Sunday 24 April 2022

Dre-XT-Stück - Easter is resurrection time

What do you mean, you didn't even know the old girl. I mean some of the last pictures were on the "very pretty" side of things. Guess I may have deliberately omitted a few things... 

Let's start off with two beauty shots of the old girl:

Next thing wasn't very drama either: Made a pair of knurled nuts to be able to remove the seat without tools.

Still not quite drama-land - even less so, when I scored a lovely Arrow Paris Dakar in stainless steel for a good price. And yes, there was a friend (made for an XR600) already waiting on the shelf... Definitely could be modified to fit an XT. 

And on this lovely day full of sunshine and loud braaaaaap noises, I decided that it would be really a good idea to finally to do the oil-change as the old girl was nearing the 3000km mark and I had planned quite a few more in the coming days with the sidecar being sold and the TR1s jetting not fully sorted at this point the old Dre-XT-Stück was my main transport... and then it happened: The oil-drain bolt just snapped. So off with the oil tank, get the bit of bolt out and then chase the M8 thread of the oil-drain. 

Not quite the amount of drama one would expect? Well, I guess for some reason I didn't tighten the oil-return properly (i.e. to 11 and a bit). On the same day, the old TR1 had started leaking from oil-drain as well, which would turn out to be a partially stripped thread... perfect timing I tell thee.

In case you're wondering: from the point where the hose came undone to where the engine died it was 1.6km or about 3-4 minutes. The old girl died peacefully with no nasty noises.

Stash the bike away out of sight, borrow a trailer and get the bike home to the workshop and ignore the old girl for a few days.

The assessment of what parts where still available on the shelves was somewhat comforting. As long as I hadn't ruined the crank, pretty much everything else was available. A new 95.25 cast piston of dubious quality (I later found out it was one of the better ones out of the cheapest knock offs), a complete head (albeit the one with the smaller valves) and a good camshaft. So really all I was missing was a well worn 95.00 cylinder as I had the current cylinder taken out to 97mm. 

Obviously there was a reason why it was so cheap - it had a stripped thread in one of the head bolt threads.

As the budget ABSOLUTELY did not allow for someone else doing the machining and also I hadn't exactly made the best possible experience with the one shop that actually works with private customers, I decided to tackle the whole job myself. 

And shockingly enough, that cylinder would have been a straight up pass with a quick deglazing and a 95.00 piston that would on the upper end of the tolerance field. (The subito has to be read in reverse, the bore is at 95.08mm, so with a straight up 95.00 up to 95.04mm piston jobs would have been a good un'...)

And so starts what would become an incredibly long Thursday morning: First hone close to size of about 95.18mm with an 80 grit stone.

Do a sanity check with a vernier caliper, just to make sure I don't make mistakes reading the micrometer or subito. 

Then a final hone to size to 95.25 with a 120 grit stone.


And then ultimately wash the bore with diesel and clean the liner with a WHITE cotton cloth. Once the diesel on the rag is still yellow after going through the bore multiple times, the hone job is finished. And if you do a proper job, you can get top and bottom to within less than 1/2 a 1/100th of a mm and yes, I obviously went with the tightest clearance Yamaha allows for the old XT600, which is +0.045mm over piston size.

All of the above results in a fit like this:

At this point it was all or nothing and get cracking. Put the old girl on my lift and started to dig into the engine. 

And the first (very) good sign: Head and cam look like new.

Yup, that's a nice seizure. Most of the burnt oil on top of the piston came from the initial run in as the cylinder technically was vastly over the max. wear limit, when I installed the piston. But then again, when the old girl was cold, she smoked and rattled quite a bit...

My professional verdict: It's f*cked.

Or to quote Star Trek: It's dead, Jim. As a matter of fact after the engine had cooled off, I could still kick it over. Tough little bugger.

Conrod wasn't blued and neither where the crankshaft webs, so when the gudgeon pin fit without unusual play, I declared the crankshaft fit for service.

Installed the piston, slapped the cylinder on...

... and then the other head bolt thread stripped.

Helicoiled it to the moon and back and tore the base gasket. On a Saturday afternoon. Decided to say f*ck it and got it on with some gasket paper. To be honest, I am not great with scissors, so this wasn't exactly a master piece but good gasket paper with lots of grease can work wonders.

Test fit on the old cylinder.

At this point I wasn't even shocked anymore when one of my M6 helicoils gave up and I had to install a time-sert.

As Mr. Postman also dropped off a set of new tyres (Mefo MFE99s, thanks for asking), I installed them the following day.

Oh the engine? 

As a matter of fact a couple of things became instantly apparent: First, there's a good reason, why Yamaha finished the cylinders on XT600 engines on the upper end of the tolerance spectrum. The old girl almost seized (luckily she only cut out) multiple times on the first two or three runs and it would take almost 70-80km of run-in time until she would hold an idle both cold and hot. But the two most notable changes were first the engine noise, the newly built engine makes almost no undue noises at all, neither hot or cold and second the amount of extra power about two full points of extra compression give you. Don't get me wrong, she ain't exactly slow and I am sure a few extra ponies will find its way into the stable once the engine is properly run in at 200-300km, but when in the past the engine would happily pull to 140kph, now the engine will pull to 110kph and that'll be the feel-well speed for this engine. Maybe once she's run in that'll increase to 120 or thereabouts, but the difference is notable.

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