Well I've had a couple of short gentle rides after the rebuild and looks like there's already marks on the bottom cylinder piston.

She should be getting heaps of oil because when I take the pipes off they're quite wet with oil.

Pictures below - confirm it's the beginnings of a seizure and it's off to the marketplace with my sweet Vanessa.

I had an MC18 all through my Ls and Ps and it was my hero. Never had a problem. Getting Vanessa has been a nightmare. Sometimes I guess we should leave our heroes of the past in the past.

The 2nd one will never live up to what you cherished.

Get a replacement barrel, new piston and all should be fine. If you want to do it on the cheap, you'd have to check if you left any debris in the barrel, but definitely new piston. Cheer up though mate, you're almost there.

That one was actually the good barrel. Plenty of nice hone marks.

I'm just not sure I can keep doing this. How do I even know a new barrel and piston will fix it?

I guess the answer is you don't. It's impossible to guarantee at any rate, but if you're certain everything else is perfect then I'd expect replacing those items to be the final pieces of the jigsaw.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Can you see something I can't?

Presuming the clearance is around 0.04mm (you did measure it, didn't you?) I'd run that.

As stated on this forum about a million times the corrct piston to the barrel (i.e. a-b, b-no mark, c-d) is no guarantee of correct clearance.
This is the result of running an unmarked piston in a B barrel. That (unmarked in B) is correct but the B barrel has been plated, it's not stamped up, but it's the wrong height (replaters tend to remove some deck height) and quite clearly a bit tight.

It ran fine, didn't seize,the barrel is barely marked and both that barrel and piston will get used again, probably not togeather though.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread

Isn't that mark in the bottom left corner a seizure just gearing up?

I'm not going to lie, I gapped the rings, but didn't think to check the piston at the time.

It is a D piston in a C barrel. Can I slide a feeler gauge up the exhaust port and check the gap now?

Do you know that the barrel has never been plated?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

I am becoming increasingly aware that I know nothing.

History of the bike is sketchy to non existent.

Well, that's what you get for giving it a female name, I suppose...
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MC21 Honda NSR250. TYGA 300 conversion.

My Vanessa is a cruel mistress, but then, most women are.

I can see what looks like a bit of a nip up.

You'll not be able to measure the clearance properly without pulling it apart. What did you set your ring gap to?

The cylinder is very likely ok. If there's any evidence of a nip then it'll probably clean up with a bit of acid and a quick hone.

Fontyyy would use that piston again, and in dire circumstances it's probably ok.

Blimey, my old TZR125 didn't bed in properly with race pistons until at least one seizure and a rub down

On that barrel, 0.40.

It's definitely rubbing in that corner. I was under the impression that any piston/barrel contact at all was bad.

For sure, if it's nipped I wouldn't use it in a racer.

My mate took a heap of used pistons and rings off me a while ago for his NSR. They were all toast as far as I was concerned, but he was happy to sand off the high spots and bung 'em in. When they got a tad too noisy (slap slap slap) he'd just bung another in.

Note to Fontyyy:

Don't worry mate, I'll save the next stash for you

Fontyyy will take your used head gakets and circlips as well Matt.

And you think I'm joking!
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If I have to take the carbs off once more...

I'm no pro at re-using stuff, you guys haven't seen anything until you've seen what Dan can put togeather and take to 2nd place in F250.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread