Why Piston and Cylinders Scratching

hartungej · Feb 8, 2012

Ok, I've had 3 engines which eventually had a cylinder get scratched. I'll explain below, but wondering if anyone can point out a problem. This is getting expensive!

#1: Grubee Skyhawk. Well broken in. Stupid newbie mistake, running semi-synthetic at 50:1 (thinking it was full synthetic). Completely locked up on me on an 80 degree day. Laugh if you want.

#2: Repaired that motor. Ran 32:1 non-synthetic. Broke in running slowly for at least 1.5 gal. Started riding harder. Fatal cylinder wall scratch to engine again, now on 20 something degree day.

#3: Bikeberry Flying Horse. Broke in for about 2 gal at about 18:1 as they suggested with a Penzoil oil. Kept around 1/2 throttleish for most of break in, varying throttle speed often. Installed expansion chamber and started riding harder with no prob. Thinned oil-gas mix to about 25:1. Cylinder scratched and lost compression after almost a gallon at 25:1 toward the end of my 7 mile commute. Fairly hard riding with expansion chamber, but not constant WOT either.

I've read a lot of things on this discussion board. It doesn't seem as if I'm doing anything too out of the ordinary with breaking in, fuel mix, etc (save for #1). Lots of people say they run WOT all the time without much trouble. Spark plug on this last one seemed chocolate/light black to my eyes. I'm definitely a newbie to engines. Can someone tell me what I'm missing, and save me some $$!

biknut · Feb 8, 2012

Did any of these motors sieze up? If not, what tipped you off there was a problem.

hartungej · Feb 8, 2012

#1 seized completely.
#2 Had progressive loss of compression until it would no longer run.
#3 Just happened last night on my 7mi commute. Rode it home 7mi. It started quite hard, then had a noticeable decrease in power. Also, it would no longer idle. If I can get it warmed up, it will run, just not idle and with approx 25% less power. I checked and noted significant scoring when I got home, and pulled shavings out of the cylinder jug

.

biknut · Feb 8, 2012

Are you running stock jetting?

scotto- · Feb 8, 2012

A simple solution to this problem is get a 4-stroke. Scoring of the cylinder of a 2-stroke can be caused by improper oil or the mixture, as well bits of carbon from within the combustion chamber can cause scratches.

Possibly engines from factory were not completely free of debris when packaged and shipped but the most common cause is oil type and mixture ratio. Gas /air mixture is also a cause if you're running it too lean........too lean of mixture (to much air, not enough gas) will make your engine run hot. Your bike will seem to be running better than ever and then........BAM! Sounds like you've been doing things right for the most part so far......bunk engines or maybe Pennzoil?

I'll stick to 4-strokes......no problems of this nature. Good luck.

dnut

hartungej · Feb 8, 2012

I'll answer both of the above in one, here.

For #1 and #2, I was running stock everything.

For #3, I had just installed a Manic Mechanic intake and high perf air filter. This made it run better than ever. Then had the expansion chamber installed. It was running great for the 1 gallon before the scratches. The plug had been a tan color, but was more of a lighter black (not really grey) when scratching took place. I was pulling to check about every 14 miles, so I was paying attention with the mods. Also, I noticed my head nuts were a little loose. I'd been torqueing about every 28 miles back to 15 (in/lb? can't remember the measurement). This time they were probably down to about 9 or 10 before I tightened them again after vibrating a little loose. I really didn't think they were that loose, but I wonder if that could have leaned the mixture out just enough.

For #2, come to think about it, those head nuts were loosening a bit, too, and were looser by the time this newbie caught them that time. I wonder if that has something to do with it? That particular engine had loc-tite on the head nuts. Didn't matter.

So, what are your thoughts about the above? Any suggestions for keeping the head nuts tight? It doesn't seem .duh.

like I should have to torque these every 20 miles to keep my engine from frying!
Wonder if it really could have been shavings from the factory or something?

