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Sportster Motorcycle Motor - Top End Discuss Sportster Motorcycle Top End issues. Rockerboxes, Valves, Cylinders, Pistons, Rings, Lift Rods, etc...

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  #1  
Old 27th October 2005
CBAS5 CBAS5 is offline
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Default Blowby Article

Here's a descent blowby article that might interest some people:

http://motorcyclecruiser.com/tech/blowby/

Here are some snippets that I find interesting and worth noting.

Quote:
Blowby occurs when the piston rings fail to do their job properly. They may be worn or they may never have seated properly in the first place. This failure to seath the rings tends to be somewhat common is large-displacement V-twins.
So how do you seal rings properly?

Quote:
it takes large throttle openings and high rpm to get the rings seated.
This seems to support this guys way of breaking in engines:

http://www.mototuneusa.com/break_in_secrets.htm

Quote:
Once the oil is pushed into the air cleaner, much of it condenses and drops out of the airstream, making a mess in the process. If this was the only problem it’d be bad enough. Unfortunately, some of the oil is pulled back into the engine along with the vapor and burnt, where it does more damage by glazing the cylinder walls and creating carbon deposits in the combustion chamber. At this stage the engine is on a downward spiral. It’s only a matter of time before you’ll be doing a top-end job.
This seems to support a good reason to drop a few dollars on a breather kit.


Quote:
Because the hot oil in the crankcase is easily displaced by the increased pressure and because it has a perfect path out of the crankcase via the breather, the first sign of impending trouble is nearly always oil migrating into the air filter.
Should some of us be worried?
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Old 27th October 2005
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Good article...

Nothing new, tho...

Stevo's been tellin us that for years!

Certainly in the case of conversions, especially, bedding the rings is arguably the most important part....

Big difference between normal venting and blowby, tho...

I'd hate to see anyone get nervous about nothing...

I like the part mentioning that TOO MUCH OIL can cause excessive crankcase pressures....puking oil is generally NOT blowby, so much as a purging of oil that needs to not be there.....
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  #3  
Old 27th October 2005
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I think he misses a number of points.

First, what exactly is ring seating? He didn't explain it at all.

Neither rings nor cylinder bores are perfectly round. As a result, when you put a ring on a piston and into the bore, there's actually very little surface area of contact between the two.

Honing puts a texture on the cylinder wall that allows the ring to machine the cylinder wall into the same shape as the ring, thus increasing the surface area of the contact patch. The ring is said to be "seated" when it has carved the cylinder into it's shape and there's contact all the way around.

Many, many times I've pulled bikes apart and looked at the cylinder walls and spotted places where the ring was never touching it. It has a huge amount to do with the accuracy of the boring job as well as the accuracy of the rings used. If it was bored perfectly and the ring was perfectly round, there would be no seating even needed.

Anyway, I also disagree with this assertion that there's something (he never did say exactly what) about large bore V-Twins that makes them not want to seal as well. The real reason these things like to spit oil out the breather is because they're a common crankpin 45 degree design. This design causes a variable volume crankcase: pistons down and the volume is smaller, pistons up and it gets larger. Most engines don't work this way, they have a piston going up for every piston going down. The variable volume design causes it to want to inhale and exhale air into and out of it's crankcase constantly. For a graphic illustration of this, take your timing plug out and start your motor.

If it's allowed to suck air in, it'll have an inhalation & exhalation effect going. Whenever air goes out, it'll carry some oil with it and deposit it. So by allowing it to inhale & exhale, you've basically created an oil pump. The reason for the check valve is NOT, as he says, to keep it from pulling in dirt. It's to keep it from inhaling air, because any inhaled air is going to get pushed out again and take oil with it.

We have check valves in the rocker boxes that are *supposed* to eliminate this. If they aren't functioning properly, you'll get LOTS of oil out the breathers. The valves themselves rarely fail, but the casting they sit in is, well, a casting. Not very accurrate. Buell had a service bulletin years ago that talked about chamfering the umbrella valve hole to make it seal better, as a fix for excess oil in the air cleaner. My whole point is that ring seal is not the only method by which you can get excess oil out the breathers.

The thing that bothers me the most, though, is the total lack of any mention of the risk of ring microwelding by getting too aggressive in the break-in. HD's break-in procedure, and S&S's, and others, if you look at them, are designed to minimize heat build-up. Be gentle on the motor, don't put it in a situation that makes it hot. The reason is simple. With very little contact area between the rings and the cylinder walls, the ring tension is concentrated and those areas that do make contact get very hot. That localized heat can and will damage the piston, and remember, the ring land in the piston is a sealing surface. Damage it and you'll never get good ring seal. This scenario happens more than you might think. Good lubrication and a gentle break-in consisting of several heat cycles to begin with are absolutely mandatory.

Anyone who does that mototune crap on one of our engine kits can just forget about his warranty. It's a recipe for ring microwelding.
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  #4  
Old 28th October 2005
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aswracing,
Wow. What a response. That article seemed good to me, but me is someone who doesn't build engines and doesn't know. Thanks for clearing things up. Since I have the opportunity, I'm going to ask a few questions.

Quote:
The ring is said to be "seated" when it has carved the cylinder into it's shape and there's contact all the way around.
Does that mean the ring will "seat" in a few hundred miles or sometime sooner like 20 miles? Motoman says (doesn't mean that I beleive him) that you have 20 miles to get the rings seated.

