Aside from all your issues, the bike looks freakin awsome.

I suspect this is the closing of your story?
It would be a great side table short book… Eloquently written sir! I’ll hoist an Ale…. Out of corn nuts now…
Thank you
Sneezles61

In the interest of continuity I intend to put the bare bones of the paranoia thread here to the present day.
We are still late summer 2022 at the moment.

Plenty of teeth gnashing yet.

Meanwhile VFR gets the same going over as the DT by the appraiser.  That bike was 10x more complex than the DT but had not been messed with by a complete red arsed Baboon.  As a result it was just a normal restoration with no real angst.  Hardest part was scouring the world for NOS original rubber brake hoses and piping as it came fitted with Goodrich braided ones.
Resto completed Xmas day 2019 when I fitted the mirrors and closed out the build.  True to form, my luck dictated that the weather would be wet, cold and horrible for another 5 months and I had to wait till the end of April 2020 to test ride it.
An interminable wait.

Dating photo from the valuation report.  At 25 years old, it will not start to be seen to be really collectable for at least another 10 years.
This is the bike I put some miles on without fear of something breaking or fear of devaluing it.  Every bush, bearing and seal is new. It was built to be as the brand new bike I had in 97, but to ride and enjoy as it should be.
The Gen 4 VFR750 was billed by the motorcycle magazines of the time as the best alround motorcycle ever made.  I will go along with that.  I never found a better one and always regretted trading it in for a Honda CBR1100xx Super Blackbird.  I knew I had made a big mistake within 20 miles.





What have I done?  By the time I got the Blackbird home, I knew I was an idiot.  A very very nice bike to be sure, but it aint no VFR.

Present day.  

For those who were watching the paranoia thread.  Bike has been test ridden and is now mechanically quiet (for an air coolled two stroke).
Clutch in or out now makes little difference to the noise, other than the slight whirr of the tranny gears rotating in neutral goes away.
​​​​​I will be coverng this little bundle of angst later in this thread for continuity, however if someone is watching the paranoia thread because they have the same problem and want to see the solution, just sing out and I will post an update there.

We are now arriving at the start of the paranoid thread.  I will quickly cover my thoughts at that time.  No point in covering that entire thread again.  It is available for anyone to see.

Over the summer of 2022 I was happily riding the bike around and put about 180 miles on it since the rebuild.  

Towards the end of the summer, I was still getting up the nerve to try another full throttle run to check WOT carburation.  I have a suspicion it is running a little on the rich side.
I resolved to run a few more tanks of fuel through before trying again.

I pulled into the yard on what was to be the last ride of the year, and the missus spoke out.. "Should it rattle like that?" .

Huh?  I lifted my visor and switched off the ignition to hear her better.

 " Does it always rattle like that? "

 "It is called fin resonance.  The cooling fins amplify the sound of the piston going up and down the cylinder, all air coolled two strokes do it".

Oh.

I trotted into the house to rid myself of helmet, gloves and jacket before wheeling the bike into the conservatory for what turned out to be its long winter sleep.

Autumn turned to winter, Xmas came and went and the bike slept on.  The only disturbance was the click on and off of the wall socket switch every month or so, to encourage the battery charger to top up the battery.  Otherwise, the conservatory being unheated was devoid of life. 
I avoid entering as much as possible, because heat from the house will enter and cause condensation. 
The ventilation windows are open all year round to avoid rapid temperature fluctuations and condensation forming on the bike.
The blinds are kept closed to help stabilise the temperature and to prevent UV damage.

I do not cover it for the same reason.  Covers prevent good ventilation and can hold condensation in.  Besides the bike is a decorative item.  Pleasing to the eye.  Better than any pot figurine on a shelf.

And the bike is now purely a decorative item to be admired from the comfort of the living room sofa.  I was completely ignorant of the time bomb ticking away inside....or so I tell myself.  Looking back, something was niggling away at the back of my mind all winter.

 

Spring is about to err spring.

Removed the rubber band holding the large plastic bag on the end of the fuel tank vent pipe - madcap idea I came up with allows the tank to breathe in and out with temperature without constantly drawing in moisture from the surrounding air.  Only do it during the winter lay up.  Both the VFR and DT are topped off to the brim over winter.
Even though my fuel stock is routinely dosed with Stabil, I try to avoid moisture in the tank.  Ethanol.  Hateful stuff.

So bike out in the yard, looked at my carpet slippered foot, shook my head and went back in to put my trainers on.
Last time I was too lazy to change my footwear, my foot slipped on the kickstart and fired one of my slippers into low earth orbit.  My how that hurt.