This is frustrating, b/c it seems like I'm treating my engine well within the range of fuel mixtures and riding it relatively hard, but not as hard as a lot of people on here who report not having these problems.

biknut · Feb 9, 2012

I'm not thinking loose head bolts caused your problem. A leak anywhere on the engine results in a oil mess. If there wasn't any oil then it wasn't leaking.

Stock jet is usually on the rich side = to about a 71 if your carb is a NT.

I have a new motor with MM head and expansion chamber. I've jetted down twice. From stock 71, to 68, and now 66. So far I only have 135 miles. Mine still looks a little bit rich, but close. It's been up to 35 mph several times already. I'm running a NGK B6HS @ 28:1

I haven't had any of the problems you've experienced. My only complaint is 4 stroking above 30 mph.

biknut · Feb 9, 2012

Be sure to only check your head bolts cold. After about 3 times retorqueing they shouldn't get loose for a long time. I keep mine at 140 in lb.

scotto- · Feb 9, 2012

Now it's sounding like your engine is sucking air in from possibly the intake manifold causing a lean condition. Any air leaking into the combustion chamber other than what the carburetor should be providing properly can cause leaness and result in eventual seizure.

I use to torque my cylinder head down near 200in./lbs.

hartungej · Feb 9, 2012

I guess that makes sense regarding an oil mess with leakage. I did have some oil with #2, but I'm certain that was a loose exhaust. Nothing around the jug that I can remember. Zero oil leaking with #1 or #3.

Yes, I had a stock NT carb on all of these engines, so I'm guessing I had the stock jetting. Richer would be better, right? So that shouldn't have been my problem. Starting with #2 and #3, I learned to keep my engine head bolts torqued. I checked both of those a few times, trying to keep them around 120-150 in lb. I sometimes torqued them hot, sometimes cold, however they never did stop vibrating loose to some extent. Any tips for keeping them from vibrating? I was thinking about replacing the stock bolts with a regular nut or 2 and a lock washer and using red lock tite. I had blue loctite on #2. Still vibrated loose! Maybe I should try 200 in/lb?

Now, I was wondering about Scotto's theory, too. But it seemed like the color of my plug was ok until it wasn't with #2, when I apparently developed an air leak. My plug with #3 looks like it was barely starting to turn from brown to gray, but I don't think it looks bad. Those nuts may have been just below 100 in lb, by my estimate, so not super loose, and had been torqued and vibrated loose at least 5 times! Like I said, though, no oil mess. I would also think the manic mechanic intake would reduce an air leak through the intake.

hartungej · Feb 9, 2012

Hey Biknut. You said you got a new engine with a MM head? I was wanting to put one of those on, too, but neither Jim or Bikeberry could tell me which engine I should put it on. They just said they were made for the 2009 GT-5 specifically and for the domed piston, but couldn't say which other engines it should work on. Just curious about what you made it work on.

biknut · Feb 9, 2012

hartungej said:
Hey Biknut. You said you got a new engine with a MM head? I was wanting to put one of those on, too, but neither Jim or Bikeberry could tell me which engine I should put it on. They just said they were made for the 2009 GT-5 specifically and for the domed piston, but couldn't say which other engines it should work on. Just curious about what you made it work on.

My MM head is the gen II head. It fit's any 80 motor with 8mm studs, that has a piston with 0.016" dome.

A lot of people have trouble with the acorn head bolts. The problem is, a lot of times the cylinder studs are bottomed out on the inside of the acorns. When you try to tighten the head bolts beyond a point, it strips the block out.

Normally the head bolts don't ever vibrate loose. Only the gaskets compress a little bit. Usually after tightening them about 3 times, and only a little bit at that, they don't tend to loosen up much after that. Always check them with a cold motor.

You either add washers under the head bolts to get more room under the acorns, or replace the acorns with flange nuts. If they never stay tight then they're probably stripped.

If the bolck gets stripped, you might be able to repair it with a heli-coil. I reccomend keeping the torque on the head bolts 145 in lb or less. Much more and it's possible the cylinder might distort.

hartungej · Feb 9, 2012

What brand/model is your motor with the MM head?

