Barrel studs and other hardware removed. Just have to take it into work on Tuesday and beadblast the old paint off before sending it for a resleeve.  Previous owner painted over the stubborn dirt between the fins, looked ok hard to tell from a casual view, but rankles.  Have an opportunity to put it right now.

I found a pic taken during rebuild that shows a "shadow" above the exhaust port where the hone missed. Never noticed it at the time, but then, I was not looking for it.
My tame injection moulding toolmaker (my ex apprentice who went back to school to retrain as a precision engineer, and would not touch the cylinder with a bargepole) had a look at the pic and confirmed my suspicions.

He thinks the boring tool chattered around the ports and they went to town with a short honing stone to try hide the damage. This explains the misalignment. I think the intake marks are where the piston has rocked in the barrel shape and has been driven hard into the cylinder wall.
Pure conjecture of course, it is an odd one.

The scoring seen in the pic is not as bad as it looks, camera has accentuated them. Can't feel them with a fingernail. I was just going to quickly hone them out. Of more concern are the imprints of the intake windows. You can feel them with a fingernail.

Initially I thought it was dirt , but the intake was clean and secure. Air filter has 200 miles on it. The cylinder itself was cleaned in a 4 stage industrial cleaning process - safety kleen coconut oil solvent, safety kleen hot degreaser solvent, pressure wash and final industrial hot wash with anti rust additive and dry with compressed air. All the ports were clean inside.

Never seen imprinted piston shapes before.  You can see one to the left of the intake port.  Dust around the bottom is just where I have been wirebrushing the old base gasket off.

Piston itself is not scored at all. It has been running hot at one point because oil has been burned black under the crown.

And the offending "shadow" above the exhaust port where the hone did not clear. Never noticed it at the time of the build. Camera is very good at picking out hardly perceptable defects.
All this happened in less than 200 fairly gentle road only miles. O Synthetic oil and plenty of it, bike always warmed up for a few minutes before riding, still started second kick when cold and first kick from hot. Only problem was the clattering.

Bad machining job as far as I can see is the cause, unless someone has seen it before.

Learned my lesson, try cheap, try twice.

How come I have access to all this marvelous industrial machinery? It makes doing restorations so much easier.

My day job is on a bit of a different scale to the DT. If you are wondering why the severe face.... A manager hove into view just as the shutter clicked. Imagine the biggest idiot you ever met, double it and add ten percent and you will be in the ballpark.
I am just getting ready to pour scorn on yet another idiotic idea.


A middling sized engine and a face that can be said to have been "lived in".

I'm so sorry you're having this issue.  I've been there.  As I've probably posted 100 times or more, finding a reputable machine shop that knows how to bore, hone, and properly chamfer ports on one of these air cooled two strokes is not easy these days, especially in the US.  For years now, I've resorted to mailing out mine to shops with good reps and hoping for the best.

One of, if not the best, were my experiences with Enduronut.  He has a large stock of pistons and used to have a source for boring.  It was great because you could send your cylinder in and he had the selection of pistons on hand...  if it didn't clean up at 0.25mm larger, they just went to 0.50mm with no delay.  Other shops tend to guess bigger when telling you what piston to obtain so you might lose an overbore.  Clearance was set properly and everything came back clean and ready to install.  First class and the pricing was better than anyone else I knew of.

Which brings me to one comment...  cleaning.  Years ago I had a cylinder done at a place that advertised a similar high zoot multi-step cleaning process like what you've mentioned.  On paper, it sounded like the cylinder would come back spotless with all traces of machining swarf and hone grit removed from the bore and ports.  It certainly looked clean!  I oiled it down with high detergent (rated for diesels, not sure that matters though) motor oil, let it sit a minute, and wiped the oil out with clean paper towels.  Paper towels were covered in black residue. Much of it in port corners. I went ahead and scrubbed bore and ports by hand (sink full of hot soapy water), paying careful attention to ports with small brushes, etc. Oiled it right away (I always get a slight light brown color of flash rust in the bore after washing in hot soapy water, but it doesn't seem to hurt anything).  Let it sit a minute, wiped it out with clean paper towels..  Repeated 2 or 3 times until towels came out clean. 

An old mechanic told me that the hot soapy water loosens debris and the oil "floats" debris captured in the pores of the iron and that it's important to repeat the oil and wipe down until clean.  I've followed that cleaning process since I was a young teen.

I suspect Enduronut (or his machinist) used a similar cleaning process because cylinders from him were the only ones I've ever had back from a shop that passed the "white paper towel test" and didn't need a good hand scrubbing. 

I know that most engines go together without this level of bore/port cleaning...  I had a friend once argue that leaving the residue was important for ring seating!  All I can say is I've very good luck with longevity and power which I believe is related at least a little to being very careful with the cleaning process. 

Oh, and on a side note, when I was racing a sponsor gave me a brand new cylinder to port.  It was squeaky clean from the factory, no residue...  which surprised me. 

CAT 4.4?
minibike sized compared to some of the big boys!

Nah.  Its a CAT C18 ACERT out of a Boeing 787 aircraft tug from Heathrow Airport.

I sometimes miss working on the big diesels...  it was a long while back and the Cummins NT855 was in almost the whole fleet. 

Tinkicker,something i have found but not rule of thumb.
Yamaha ( and i suspect other mass produced company pistons) pistons them selves can be all shapes. To the point of being slightly narrower at the crown than the skirt. I would be sure you have the piston of choice present when you get the bore done. I have assumed before and you know that assuming makes an ASS out of U & ME.
Probably didn't have to, but worth a mention.
Also if you're in the UK  i would recomend PJME for reboring. I have had over a dozen two stroke barrels done by them in the past few years and no trouble at all.
I take it you are as i have never seen Americans with what looks to be an aging Bedlington Lurcher (Avatar)
See that tin HT cap,if it's a genuine part,take it off and sell it on EBay. Nowt but trouble.

Lurcher? I will have you know I am a pedigree Irish Wolfhound.

As for tin ht cap. Yes that went in the bin before the bike ever hit the road.

Piston (std), yes, that has gone with the cylinder for correct sizing of the bore.

Cylinder went to PJ Motorcycle Engineers in Wolverhampton yesterday for a new sleeve making and fitting.  A sub 2000 mile bike should be on a std bore, not max o/s.

So all boxes ticked apart from your dog identification needs a little work.