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Author Topic: engine piston clearances and ring groove clearance  (Read 953 times)

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Offline southdakotasolo

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engine piston clearances and ring groove clearance
« on: July 20, 2017, 07:03:53 pm »
any one have or know what the piston to cylinder wall cl and ring groove and end cl is for a ms 250 stihl. any one have experience with a ms 311. on my second one, the first one had a bad bearing from the fact so replaced it on warrenty, no guestion's.  have about 3 hours use between both. just wondering about using a richer oil mix (40:1) as we are running around 95 to 105 temperture's here in sd this year.   

Offline joe_indi

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Re: engine piston clearances and ring groove clearance
« Reply #1 on: July 21, 2017, 03:01:54 am »
If the piston is good, with the spark plug tightened, a dry piston without the rings, when dropped into an upright cylinder should slid down slowly.
Rings when pushed midway in the cylinder, should not have a gap over 0.01mm. These are not specs but guidelines I made up myself

Offline southdakotasolo

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Re: engine piston clearances and ring groove clearance
« Reply #2 on: July 21, 2017, 09:56:38 am »
thks for the info joe. having not worked on any 2 stroke engine since my go kart days, a Clinton 6.5 hp and a 2.5 hp,  I have spent the past 40+ yrs working on mostly large diesel engines. use to large clearance's on ring and piston gaps by comparsion. 

Offline Kel71

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Re: engine piston clearances and ring groove clearance
« Reply #3 on: July 21, 2017, 03:00:15 pm »
Your saw will love anything between 32:1 and 40:1.
For me my easy to remember mix is 6 quarts of gas and 5 oz oil. That puts my mix at 38.4 : 1.
I sent you a pm about ring end gap.

Offline southdakotasolo

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Re: engine piston clearances and ring groove clearance
« Reply #4 on: July 21, 2017, 04:15:43 pm »
thks kel71  my old Clintons ran 28:1 if I remember rightly. it was a good mosquito fogger. lol  reason I asked was that I felt a 40:1 would be a good brk in mix for the 311. I thougt I would increase oil till it started to smoke to keep the mosquito's at bay in the shelter belt I am cleaning out on the farm.  lol

Offline PNWRusty

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Re: engine piston clearances and ring groove clearance
« Reply #5 on: July 22, 2017, 12:09:22 pm »
thks kel71  my old Clintons ran 28:1 if I remember rightly. it was a good mosquito fogger. lol  reason I asked was that I felt a 40:1 would be a good brk in mix for the 311. I thougt I would increase oil till it started to smoke to keep the mosquito's at bay in the shelter belt I am cleaning out on the farm.  lol

There is no special break-in oil mix. For best results use the manufacturer's recommended 50:1 and the HP Ultra synthetic. Adding extra oil is not recommended. 2-stroke oil lowers the octane. That's the last thing you want when it's over 100 degrees!

Make sure the carb is properly adjusted. You don't want a lean condition or detonation regardless of whether it's caused by too much oil or misadjusted carb. 

Offline Kel71

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Re: engine piston clearances and ring groove clearance
« Reply #6 on: July 22, 2017, 03:15:55 pm »
I live in Fl. All summer long my saw gets run in 100+ THI( Temp Humid Index).

50:1 is 2 oz of oil per 100 oz of gas.
40:1 is is 2.5 oz of oil per 100 oz of gas.

PNWR how much is that going to drop the octane rating of my 89 octane real gas?

My math might be off but I don't think my saw can tell the difference between 88.23 octane vs 88.04  .  For that little drop in octane my saw's rings and bearings get to enjoy 25% more lube.

I don't know my oils octane so I used 50.

Offline PNWRusty

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Re: engine piston clearances and ring groove clearance
« Reply #7 on: July 22, 2017, 03:57:37 pm »
I live in Fl. All summer long my saw gets run in 100+ THI( Temp Humid Index).

50:1 is 2 oz of oil per 100 oz of gas.
40:1 is is 2.5 oz of oil per 100 oz of gas.

PNWR how much is that going to drop the octane rating of my 89 octane real gas?

Probably not a lot but that's just a guess. Even if you knew the octane of the oil it's not something that can be precisely calculated, it would need to be tested. Every oil and fuel is a little different due to the complex interaction of the various hydrocarbons. And probably only Stihl has done the testing on their oil.