Got the bike started and sat on her holding the RPM at around 2000 to warm her up gently and once she would hold an even tickover, went into the house to get my jacket, helmet and gloves.
30 seconds later I was at the back door, bike ticking over in the yard.

Yikes, she sounded absolutely horrendous.  Like a handful of change getting shook up in a coffee can.  The missus was right, that was not a rattle, it was a downright knock.

Paranoid thoughts were flipping across my mind.

Knew I should have pressed the crank apart and fitted new big end kit.  Had the big end failed?.....
Was it the little end breaking up?
Did I blow the piston on the failed wide open throttle run?
Has the oil pump stopped delivering and blown the motor?  I know I should have checked the output when I bled it.

It did not quite sound like the crank, but what was it? It had a rebore and it kicks up straight away, it idles reliably and seemed to pull OK.
Then I remembered it had developed a slight misfire above 5000 rpm when I put it away.   Was it related?  Was it metal on the plug electrodes?
Has the timing slipped and the engine is busily eating itself a neat hole in the piston crown?

Whatever it was, this is the devil bike and it would be the worst case scenario.  How I wish I never set eyes on it.
No matter how much care and attention is lavished on it, it completely refuses to be a reliable machine.
Sharing the conservatory with the bike is my watch tinkering lab and I put as much care and attention into that bike as I do with watch movements...

Shared space.


That bike should run like a swiss watch, yet it absolutely refuses to.


So I pulled off the head and had a look.

No hole or signs of detonation on the piston crown.  I suppose thats something to be glad about, but no, it meant it is a bigger problem.
Looking down the bore revealed no big score marks and bringing piston to the top and giving it an exploratory push back and forth revealed little slop. Oh no. Job is getting bigger.

Time to lift off the barrel...
Nothing jumps out at me.  Oh jeeze.  Is it the crank or the tranny?

Crank still feels fine.  Smooth and free turning.  No slop in the big end.  No slop in the little end and no excessive side play.
Piston is showing no scoring or any signs of the black death of incipient seizure.
Rings not scored or showing any sign of being gap bound.

I look up the bore from the bottom instead of the top.  What the hell am I looking at?

I am looking at two perfect imprints of the piston intake windows embossed on the cylinder wall.  I never in 40 years of spannering saw the like before.  And this has happened in less than 200 miles.

Japanese made, quality TKRJ piston.  Nothing untoward other than a little blowby on the thrust side.



Never ever saw the like before.  Then again, I never bought a bike from a demon before either.
Intake window imprinted on the cylinder just above the intake port and strange scuffing marks.  I know the ports were radiused top and bottom after the rebore because I took off the sharp edges  myself at work with my riffler files, to make sure, so that is not the cause.
What on earth.....






 

I seem to remember you mentioned getting a cylinder relined? If the cylinder was lined, then the issues now are related to the lining coming loose when the bike gets hot.

Yamaha cylinders have cast in skull type liners, and there simply isn't enough material to properly support a pressed in straight sided liner.

There are new Chinese DT175MX cylinders listed on ebay, but no idea if they are any good or not?

I have had a cylinder with that ghost mark on the piston.  It had been lined, and the motor rattled badly (I think the piston gets damaged?). Fitted another cylinder and the noise went. 

At least I now knew where the knock is originating, but my now paranoid brain is going into hyperdrive.

Apart from being a bike cast out of hell for becoming plain too evil and driving Beelzibub into a wild frenzy, why has it managed to destroy a fresh rebore in less than 200 miles?

My private thoughts were that it was poor lubrication.  The damage is confined to the thrust side of the cylinder.

The pump was working when I bled it and I am of the old school that still adds a splash of two stroke to a full tank just.... because.  I tell myself it is to prevent fuel tank corrosion and lube the carb slide.  But I know it is just a bit more paranoia.  I cannot trust the bike to hold together without my lucky rituals.
As said earlier, should have checked pump max output instead of just setting the cable and checking min pump stroke clearance.

Was it the BelRay SI7 I was using?  Is it naff these days?
I used BelRay lubricants exclusively back in the day when I had a stable of two strokes with no problems.  
BelRay MC1 in my pure competition bikes at 50:1 and SI7 in my autolube equipped road bikes.  Thousands of miles / hours run without problems and never needed a rebore.
My old, last of the air coolled YZ250 was a bit harsh with the rings and would lose its crispness after a few months, requiring replacement rings, but never any serious bore wear.

Had the pump just stopped delivering and the only oil the motor was getting was the splash I put in the tank, just enough to keep the bearings lubed, but not enough to prevent bore wear?

