corroded bolts and butchered threads and rear engine mount and sub frame

[cc]

The crossrail or subframe for the front suspension is attached by 6 bolts to the car at its trailing edge (3 on each side ) and by one bolt next to the front suspension on lhs and rhs.
The power steering is also bolted to the subframe.
When taking out the subframe bolts to get access to the alternator i was surprised how much surface corrosion there was on the 2 longer bolts, that attach the steering rack to the sub frame. 1 each side. See pic.

When removing one of the bolts for the rear engine mount i damaged the bolt hole in the gearbox housing it attaches to. see other posts how i did it. Im buying a tap to clean out the damaged threads before attempting to put it back in.
Theres a $2 coin for scale in the pic and the 3rd longer bolt is the one attaching the the cross rail at the front near the suspension. Showing the uncorroded appearance.

[johnl]

I'd refit using plenty of 'never-seize' (or similar product, but grease will do). I grease most bolt and nut threads, avoids problems later, and the clamping force for X tightening torque will be greater.

Regards,
John.

[cc]

Hi John
Have read a few points of view over the years on whether to lubricate a bolt and how it affects the torqueing up of it.
Im in the clean it up and replaceas is camp  if its going to be significantly tightened.

Re the buggered/damaged rear engine mount bolt hole in the gearbox housing...

I didnt totally destroy thread and would have to remove the engine to get access to helicoil..
Its a M16 1.5 bolt and a local tool store sells the tap for less than $10, so am having a go.
Ive never done this before, should I use a light oil? am going to run it in and out progressively and hopefully clean the thread up/out.
Its 1 of 3 bolts and im hoping the other 2 with the weakened 3rd will still do the job.

[Colin Edwards]

Hi CC,

Trefolex cutting compound!

[johnl]

Hi CC,
FWIW (and I'm no expert in this field), I've lubricated all sorts of fasteners for literally decades, and never had a failure that could rationally be put down to the lube (old worn / damaged threads yes, over enthusiastic tightening yes, stress risers from damage to the fastener yes). Having said that, I have had a few thread failures where lubrication has been stipulated / used and the fastener carefully tightened to the manufacturer specified torque, though this has been failure of the thread in the component into which the bolt is fitted, not failures of the bolt itself (some application specs sail very close to the wind it seems).

With fasteners that I've installed (wet or dry torqued, whether lube was recommended / used or not), I can't recall having a thread failure in use, only as the fastener was actually being tightened. You can feel the thread start to go as the fastener is being tightened, the 'feel' subtly changes as the thread starts to stretch ('creep'). You need to pay careful attention to the feel as the fastener gets tight, if it's a remotely critical application and the feel starts to oddly change during tightening then it's either time for a heli-coil or a part replacement. I can't recall snapping a bolt as I tightened it, other than very small ones and old rusty ones that I should have changed anyway. I also cannot recall any nuts / bolts that I have installed (lubed or not) ever coming loose in use, other than the ones I have forgotten to tighten properly...

I also make sure I lube under the head of the bolt and / or on the nut face, a lot of 'stiction' lives there. 'Stiction' is hard to quantify and account for, and may vary considerably from individual fastener to fastener, so a given torque as seen at the bolt head may equate to significantly different bolt tension as seen in the bolt shank, and thus a variation in the clamping force created, i.e. two bolts tightened to X torque may not both produce Y clamping force (one may be Y+ or Y-). Lube reduces such difference (but doesn't eliminate it). Which is why super critical applications tend to measure fastener stretch rather than use a 'tighten to X torque' specification.

Stiction in the threads may also allow the fastener shank to twist significantly, especially longish bolts where the thread is a fair distance from the bolt head (why studs and nuts are generally better than a long bolt). Twisting in the shank will cause a lessening of the clamping load the fastener can exert for X torque tightness at the bolt head. Again, lube lessens differences and increases clamping load.

Breaking fasteners on disassembly is another matter, and this is I would suggest mostly be due to the fastener having been originally installed with no grease etc. My gut suspicion is that many fasteners are not lubed at manufacture because in that specific case 'dry' is sufficient, grease tends to attract dirt, and because lubing adds time and cost to assembly(?).

Regards,
John.

[bazzbazz]

I'm trying to picture in my mind the placement of the bolt you have stripped, is it possible to get a slightly longer bolt so it protrudes through and get a high tensile nut on the other side and re-bolt in place. I can tell you right now if you've damaged the threads to the degree you're describing there is little hope of being able to torque it back up to specs.

(Been there, done that) 

[cc]

Hi guys
will take some pics as it goes back together...
Went to trade tools Coopers plains brisbane to get the die. $7.90 and i couldnt help myself.
A Warren & Brown torque wrench for tightening up to 25Nm. So easy to under/over tighten/strip small bolts in aluminium alloy engines.
Am thinking about the bolt on the timing belt tensioner.
Whats trefolex cutting compound?
update..in regard to lubricating a bolt before torqueing it...Warren & Brown in their instruction pamphlet. That came with the 1-25Nm Torque  wrench, a model 320500 Advises re lubrication, to use a light oil. See pic.apologies for the poor quality.

[cc]

Have returned the rear rigid engine mount and the bush to position. Pics show same and the 3 bolt holes in the rigid mount, the centre one of which was the location of the interference between the exhaust mounting bracket and the 19mm socket used to wind out the bolt. Check pics to see how they overlap. The centre bolt is the issue. The exhaust mounting bracket prevented the bolt from exiting as I wound the socket.... as the bolt was prevented, the socket became harder to turn! damaging the threads in the mount. To remove correctly i should have removed the 2 outer bolts of the 3 bolts first...
Unusually accessing eper, with my vin number shows only a 2 hole rigid bracket engine support. The first time my 156 has differed to eper. Also on elearn, its a 2 hole bracket. Which means if i cant get the centre bolt to start. I will go with 2 bolts until I take the engine out and helicoil it..
The weight of the rear of the engine is curiously suspended from the sub frame by only 2 M8 bolts that look a little slender for the job. See pic. The 2 holes in the sub frame they slot thru are a much larger diameter than the bolts. See pics with witness marks. Its as if the sub frame was made to take a larger bolt..
Starting the 2 M16 rigid bracket bolts into their holes wasnt a given and the engine had to be in its original position before they would start. Tried starting them with the engine rear about 20mm low and they wouldnt start..

[cc]

more pics of witness marks and rear mount