But small changes can and do add up and more than one saw has been damaged by little things adding up. Old fuel has lower octane than fresh fuel. Adding extra oil not only reduces octane but leans the mixture. Carbon deposits on the piston crown and top of the combustion chamber increase compression and octane requirements.

It's best to use fresh fuel mixed 50:1. Then you don't have to worry. Extra oil is of no benefit and can actually make things worse.

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My math might be off but I don't think my saw can tell the difference between 88.23 octane vs 88.04  .  For that little drop in octane my saw's rings and bearings get to enjoy 25% more lube.

Well, we don't know what the octane difference would be. But, yes, if your saw is on the ragged edge of detonation, any lowering of octane is sufficient to make it detonate. And saws do self destruct via detonation. It's most likely to gappen when it's very hot. If by "enjoy 25% more lube" you mean create 25% more carbon deposits, yes, you probably have something there. The extra oil will not be of any benefit to the engine unless you consider more carbon a benefit.

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I don't know my oils octane so I used 50.

Yes, that's your safest bet. I've run my 50cc 026 hard for 20 years using nothing but 50:1 and it's still going strong. All it's needed is chains, air filters, a new fuel pick-up and gas/oil. I did buy a spark plug for it once but this week I found it uninstalled. More oil carbons up the engine without any benefit.

Offline Kel71

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Re: engine piston clearances and ring groove clearance
« Reply #8 on: July 22, 2017, 05:01:35 pm »
Rusty just incase it was not clear. I used 50 as my oils octane rating in my calculations.
Is it to much for you to admit that you had no idea that the octane difference between a 50:1 mix and a 40:1 mix was so close?

Offline PNWRusty

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Re: engine piston clearances and ring groove clearance
« Reply #9 on: July 22, 2017, 10:49:20 pm »
Rusty just incase it was not clear. I used 50 as my oils octane rating in my calculations.
Is it to much for you to admit that you had no idea that the octane difference between a 50:1 mix and a 40:1 mix was so close?

No doubt the effect on octane is not huge. And I hope I didn't imply it was.

I simply said it lowered octane, created more carbon deposits and made for a leaner mixture.

The first two are undesirable, the third can be tuned for but is still a slight negative anyway (due to lower embodied energy). If extra oil had any benefit, we could weigh that benefit against the negatives and decide whether it was a worthwhile trade-off. But since there are no benefits, only negatives, it shouldn't be difficult to determine the appropriate mix to achieve better performance as well as more longevity. Because hard carbon deposits do gradually build up in proportion to the amount of oil used.

Offline Mad Professor

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Re: engine piston clearances and ring groove clearance
« Reply #10 on: August 07, 2017, 08:08:51 pm »
I measured NOS OEM P/C clearance on stihl 038M parts.  It's about 0.0025". That is a 52mm cylinder. An 025 might be a little tighter, new.

You can get a crude estimate using feeler gauges.  Measure at the skirt, pistons are tapered.

Offline ehp

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Re: engine piston clearances and ring groove clearance
« Reply #11 on: August 07, 2017, 10:02:39 pm »
the oil ratio is all on how you set the carb and you have to set the carb for 50-1 as you have to set the carb for 40-1 , the carb is not the same setting . Now on oil ratio big thing is WHAT TYPE OF OIL, I have had saws sent to me from out west using that maxima stuff I could not get the cylinder off the crankcase cause the oil left such a hard ring of junk at BDC is was unreal, All my saws run 32-1 or 40-1 , it depends on what oil Im running and you will not find a single carbon spot anywhere in any of the motors and my motors get a ton of hours on them .  As far as piston clearance unless you got real good measuring tool for that job your just guessing , feeler gauge between piston and cylinder wall is not accurate , .002 is close ,lots of motors are at .0015 but those numbers can change mainly because of what material the bore has sprayed on, chrome bore is tighter than the old cast iron/sleeve style but niskasil bore is tighter than chrome bore because it gets rid of the engine heat better . Also a piston is not round like the bore and if you measure a new OEM piston you will find a piston diameter changes a lot as you measure from the top of a pistons bore to the bottom of a piston say measure every .050 from the top to the bottom, on say a 60mm piston those diameter numbers change pretty much around .010 , reason is because of engine heat , above the ring grows the most as it has the most heat 

Offline Mad Professor

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Re: engine piston clearances and ring groove clearance
« Reply #12 on: August 08, 2017, 03:06:20 am »
You measure piston clearance at the skirt, where the tolerance is smallest.  This is also where the wear occurs, thus "piston slap".  The rings hold the crown away from the bore so more clearance there is fine.

