Brown truck guy just delivered it and as I eyballed it, the piston has many scratches on it. Worry or no?

machine marks are normal

Thanks but these arn't machine marks. These run vertical and look like they came from either improper ring installation or more likely from not de-burring the exhaust port openings within the cylinder.

Can you post pictures of the scratches??</p>

Ken</p>

Ill try Ken. I don't have a macro lens though.

Sorry about the bad shot. I had to use my 200mm lens standing on one foot with one hand holding a CFL light in one hand and I musta looked really goofy. lol

I used to race 2 stroke dirt bikes and these scratches on the piston would be a bad thing.

Does your fingernail glide over the "scratches"? In other words, is this a slight marring or does your fingernail actually pick up the scratch?

If you can't feel the scratches with your fingernail don't worry about it.

Edited - Your picture wasn't posted when I posted, I assumed you had disassembled the engine.

I'm an engineering lab tech and can guess the scratches are 0.002" to 0.003" deep. I would have to pull the piston out to do the feel test, but yes, I would be able to feel the scratches using the fingernail method. Rotating the crank I can feel some sort of grinding/resistance through the stroke. Maybe I am being over anal for a $100 engine?

I would pull the head and backplate off and give it a good visual inspection for swarf (machining debris ) and flush it out with "jet spray" type carb cleaner. Use Q-tips to clean all the little nooks and crannies too...You'd probably be surprised at how many new engines have some sort of swarf in them.</p>

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If the scratches are not in the sleeve and ring, it should be OK.</p>

So far I don't see any scratches on the ring. But I like your suggestion. It's only a $100 motor. I could tear it down and do some port polishing. Take out the scratches on the piston with some Scotch Brite crosshatch style and shorten the rear piston skirt to allow earlier fuel entry? This is a piston/port timed 2 stroke right?

No, it's not piston ported...so you wouldn't gain any performance there. (except for marginally lightening the piston )</p>

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The ST .90 is more of a torquer, not really a revver...but some "softening" of the sharp edges in the mixture's flow path might prove beneficial?
You might find a burr or something along the edge(s ) of the ports that caused the scratches, so dis-assembly might be worth while.</p>

Once broken-in and the carb is adjusted properly, they are (or can be ) nice running engines.

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I must be getting old. I thought 2-strokes were Piston Port timed, rotary valved or reed valved?

Well...OK...piston port timed, but it's the top of the piston, not the skirt...

Shaft rotary valved, piston port timed...

Pull the head off and have a look...or you'll see exactly when you take 'er apart, aye

That engine should be sent back to Tower. something is not right

Scratches (actually grooves) like that are definitely not normal, especially on a ringed engine where piston clearance is much larger than with an ABC type. Whatever has caused that damage will have to be trying to damage the ring as well. That seems to be the ring showing at the top of the port and I can see slight marks on it too. I'd suggest sending it back for a replacement because the piston skirt is needed to seal for crankcase compression and those grooves would allow leakage.

I'd agree it's most likely there's a rough edge on the liner ports which should show up by turning the engine very slowly and feeling for a slight catch as the ring reaches the port. I felt a catch like that on a new Enya 60 so I radiused the edges of all ports with a jewellers file but there was no damage to the piston or I would have sent it back.

That's another option of course...

Beg to differ bro. The fuel/air mixture goes into the crankcase and then up through the transfer ports. If you shorten the rear skirt of the piston it allows earlier entry and more volume into the crankcase and up into the top end through the transfer ports. I admit though, I don't know anything that shnele thing. &lt;sp&gt; I could be very wrong.

Thanks guys.  Ill send it back.  Tower hobbies has allways been straight up and great customer service in all the years I have been buying from them.</p>

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Thank you and sorry about the bad photo.</p>

I know the types of engines you're talking about...certain motorcycle and snowmobile (etc. ) engines, but our model engines are different. The port timing is controlled by the location of the ports in the sleeve and the top of the piston as it uncovers/covers them, not the skirt. So they're not exactly what you'd call "piston ported" in the same sense as the motorcycle/snowmobile (weed whacker, leaf blower, etc. ) engines.

As far as reed block, rotary valve and piston port goes, in this engine it is the hole in the crankshaft that periodically opens and closes under the carb that handles the job of allowing air/fuel in the crankcase, then sealing it off on the downstroke to allow the backside piston pressure to pump it into the cylinder through the ports.

Before I start hacking away trying to take the motor apart I thought I would check with yall.  It's been many years since I tore one of these apart.  Suggestions for dissassembly?</p>

After sleeping on it, I think Ill try and fix it myself rather than go through all the trouble of sending it back, boxing it up, shipping etc.  I'd rather buy another $25 piston than spend $10 shipping and an hour of my time.</p>

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And thanks for the help guys.</p>

Well the culprit is most likely the sleeve

Therefore you better figure on a cylinder sleeve and ring also

It is bad when you have to rebuild a new in box engine

Here are a couple how to vids on taking the engine apart.

http://www.youtube.com/watch?v=TuzEco3EDUMpart 1

http://www.youtube.com/watch?v=OLSoXPLGzOIpart 2</p>

I would send it back immediately.