production and the QC people or the CNC operator may have been sleepy that day. by the time you hone it you will be a bit more oversize.after the block is honed recheck the clearences, also check for taper. if the motor is ever overheated piston skirts have a tendancy to collapse.<br /> the machinist I use to bore my blocks is good. all I ever do is check the skirt diameter. and check the ring end gaps. in 30 years or so of engine building I have seen a few that were out of spec both ways .

 

[rabidfish]

Re: is piston wear normal vs. bore wear?

I concurr with Rodbolt. Especially the piston skirt issue. <br /><br />Most folks don't realize why the skirt clearance is so important. The rings make the compression in the combustion chamber, the skirt makes the comression in the crankcase. You can probably de-glaze and re-use the cylinders ( with new pistons ) without boring. Just remember they wont last as long as fitting new pistons into a fresh bore. <br /><br />Curious: How did you find a tolerance for piston-bore dimensions... OMC/BRP never published such a spec... They work like Rodbolt stated... Bore the cylinder to spec. and assemble.

 

[R.Johnson]

Re: is piston wear normal vs. bore wear?

Rabidfish: Where did you come up with the piston skirt, crankcase compression idea?

 

[rodbolt]

Re: is piston wear normal vs. bore wear?

R.Johnson<br /> cause thats how it works. the early 2 and 3cl loopers were notorious for it. the first sgn of a piston going bad was an unstable idle. traced back to worn/collapesed skirts. the primary compression in a two stroke must be as well sealed as the seconday compression. the rings are designed mostly to seal in one direction.<br />the cly sealon the seconbdary side would pass a leak down test yet the idle could not be controled. usually due to piston skirts.

 

[rabidfish]

Re: is piston wear normal vs. bore wear?

Make no mistake: It wasn't my idea, I do not claim to have invented it, nor to have introduced it. My only claim is that I do understand it.<br /><br />It's the crankcase "compression" that forces the fuel/air charge through the transfer ports, into the combustion chamber. Without this positive pressure, the air/fuel does not flow properly. This is aggravated even more at low speeds. <br /><br />This is where Rodbolts statement comes into play...<br /><br />The most common cure/patch is to raise the idle speed to keep it running when you put it in gear. Now we have more stress on the engine and potentially even gear/clutch damage. (There is a downward spiral here that all started with piston wear...)<br /><br />The good news is: It can be avoided with proper maintenance. This is why winterization, proper oils, regular water pump replacements, and annual maintenance is so important. <br /><br />We all know that if you do not winterize an outboard, it will still run in the spring. These items are the things that keep the engine running well for years. And years, and years, and years...

 

[reeldutch]

Re: is piston wear normal vs. bore wear?

i don't like outboards to run for years and years and years.<br />i'd like to rebuild them<br /><br />haha

 

[rodbolt]

Re: is piston wear normal vs. bore wear?

hahahahaah<br /> I love that

 

[mnw001]

Re: is piston wear normal vs. bore wear?

Hey Reeldutch or anyone<br />What makes one carb more lean than the other except maybe float level or plugged main jets?

 

[R.Johnson]

Re: is piston wear normal vs. bore wear?

rodbolt: are you telling me that the piston is acting like a parachute seal, such as the rubber seal in a bicylcle tire pump on the down stroke, and those sguare faced ring's know up, from down?

 

[rabidfish]

Re: is piston wear normal vs. bore wear?

The rings are not all square. Usually the top ring is keystone. <br /><br />The fact is the piston is doing exactly that... It is pressurizing the cankcase on the way down. The piston skirt clearance is what keeps it sealed tight.<br /><br />This isn't just mine and Rodbolt's opinion. It's a fact known by 2 stroke engineers the world over.

 

[walleyehed]

Re: is piston wear normal vs. bore wear?

R.J., Notice how the rings on most have a chamfer on the inner, top side??? combustion forces them out even harder than compression does on the up-stroke, so yea, they have more pressure after combustion starts than from the bottom-up, to that point.