Was it dirt in the intake? Did the silly old sod leave a ton of dirt in the intake?  Stripped the airbox fittings and carb off. Nope, everything was clean as a whistle. No dirt was jammed into joints or little nooks and crannies.  Air filter was new, freshly oiled and fitted less than 200 miles before.  In addition, the piston skirt was not scored.  If it had been eating lumps of dirt it would have been.
So no, I could not blame the demon who sold me the bike.

So why had I not noticed it before? 
When the bike was first started and test run, everything seemed normal. 
When I started riding it, my typical ritual was to get the bike out, go put on my helmet and gloves, start the bike and warm it up till it would hold a steady idle. 
Then I would go open all the gates to the outside world. 
The back yard is like Fort Knox.  By the time I got the three gates open to the road, the bike was warmed up enough to ride away.
Coming home, I would switch off the bike, then go get rid of helmet and gloves before putting it away.
So I suppose it is a combination of a gradual worsening of the noise, not expecting trouble and always having my helmet on when the bike was running.

The Missus sure put me in my place.

I took the barrel to work and put one of my bore gauges down for a comparative check.  The bore appeared round at the top and round at the bottom.  It was NOT round in the middle and furthermore was .25mm or so bigger in the middleve than the bottom.

What the feckity feck.

I jumped to the conclusion immediately that my friendly local car engine reconditioners had mucked up the job and ruined my cylinder.
When I fitted the cylinder, nothing seemed untoward, but then I never checked.  Just trusted them to have done it right.

I knew I had a pic of the build after I fitted the barrel in my provenance file.  I dug it out and there it was, the gently smoking Colt Government 1911 model .45.

The camera had picked up something that was not visible to the naked eye.  I suspected the boring bar had chattered badly around the ports, either because of inexperience of boring a two stroke barrel by the operator and going too fast, poor or worn equipment or both. 
The final hone had missed a portion of the cylinder wall because it was too deep for the hone to reach.  You can see the difference in the hone marks almost half way between the top of the cylinder and the exhaust where they had done it by hand.  A definite band different from the hone job above.

I showed the pics to my tame precision engineer and he confirmed it.  They had screwed up the bore and tried to cover it up by hand honing the chatter marks out in the localised area.  They had used very course stones to speed things up.
Must have taken hours with an expanding hone in a drill.....

My luck has held to its usual form.  Screwed up rebore attempt.  The dark shadow above the port is where the hone could not reach.


So we have a bike that has now around 2000 miles on the clock from new. 
It had a completely unworn, but rust pitted barrel necessitating a rebore.  Something that the demon should have done as part of his "rebuild".
To ensure that the deep pitting would clear in one rebore attempt, I made the decision to jump immediately to a 0.5mm oversize piston.
The rebore was not a success and 200 miles later,  it was touch and go whether a 0.75mm over bore job would clear, such was the damage caused by the piston tipping over and ramming itself into the cylinder wall.
So was looking at a max oversize rebore... On a bike with 2000 miles from new?

That is not right.
I looked at my options.

1.  Send it to the foremost two stroke specialist in the UK (PJ Motorcycle Engineers) to inspect the cylinder, do the rebore and suck it up.

2.  Send it to the same specialist and have a new liner made and fitted at three times the price?

Of course, to be fair to the bike and its future custodians, there was really only the one option.

Barrel was stripped of all the hardware, taken to work, beadblasted, cleaned out and all the threads tapped out before being parcelled up and sent for a new liner fitting.

 

The majority of people are unaware that fitting press fit liners, to cylinders which originally had cast in liners, will often end in tears, as there is simply not enough material in the cylinder casting, to properly support a pressed in liner.

Sometimes lining a cylinder works, but problems are very common, and It's something that should be avoided if new cylinders are available to a reasonable price.

Glad yours seemed to work out ok.................lots dont!

So we are ready to box up the cylinder and send it away.  I needed to select a new standard piston.  I have used TKRJ pistons a lot in the past and always rated them, so same again or do I bite the bullet and fit OEM?

Since it is going back to std at great expense, it seems a little parsimonious not to fit an OEM piston, rings and small end bearing.  A standard bore on a low mileage bike should have the same piston and hardware it left the factory with.

So all OEM equipment ordered and time for a like for like comparison between genuine and aftermarket.  Will they be the same?

TKRJ piston and small end supplied by a reputable UK Yamaha aftermarket parts supplier. 
The piston pin bosses were spaced several millimeters wider than the OEM and the small end was of a different design and not as wide.  Some of the more knowledgeable piston experts on here opined that the supplied small end was in fact a 125cc bearing that some think are interchangeable, but are in fact narrower than the 175 bearing.
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And the genuine articles in comparison.  The ends of the bearing are far closer to the pin bosses.  Not happy with the aftermarket one being allowed to migrate side to side by several millimeters.