Yeah, I'll have to try getting some washers to see if that helps. I'm a bit perplexed as to why my nuts keep coming loose and others apparently do not. I suppose they could be bottoming out on this motor. I'll have to look closer.

So, it sounds like the theory is that I may have had another bit of an air leak? I've recently ridden this motor up to about 30 miles/hr, but not probably sustained for more than 30-45 sec at a time, then backing off. I ride about 7 miles at a time, normally. My fuel mix seems reasonably normal.

I guess I'll try lock washers on the head nuts. I might switch to synthetic oil around 24:1ish after break in before I run my next motor too hard. Maybe I'll go down to a 36 or 40 tooth sprocket from a 44 to keep the motor from having such high RPM's at higher speeds. Any other suggestions from anyone? Do I need to not run the motor quite as hard-though it seems like others do. This is getting kinda frustrating with the same expensive problem over and over.

biknut · Feb 9, 2012

hartungej said:
What brand/model is your motor with the MM head?

Yeah, I'll have to try getting some washers to see if that helps. I'm a bit perplexed as to why my nuts keep coming loose and others apparently do not. I suppose they could be bottoming out on this motor. I'll have to look closer.

So, it sounds like the theory is that I may have had another bit of an air leak? I've recently ridden this motor up to about 30 miles/hr, but not probably sustained for more than 30-45 sec at a time, then backing off. I ride about 7 miles at a time, normally. My fuel mix seems reasonably normal.

I guess I'll try lock washers on the head nuts. I might switch to synthetic oil around 24:1ish after break in before I run my next motor too hard. Maybe I'll go down to a 36 or 40 tooth sprocket from a 44 to keep the motor from having such high RPM's at higher speeds. Any other suggestions from anyone? Do I need to not run the motor quite as hard-though it seems like others do. This is getting kinda frustrating with the same expensive problem over and over.

My newest motor is a T80 from BGF. It's just a very generic China motor. The way you've been breaking in your motor is about the same as me. I might even be running it a little harder. I started at 24:1 the first gallon. Then 28:1 second gallon. I'm still burning through the second gallon.

2door · Feb 9, 2012

Get rid of the acorn nuts. As was said, they can, and usually do, bottom out on the studs and tightening them will distort the threads of the stud making it hard to get them off without removing the stud from the case. Question: Are the nuts loosening on the stud or is the stud loosening in the engine case? Or both?

I don't use lock washers under the head nuts but I do use shouldered nuts and a flat washed torqued to 120 to 140 inch pounds and check the torque after the engine has reached operating temperature a couple of times. Never had one loosen in over a thousand miles of moderate to hard running. I don't use any thread locking compound.

Tom

hartungej · Feb 9, 2012

Good question regarding which one is loosening-the nut or the stud.

I'll have to try the shouldered nut and washer idea. I really hope that'll do it.

If not, maybe I go to a 4 cycle?

scotto- · Feb 9, 2012

hartungej said:
Good question regarding which one is loosening-the nut or the stud.

I'll have to try the shouldered nut and washer idea. I really hope that'll do it.

If not, maybe I go to a 4 cycle?

4-stroke or go broke........you'll never go back!

dnut

thegnu · Feb 10, 2012

I dont have many miles on mine yet but , I also use shoulder nuts , blue loctite, an use the acorns to double nut an lock my head bolts ,Last time I checked them they had only loosened a very lil but an attribute that to the motor settling in my tourque setting is at 150 in/lbs ,I had to make a wrench to hold the shoulder nut while I tourqed the acorns to 5 in/lbs .just enough to lock them . good luck .
Gary

skipu · Feb 10, 2012

crappy champher job i pull all my motors apart and go through them deck them champher them squish clearance them clean and deburr ring grooves clean up ports = zero problems on the mechanical end

skipu · Feb 10, 2012

i like the 4 strokes too but they dont look asthetically correct to me just my taste i spose

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