Torque and Shear

[tompearson]

I removed my sump last week to take the bits of dipstick guide out of it that I had dropped down there when I broke it, and also to give it a bit of clean after 10 years of use. Putting it back on I applied the sealant and then only lightly bolted it in due rain starting to fall (I'm doing it on the drive)

I went back today to torque it to the correct settings with a new torque wrench and following the advice of the Haynes manual tighten the bolts going into the block and the gearbox until they were firm-ish. I then torqued the gearbox bolts to 40NM before starting on those going into the block in a diagonal pattern with the wrench set 15NM.

The wrench seemed to behave a bit funny at this lower setting, jumping rather than smoothly switching to a 'spin' type mode as it had done at 40NM - (It also felt like it was taking more than 15NM, but that could easily just be me not knowing what 15NM feels like) Anyway on about the 8th bolt it jumped again as it hit the torque setting and sheared the bolt off, not good but it was about the most accessible bolt so I figured that whilst I wasn't going to look forward to trying to get it out it wouldn't make any difference that the sump was attached and therefore carried on.

Two bolts later and the same happened again and I now started to question the wrench so tried it on some bolts that I had used it on to get the correct torque. In some instances it clearly wanted to apply more torque without stopping, although not on the 40NM bolts, only the 15NM ones. I then decided that shearing further bolts on the sump would probably not be a good idea and instead used it on one of the a nuts that hold the charge pipe on and this time the jump caused the nut to cross thread so that it now just spins on the bolt (not the best result either as the charge pipe is currently not attached).

Sorry for the long story, buy my questions are -
Is this the product of using a rubbish torque wrench (it is a £45ish draper one) or have I done something wrong?
Any pointers on removing sheared bolts?
Is it safe to tap a large hole in the block if I need to drill the ends out?

Thanks

 

[jonny87]

thats a bit weird, i think you should take it back to where you got it from and ask for an exchange because it defo shouldnt do that, there must be something wrong with it

i wouldnt drill the bolts out, best bet is to get a stud extractor, drill a small hole in the middle of the sheared off bolt enough to get the extractor started and then you can twist the extractor to un screw the bolt (extractors are left handed threads so wont un screw itself whilst you are twisting it) something like this:
http://www.screwfix.com/prods/20084/Van-Car-Accessories/Mechanics-Tools/Stud-Extractor-Set-8-Pc

 

[tompearson]

Ok, so it might not be me being a numpty which is good.

I have heard varying things about these types of extractors. Some people rave about them, but quite a few suggest that if anything goes wrong you are in a whole heap of pain since they are very tough but brittle and therefore can snap which makes the removal significantly harder.

 

[jonny87]

pfft mate im a mechanical engineer, used extractors tuns of times at work and have never broke one, dont be ham fisted and you'll be away

get some sort of dismantling spray/lube spray/wd40 and soak the sheared bolts in them before starting to give you the best possible chance, i would much prefer to try this way first before drilling out and re tapping new holes, at the end of the day if you ****** up the drilling and re tapping then you are going to be royally ******!

 

[sportstractor]

So is the bolts that snapped under any tension?

I had an engine mount come flying off an old car and a bolt sheered in the block and i managed to wind it out with my finger. If theres no tension theres a chance uou could wind it out just by slotting the top, if any is exposed. If not an extractor will sort it but they can be a pain doing the initial pilot, and they do tend to snap, do have to be careful like johnny87 mentioned.

Are you sure your torque wrench is designed to go that low, Iv got a specific torque wrench for low torque applications, you probably would of been better off judging it.