Quote:
That localized heat can and will damage the piston, and remember, the ring land in the piston is a sealing surface. Damage it and you'll never get good ring seal.
Does that mean you should run your engine for short periods of time while it's still breaking in? Meaning don't go out and put 500 miles on it in one day without shutting it down. Also, if your engine is running hot (250-270 degrees) then will that damage the pistons as well. What difference does that running temperature (250-270 degrees) make during breakin vs. after breakin?

Finally, should short burts of heavy throttle/rpm be applied during breakin or will that still risk microwelding? Should it be applied at any point during the breakin? For example, after 200 miles or so.
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  #5  
Old 28th October 2005
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Quote:
Originally Posted by CBAS5
Does that mean the ring will "seat" in a few hundred miles or sometime sooner like 20 miles?
Depends on how good the machine work is. If everything is perfect, it'll be seated when it's put together (it won't be though). If the bore and/or rings are out of round badly enough they may never seat.

Quote:
Originally Posted by CBAS5
Does that mean you should run your engine for short periods of time while it's still breaking in? Meaning don't go out and put 500 miles on it in one day without shutting it down.
The first run-ins are the most important. We like to start the motor and run it for about a minute, with fans blowing on it. Then let it cool ALL the way down. Repeat but the second time run it 2 minutes. Next time 3 minutes, etc. The idea is to let the rings do their job without overheating. They're very sensitive to localized overheating when only a little bit of them is touching the cylinder wall and you can damage the pistons easily.

Quote:
Originally Posted by CBAS5
Also, if your engine is running hot (250-270 degrees) then will that damage the pistons as well.
Not if the rings are already seated.

Quote:
Originally Posted by CBAS5
What difference does that running temperature (250-270 degrees) make during breakin vs. after breakin?
You get the motor that hot before the rings are seated and you've probably already microwelded them. The thing with microwelding is that you may do it and never know it. It's primarily the top ring that does it, so you may not get any oil issues. It'll just be doomed to be a mediocre performer.

Quote:
Originally Posted by CBAS5
Finally, should short burts of heavy throttle/rpm be applied during breakin or will that still risk microwelding? Should it be applied at any point during the breakin? For example, after 200 miles or so.
Not just no, but hell no!

Ron Dickey has an interesting case history of microwelding on his web site, it's worth a read:

http://www.axtellsales.com/RonsDocs/Ring%20Seal.pdf

Ron knows more about cylinders, pistons, and ring seal in his little finger than I'll ever know.
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  #6  
Old 28th October 2005
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Great article, but this just leaves me with a few more questions. Wouldn't you know that the vicious question cycle would continue?

If short bursts are not allowed, then is the run at the factory a bad thing? What do I mean by that? I have heard that once a bike is fully assembled they will run it once before they pack it at which time the bike can reach speeds of up to 70 mph.

Do you know what he means by "highway miles would have polished off the speckles". Does that just mean covered up the traces of microwelding the rings or they would have improved slightly over miles? I'm leaning toward the first since he said that "the engine is forever a mediocre player until the pistons and rings are replaced."

Finally, would running the bike on a dyno or running a compression test confirm proper ring sealing? Meaning poor horsepower would mean microwelded rings and poor compression would mean poor seated rings (of course a compression test can also reveal bad valves).
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Old 28th October 2005
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ASWRACING glad to hear from people that do the work everyday, experience is gold. Thanks
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Old 28th October 2005
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Starbuck, thanks for the comment, happy to do so, when I have time.

Quote:
Originally Posted by CBAS5
If short bursts are not allowed, then is the run at the factory a bad thing? What do I mean by that? I have heard that once a bike is fully assembled they will run it once before they pack it at which time the bike can reach speeds of up to 70 mph.
Yeah, I've seen those run-ins during the factory tours. Not much load on the motor and they don't run them very long. They're not hurting them.

Quote:
Originally Posted by CBAS5
Do you know what he means by "highway miles would have polished off the speckles". Does that just mean covered up the traces of microwelding the rings or they would have improved slightly over miles? I'm leaning toward the first since he said that "the engine is forever a mediocre player until the pistons and rings are replaced."
Yeah, he actually showed me those rings when I was up at his shop several years ago. Aluminum had literally smeared off the piston ring lands and stuck to the rings themselves. The damage was to the pistons. The aluminum speckles may have cleaned themselves off over time but the damage to the ring lands in the pistons was permanent, and causing poor seal.

Quote:
Originally Posted by CBAS5
Finally, would running the bike on a dyno or running a compression test confirm proper ring sealing? Meaning poor horsepower would mean microwelded rings and poor compression would mean poor seated rings (of course a compression test can also reveal bad valves).
A better test would be a leak down test. Yes, it'll show up as less horsepower but if you have low horsepower, there are a dozen things that can cause it, you don't know which. A leak down test is the best way to check the mechanical integrity of the motor without tearing it down.
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Old 28th October 2005
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What makes your explainations so outstanding is your ability to take very detailed, trade specific information and relate it to us common folks (namely me). Always great stuff!

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Old 28th October 2005
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aswracing:
Thanks for sharing your experience with us. I feel that the air-cooled v-twins are in a class of their own and am always suspicious of articles that are not written specifically about v-twins.
Thanks Again
Bob
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