Dre-XT-Stück - Cylinderheadboltthreadsshouldhavedonethemrightthefirsttime

It started like those stories always seem to start - when things are actually going well. Or to put a date on it, on May 24th, 2024 after enjoying a lovely little rideout on the old girl, the front left of the cylinder was covered in oil. 

So I did what any sensible person would do: clean it off, do nothing, hope for the best and get the inspection out of the way and then as the problem didn't magically solve itself, mumble something and park the old girl in the shed and ignore it for about half a year. Which is roughly when I realized that the old girl would come in a bit handy right about now.

Apparently stripping down the engine never goes quite without a bit of extra hassle, so the opportunity arose to fix all the remaining camshaft journal threads as well to improve my chances of making it through a full season without a rebuild of the engine. 

 

 

The piston has sucked in quite a bit of oil through the engine breather and was covered with a nice crust of burnt in oil.

... and this is from right after I rebuilt the engine and it felt a bit doughy.

 What looks like a fairly usable headbolt thread actually isn't and is the reason we're having all this fun now.

The solution will be the same as used on the my dad's XT500 4-valve engine - stainless steel M12-to-M8 adapters with an overall length of 27mm. Why not 30, you ask? Well because the stock threads are only 26mm long and with the steel inserts they should be very much strong enough. Also 25Nm on M12 is so far away from the maximum torque that is permissable in aluminium, I was actually going to use threadlock just to make sure they won't rattle loose. The other, quite valid point is: why not use a commercially available insert? In short: I tried Helicoil and they failed as they are too short and thus too weak and the top most is a commercial product, but again only 15mm long AND about half of it is slotted for installation, meaning it will most likely not be able to carry the full load.

 Installation is very straight forward, center on the old thread with a 6.5mm drill lock the table on the mill and clamp the cylinder down tight and go at it with a 10.5mm drill. 

That's not quite looking right, is it? It is not indeed. For some reason the guy in charge of tightening up the drill chuck didn't do his job very well and the drill came loose, wandering about and making a very interesting tapered 11.2 to 11.8mm hole. Not good. 

After some thinking, I decided to make an oversize insert as luckily I had started in the one corner that had plenty of meat around the hole and settled for M14x1.0.

Now the cheap thread die left a remarkably bad finish, which in hindsight was a good thing as it was rather unpleasant to thread in, which in turn most likely meant it would be rather unlikely to come out as well. (Or as someone else said: cross-threading is a kind of thread locker.)

Measured the thread depth and decided where to part it off, so it would end up flush or slightly below the headgasket surface.

Then it was just a matter of doing the other three WITHOUT cocking it up again. 

Basically do this until you're done and test it with a headbolt.

 Because the department of fun and games hinted at the fact that on more M6 thread was stripped and at least another one was feeling a bit dubious, I decided to install timeserts on all of them.

Obviously not without making another mistake: I didn't quite countersink the first one deep enough as I thought the chamfering tool had already bottomed out.
 

Having gotten this far and not being sure just how well that insert was seated, I decided AGAINST any attempt of getting it back out. Instead, I decided to work around the issue. 

First I put the cover back on, centered it with two additional bolts and tightened it down.

This left an imprint (hardly visible in the picture) on the boss.

 Which was then chamfered to provide the necessary clearance so the cover could be tightened all the way down. 

Gave the cylinder a quick hone and then wondered whether installing it without a base-gasket would improve the squish gap. For those interested: Nope, stuff gets too tight (again), between 0.45 and 0.55mm of the piston sitting low in the bore. With an admittedly very pliable base gasket I ended up with 0.70 to 0.80mm, which has to be added to the 1.00mm of the cometic head gasket. 

To quote a certain repair manual: Assembly is the reverse of disassembly, with the two most important things to bear in mind being that the cylinder base nuts need to be tightened to 40Nm (42 in the manual) and the headbolts to 25Nm. (29 in the manual which is just too high and the cause for many a stripped thread!)

 

 

First start was rather anti-climactic - not much smoke just a two or three kicks, pretty much standard procedure as if the old girl would have just been parked over the Winter.