Feeler gauges will tell you within about 0.001".  When I checked the 038M P/C I got 0.0025" using precision machinst micrometer and bore gauge, the bore gauge was measured against the micrometer. A 0.002" feeler would fit, a 0.003" wouldn't.  I didn't have a 0.0025" feeler to check that. 

I also measured the taper on the piston, they are round, unless of poor quality or worn.  The taper of the 038M piston was 0.005" from the crown to the skirt.

Offline Al_Smith

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Re: engine piston clearances and ring groove clearance
« Reply #13 on: August 08, 2017, 04:03:21 pm »
Hmm,matter of fact for some reason I don't get a carboned up cylinder either and I run 32 to 1 mix .Maybe I'm just lucky .
I never heard that too much oil altered the octane rating of gasoline .I'll have to check that out .It can't lower the actual iso octane because there is none in modern gasoline .This is not your grandfathers heptane /iso octane gasoline formulation . ;)

Offline ehp

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Re: engine piston clearances and ring groove clearance
« Reply #14 on: August 08, 2017, 05:29:29 pm »
Mad, I have 4 different piston lines that I make and sell to engine builders and believe me  pistons are not round , a round piston will seize every time once it gets hot . A lot of things most guys donot even think about goes into building a good piston , big things like skirt thickness , dome thickness , the taper of thickness inside the piston skirt , these all have to be correct as once the piston is put under load and heat it gains size so you need to figure that out before building a punch of costly door stops . No Im thinking your not lucky Al, your just correct and have always set the carb correct . Spend as much time as I have on a dyno testing what fuel or oil does . Octane numbers are just that numbers and till you find out what is in the fuel or oil to give those numbers really does not mean much . Lots of fuel today uses ethanol base in it to cause octane numbers which it will but to make the engine live you have to increase the amount of fuel being burnt by the ratio of ethanol in the fuel. I also read lots of times that the best fuel for 2 stroke motors in high test pump gas with no ethanol in it , well that depends on what your wanting out of the motor , if your racing but cannot use methanol but they allow stuff like E-85 or E-98 you run that stuff after you drill the carb out to pass that amount of fuel  needed to make the motor run proper   

Offline Al_Smith

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Re: engine piston clearances and ring groove clearance
« Reply #15 on: August 08, 2017, 06:54:04 pm »
Interesting most people have no idea what it takes to make a piston .The ring portion is round,the skirt has ovality and taper .Years ago that was cut on what's called a "cat head " skirt cutter where the ovality is cut using an eliptical with a mechanical ramp set to cut the taper .Now of days it's all done on CNC machines .
Some automotive pistons are what they term "barrel shaped " .In other words instead of the skirt being the widest at the bottom it's about 1" up and tapers off slightly more narrow towards the bottom .It was all about heat dissipation .Those have been replaced with a teflon coated type allowing tighter fitment without binding .What used to be measured in thousandths of an inch is now in microns,thousands of a millimeter.......trivia 101----- :)

Offline Texas-Jim

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Re: engine piston clearances and ring groove clearance
« Reply #16 on: August 08, 2017, 07:29:01 pm »
Ah the great oil debate again, i love the oil debate!

Regardless of the oil mix one fact stands out, oil Molecules are heavier than gas Molecules so in theory higher oil mixes will lean a system out. Now the question is by how much.  Gasoline weighs 6.3 pounds per gallon, oil weighs 6.84 per gallon. You can not alter one fact without it effecting another one.

example: you go from a 50:1 ratio to a 20:1 ratio. Your engine will now run leaner, and you’ll have to make jetting changes. You’ll need bigger (in number) jets because the oil molecules are thicker and the flow rate (the amount coming through the jet) is less. The volume of fuel has changed. The oil takes up some volume that the gas used to occupy, so your engine is getting less gas and needs to be richened up.  Carb adjustments plays no role in that flow rate.   They are designed for 50:1 so run 50:1 in it.