 

[R.Johnson]

Re: is piston wear normal vs. bore wear?

Kenny: I realize that most ring's now used are tapered on the top surface, and are referred to as "keystone" which really is not correct. What ever compression is going into the crankcase is a direct cause of the seal of the ring. I fail to see where the piston skirt has anything to do with the sealing effect. As that piston is going up, and down in the bore, those ring's are the only thing that are probably staying reasonably straight. To carry this argument on, we would get into lubrication seal, piston exspansion, etc. If that piston skirt is acting as a seal, I have never heard of this. As for the tapered ring, that would be an inclined plane. They would act only one way. Combustion forcing them out on the down stroke, compression forcing them out on the up stroke. I see that piston only as a carrier, not as a seal.

 

[rodbolt]

Re: is piston wear normal vs. bore wear?

the piston aids in sealing the primary side. the skirt also aids in keeping the piston straight in the bore. but like the old 3 ring pistons, skirts add drag. its a trade off. the rings also aid in heattransfer from the piston to the walls. but most dont seal as well on the down stroke. this happens in twostrokes. in four strokes its not nessasary. the oil grooves on the piston serve a dual function, they help retain lube and the oil acts as a seal. kinda like the labyrinth seals on the mains. that was a trade off we did when hydroplaning. cut up the skirts and run TRW single ring pistons. idle quality was not an issure it usually only had to idle for seconds. we had soilid machined heads and no water pumps sowe could not sit still long. but we did not troll for rockfish for hours at a time.

 

[rabidfish]

Re: is piston wear normal vs. bore wear?

.

 

[captweed]

Re: is piston wear normal vs. bore wear?

The spec for clearance was in the manual,min of .0045,max .0065.Our measuring gave a total of .011,pretty sloppy.I've seen a lot of motors come in my buddy's shop where you can rattle the piston around after pulling the heads off.My input to the ring discussion is simply that carbon buildup is bad,it sticks the ring in the groove and gets behind it and forces it out.Sierra and Sea foam are very good carbon removers.Since the piston is not supposed to contact the cyl. wall,I was surprised to see so much piston wear,the bore was only.001 over.While it's apart,going with all new piston,rings and bearings,good for another 20 years! thanks

 

[rodbolt]

Re: is piston wear normal vs. bore wear?

for just a few coins more you can bore it and do it right. taper is also a factor.

 

[rodbolt]

Re: is piston wear normal vs. bore wear?

hello again<br /> what manual did you get your service specs from. most modern 2 strokes are worn slap out at .004 piston to wall clearence. double check your measurement methods and the maximums and minimums with a factory shop service manual. <br /> some times the clymer and seloc are way off.

 

[rabidfish]

Re: is piston wear normal vs. bore wear?

His 3" bore can only be a 49.9 Cu In 3 cyl.<br />The older manuals had a spec in the book of 4.5 to 6.5 thousandths.<br /><br />Those old 3 cylinders had alot of piston expansion. The truth is, the piston transfers heat to the cylinder walls. That's why it doesn't melt at hi rpms. Hence the collapsed skirts from severe overheat.

 

[Dhadley]

Re: is piston wear normal vs. bore wear?

Todays pistons are tapered, cam ground and sometimes barrel shaped. It makes a great deal of difference where you measure them. Different motors have different skirt to wall requirements. The normal wear tolerence of a cylinder is usually .004 max. The largest part of the piston skirt is rarely at the nominal size. IE -- a standard piston for a 3.00" bore is not 3.00" at the largest part of the skirt. <br /><br />Some motors have a minimum skrt to wall clearance of .0055". Some may be .0068". Some may be .0045". A lot of the skirt to wall clearance depends on the shape of the piston, the cylinder material, how well the motor cools, ring material, dome thickness, fuel, and on & on.<br /><br />There are also many different designs of rings. Some square, some keystone, some semi-keystone and some (like the E/J top ring design) are pressure back. Lots of theorys about how the ring is to disapate the heat and seal on the compression stroke. And just as many theorys on piston ring groove design and ring tension.