Bringing M8 bolts down to M6?
 

All the bolts on my crankcase are M6's except for the ones going through the oil filter cover, these were once M6's too but thread was stripped and they were tapped up to M8 (only option on the road at the time). The metal around one of the bolt holes through the oil filter cap has cracked open because the increased bolt size means bigger hole for the bolt means less metal around it means less strength.
There seems to be a bit more metal around the holes going through the crank case cover and crank case itself so I'm not worried about them (yet!) :unsure: but if I replace the oil filter cover and drill it bigger again it's probably going to happen again.
I was looking into helicoils and time serts but they seem too thin to take hold in the enlarged holes.
Anyone have any bright ideas?
I had thought of a time sert within a time sert or something like that but I have no clue about these things so I'm presuming that'd be a bad idea. :huh:
Thanks!

8mm aluminium bar threaded, screw it in tight, cut it off and file it flush, fit the outer cover, use a snug fitting drill to dimple/centre the plug you've installed, drill it 5mm then tap it 6mm........

Yeah.. That sounds like a good plan.

However, I would put some serious thread locking compound or even some chemical metal on the threads of the bar to lower the risk of it eventually spinning..


You can actually fill and tap the higher quality metal putty compounds. Although I haven't tried it personally, I've read plenty of cases of it being done successfully. The oil filter cap is low load and low torque so it 'shouldn't be a problem.

Or, fit the aluminium bar then file out where the crack is and have someone fill it with a fat seem of weld. Then file down flush on the face, obviously.

I think that would be the most secure in the long run but also the most work/cost.

Great idea! I knew I was thinking of it arseways! Will use a locking compound alright.

The work I don't mind but cost is another story! But if you're going to do something you might as well do it right.

Thanks for the info, it'll take me a while to get it done but I'll post back the results.

Yes, vee out the crack and weld, just make sure the welder knows their stuff, and disconnect the battery if you're doing it in the bike.

I was looking at Loctite 270 to lock the thread of the 8mm rod in the crankcase, its service temperature range: -55°C - +180 °C. It's location though is right below the cylinder, think this temperature is good enough? Recommendation for alternative if needed?
As for filling aluminium cracks I was looking at Loctite 3479, I have a few small chips and cracks that need fixing so this might be just the thing for me. Anyone got any experience with it or similar products?

I don't know where you are but I've got some 270, I bet the filler won't be cheap, but if you are going to shell out your hard earned on that, why not just use that on the threads as well? you could also "pop" on the casing, using a sharp punch, just on the outside of the hole/threads in two or three spots, you'd have to smooth back the mushrooming.

Thanks but I'm in Ireland which is a bit of a trek from York!
Loctite says "The threadlocker from Loctite functions by curing in the absence of air." so I was guessing the metal-filled compound does require air to cure?
The 3479 isn't cheap, quick google search shows about 60 euro but it's still cheaper than bringing the bits n pieces I have in for an aluminium weld, plus I'd like to gain the experience.
Don't really undrstand what you mean about popping the casing and al that, could you explain?

A centre punch is used to mark/start/guide a drill bit accurately, try drilling steel without and the bit will run all over the place, the punch is ground at 120 degrees, the same as the bit, it helps the bit start to cut. To accurately place the centre punch you use a centre pop, a lighter, sharper punch, with this you can feel and pick up on scribed lines which mark the position of the holes so get an accurate placing, scribe it, pop it, punch it then drill it.
Anyroad, I'm saying use a sharp punch just on the outside of the 8mm threaded hole (after you've plugged it) to distort the threads so securing the plug, it can't unscrew easily because you've messed up the top thread.
Does this make any sense?
It looks to me that the filler you are considering is a two pack epoxy, this uses a chemical reaction to harden so it will harden in air or in the absence of air.

Hi oothef, that makes sense now thanks. Do you scribe it by eye? I've seen center finders but don't think they'd be accurate enough for an 8 mil rod. I don't think I'd need to "pop" it as it's on the crank case and once the crank case cover goes over it there's no open hole for it to come out as it'll be sitting flush with a surface. The filler should be enough then (saves me buying thread locker for just one job). Also the only time there's going to be a strain on it will be when loosening the new inner 6mm bolt which should be free-spinning.

Sorry, got carried away with punches, use the outer cover as a drill guide to centre the hole, use a snug fitting drill to put a dimple in the plug and that's where to drill 5mm. Use a centre pop (a sharp punch) to pop around the outer thread, slightly damaging it and securing the plug, along with thread lock or filler, you're right once it's in, its only when undoing the screw that it could loosen, make it the first one you slacken and the case holds the plug secure, as you say.
To pop or not to pop.............
When you come to drill 5mm you can see if it's square left/right from above, you may want to get someone to check you are square up/down, that's always hard to judge on your own.

I think you should take your M8 studs to an engineering company and get them to drill and tap the M6 thread on a lathe to keep then concentric and square, this will add to cost but trying to drill them in situ will not be easy, the core diameter of the stud is only about 6.8mm in the first place.
Locitite 270 should hold them in fine, degrease everything with a solvent cleaner or petrol first.

There's not enough meat on an 8mm stud to drill and tap (i.d. of m8 @6.75mm o.d. m6 6mm so there's only @ 0.375mm between threads). Done in the hole the outer metal will support it.
You've then got the problem of installing it securely.
You also don't know how square the m8's been tapped

Put an M4 in then and go easy on it. It's a very low torque bolt anyway.