Yes I know there will be those that yell " I have run 32:1 or 40:1 for years with no problems" Well sadly you have been doing it wrong. More oil does not make a saw last longer, newer saws with chips can actually kill themselves because the saw cant compensate for that.

A properly jetted engine will run better, last longer and develop more power at a lower oil ratio than at a higher one.


I tend to put more trust in the men who designed and built the saw then Billy Bob down the road.










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Offline Al_Smith

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Re: engine piston clearances and ring groove clearance
« Reply #17 on: August 08, 2017, 08:15:59 pm »
I might love the oil wars more than you .It's the ratio of fuel to air not gasoline to oil .Using the theory less is more  is better only applies to 4 cycles .
If less were more how come most saw racers use at least 32 to 1 or heavier mix ratios ? It seems to me those race engines have plenty of power .
Extremely  rich tuned engines will in fact carbon up but it's from the fact you can't burn extremely rich mixtures of fuel,oil or no oil . Proper tuning they don't carbon up .

I have an old McCulloch 650 geardrive with open exhaust that blows black smoke at idle because it has no back pressure ,has to be rich at idle to run .Yet at cutting speeds it cleans right up ,no carbon .BTW that engine runs at close to 10 thousand RPMS .I've got two saws I've owned for over 40 years always ran on 32 to 1,no carbon and run today as good or better than when new .

Offline ButchC

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Re: engine piston clearances and ring groove clearance
« Reply #18 on: August 09, 2017, 10:07:25 am »
I also enjoy the oil threads,, to a point.  Personally I have never had reason to make science out of it. Over the years I have leaned the oil % from 32 to 40 to now 50-1 as the oils have improved. I have never seized a saw, never scored a cylinder and never had one carboned up so what's for me to make science out of? ;) Now if we are going to talk about saws that have been brought to me for repair that's different. I either live in the land of misfits or people in general dont much take care of equipment nor do they know if their saw is running rich or lean. Every carboned up saw I have run into was also running stupid rich, mostly from air filters that has never seen service but now and then a mis adjusted carburetor. Guess what I am saying  is I agree that a carbon problem isn't specifically an oil chemistry problem,  it's a mixture problem,, in my experiance,,,
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Offline PNWRusty

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Re: engine piston clearances and ring groove clearance
« Reply #19 on: August 09, 2017, 11:47:59 am »
Over the years I have leaned the oil % from 32 to 40 to now 50-1 as the oils have improved. I have never seized a saw, never scored a cylinder and never had one carboned up so what's for me to make science out of? ;)

That's been my experience as well, with one exception. I did seize a Poulan many years ago. It was running 20:1 fuel oil mix if I recall. A smoky little bastard too!  But I've been running 50:1 for over 20 years without issue. Just reliable starting, great cutting performance and no fiddling around. It makes me wonder why anyone would run a different mix than the one that is recommended, the mix the saw was engineered to run on.

Quote
Now if we are going to talk about saws that have been brought to me for repair that's different. I either live in the land of misfits or people in general dont much take care of equipment nor do they know if their saw is running rich or lean. Every carboned up saw I have run into was also running stupid rich, mostly from air filters that has never seen service but now and then a mis adjusted carburetor. Guess what I am saying  is I agree that a carbon problem isn't specifically an oil chemistry problem,  it's a mixture problem,, in my experiance,,,

True, running too rich is the #1 cause of excess carbon. But all engines get a little carbon. If it's running properly, the small amount of carbon is not an issue. Running extra oil will cause more carbon but perhaps not enough to matter as long as it's properly jetted/adjusted. Anyone who says their saw has NO carbon is lying or doesn't know what carbon is.

The people who see it all are the people who have rebuilt and repaired literally thousands of saws for professional loggers. You can learn a lot from people like that because they have tons of real world experience and examples. Let's see what people like that have to say about the best fuel/oil ratio for professional saws:

http://www.madsens1.com/saw_fuelmix.htm

In fact, I have never seen a reputable source recommend a higher oil concentration than 50:1.