Left Hand Oil Drain bolt stripped thread - whats this little hole?

WR450 2008 model

The threads have stripped on the left hand oil drain bolt, which drains the oil from the integral tank - the bolt has always been tight and wouldn't freely rotate by hand. When I had a look and started poking around, the thread peaks just starting breaking away from the crankcase main section. It hasn't been over tightened as I use a torque wrench and tighten it to the factory spec of 20Nm, but this seems a little high now, especially and the other drain bolt which is M10 (this bolt is M8) and that is 20Nm as well!


I am getting ready to repair it by tapping it out to M10x1.25, so it is the same as the other drain drain bolt.

There appears to be plenty of metal in the area to support the larger diameter bolt.

I have decided against helicoiling it back to the original M8 sie as I'm concerned about breaking the tang off the helicoil and it going into the oil tank and not being able to get it out, and also how to stop swarf getting into the tank when drilling and tapping the new thread (I know the trick about putting grease on the tap so the swarf sticks to it, but this isn't 100% effective and some swarf always escapes) and hence getting sucked down into the oil pump and possibly blocking an oil way.


To prevent swarf getting into the oil tank, I have made a little plug, from an M8 bolt, which I have filed a screwdriver slot into, and inserted it into the few remaining threads that are left - this blocks up the hole and will prevent swarf getting in to the oil tank. It is nothing more than 4.5mm of threaded portion from a normal bolt, screwed into the drain hole - you can just see it in the photo below. I can then drill and tap the hole knowing no swarf can get into the tank. The plug will then unscrew and as its M8 (ie has an OD of 8mm, and the minor dia of an M10 fine pitch internal thread is 8.80mm), it will come out easily past the new threads. Depth to this plug is 14.5mm which is just under the length of the new bolt (which is another bolt as fitted the the other drain hole), which I can shorten slightly so it will tighten up against the copper washer and make an oil tight seal




My question, is what is that little hole in the threads, circled in red?

I've tried poking a wire into it, and it appears to be a blind hole, so may be a bit of porosity in the original crank case casting, but I can't be certain.

Does anyone, who has split one of these engines, know if this hole goes anywhere?

Edited by GuyGraham

I have no idea, but you are right about that torque number being high.  I have never used a torque wrench on the drain bolts, just a normal sized 12mm wrench and do it by feel.  I think the bolts that cinch my forks in the triple clamps are 21nm and I thought that was a bit excessive when torquing it down.  I really couldn't imagine tightening the drain bolts similar to what they indicate for the forks.  I'm assuming they are listing that number with the expectation you change the crush washer every time which might take some of the stress off the threads.

Dont use a new washer everytime, as I anneal the exisiting one as its copper and therefore reusable.

Nobody knows what that hole is or even if your bike is the same?

Mate I'm pretty sure my sons TTR230 has that same type of hole in it too but maybe a little bit bigger. Hopefully someone here knows what it does.

Well. its done now


Drilled out the stripped threads to 8.8mm with pilot drill (I ground a flat on the end of the drill so it would get closer to the plug which was sitting in the bottom of the threads

This is the plug - a 4.5mm length of M8 bolt with a slot filed into it


Because the drill was taking out so little metal, I knew it would try and pull itself in uncontrollably, so I made a stop for the drill which would stop the drill being pulled in too far

I made this out of an M10 Nut with a couple of M5 grub screws through opposing flats, and set it to 14.5mm which is just shorter than I measured the depth to the 'plug'




With the hole drilled out to 8.8mm, I tapped the new M10 x 1.25 thread into the hole. Iocked two nuts against each other on the tap, as a depth stop (like on the drill) so UI knew have far to go in with the taps.

The amount of swarf made me glad I'd made the little plug to block up the hole, because some would easily have got pushed into the oil tank, perhaps never to be seen again and blocking the oil way down to the pump :confused:

Once I'd tapped the thread, and used the plug tap to get the threads to the bottom of the hole, I removed the 'plug', which came out real easy



Heres the new larger thread, fully formed, waiting for its new M10 bolt



That little hole in the side of the thread is still there - just don't know what its for


and here we are with the replacement bolt fitted and torqued up to 15Nm, and oil tight!



So, if anyone else strips the thread on the left side oil drain, you can safely drill it out and re-tap it to M10

Edited by GuyGraham

Have got about 30hrs on the bike since repair and all is good, so that little hole in the thread appears to have no funciton

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