Stripped threads in block... Aka Why I hate old aluminium blocks

[pjo046]

Well, after 5 hours in the garage, 5 helicoils have been installed, the water pump and lower balance shaft cover is finally properly torqued down, and so is the oil pan. :-)

As for the new torque wrench versus the old: Where the old would break loose some bolts just above 4 Nm, the new wrench required 5.5 Nm to do the same. On the oil pan gasket where I tightened all bolts to 2Nm yesterday, it took 4Nm with the new wrench to move the bolts slightly.

[DannoXYZ]

Originally Posted by pjo046

Turns out the newly aquired 2-24Nm Torque Wrench must be the culprit here after all..

I went out to install the oil pan just now. First I fingertightened all the bolts. Then I started on the 4Nm torque step (Lastly there is supposed to be a 10Nm step). I followed the torque sequence, but I kept having to redo it again and again as by the time I got to the first bolt after finishing the sequence, the first one wasn't even fingertight. I know you have to do it some times as the gasket settles etc, but surely not again and again and again on 4Nm? I ended up backing the wrench down to 2Nm to be able to tighten all bolts to the same tightness. Took a few turns this time as well, but got there in the end.

This is actually not the wrench's fault. Due to the weight of the oil-pan and its changing angle as you tighten the loose-bolts, the pan will lift up and relieve tension on the 1st bolt tightened. Just the weight of the pan and thread-friction will cause the wrench to click at 4Nm. As the other bolts lift the pan up the 1st bolt will be completely loose by the time you come back around to it.

Personally, I like to use Time-serts or E-Z Lok inserts to repair damaged threads in the block. Have done close to a hundred of these repairs and they hold up well to repeated removal & tightening of the bolts. I don't like Helicoils aren't as strong and often back out when you remove the bolt.

[AKCJ]

I agree with not using antisieze.

Regarding the stripped bolt holes. Sometimes you can get lucky and remove the bolt and then replace with a slightly longer bolt. There may still be some good threads further down the hole. You need to be very careful doing this because you do not want the bolt to bottom out in the hole - that could make things worse. You may have to get the next size longer bolt and then cut it down to the max length that will fit to take full advantage of the available threads but not bottom. Also, instead of using a bolt, you can switch to a stud. Install the stud with the type of Loctite made for this. It would make it very hard to get the stud out in the future but you probably wouldn't have to remove the stud in the future (just use the nut).

Note that the above methods may not be the absolute best solution, but they may be good for some cases.

[pjo046]

Good advice, AKCJ! Hopefully I won't need it, as I hope no more threads will strip from here and on.

I remember reading a thread here on the forum with different good recommendations when undertaking a rebuild. Stuff like "Put the RMS on the crank before you install it in the block/put the girdle on", "Bolt the oil pump in place before tightening the girdle to ensure proper alignment" etc etc.
However, one of the recommendations was (I copied them and saved them in a word document for when I started my rebuild):
"9. The factory shop manual's torque specs are a good starting point, but once a bolt has been inserted/removed a half-dozen times you'll have to snug them down a bit more than the FSM says. "

I'd strongly advice AGAINST doing that! Torque is torque, no need to overtorque, as it will surely strip threads.

My torque recommendations for anyone undertaking a rebuild (Based on my own faults and experiences):

- Follow the FSM torque specs and procedures

- Install all bolts DRY, unless otherwise stated. (The 944 Turbo FSM specifies to lightly oil the balance shaft cover bolts, I recommend NOT doing that. The regular 944 FSM does NOT mention oil on those bolts! I oiled the bolts on the first cover I did, and stripped several threads. On the other side with dry installation, there was no stripping (Well, one started but that was due to the torque wrench beeing off)). There is a reason for this, lubrication on the threads means the same torque value will put about 25% more force on the threads. Thats why more or less all M6 bolts are specified to be tightened to 10Nm in the FSM (Dry), while the M6 bolts for the water pump is specified to 8Nm (They are to be installed using Loctite which acts as a lubricant, thus necessitating lower torque to achieve the same clamping force). Alternatively, if you feel more comfortable putting antiseize, loctite or similar product on a fastener that should be installed dry, then reduce the torque setting accordingly (A 25% reduction should suffice).

- Clean the threads on both the holes in block and on the bolts themselves before installation. DON'T use a tap and die as this can weaken the threads, only use thread cleaner tools (They might have an english word, but I don't know it)

- After cleaning them, wipe off both the bolts and the holes in the block with acetone or similar non fatty cleaners (White spirit etc will not do, as it will leave a thin fatty coat on the threads which will actually act as a slight lubricant) to ensure they are completely dry

- On the fasteners that have only a 1 stage torque procedure, first torque all fasteners by hand, then to about 75% of the specified value, then torque them to the specified value. This way you ensure more even tightening of all the fasteners, the wrench movement on the last torque step won't be very large so it is easier to notice if any of the fasteners starts spinning and thus easier to stop fast enough if this happens.

- Be sure to use a good quality torque wrench, one that has a calibration sertificate included and a specified maximum margin of error (Usually 3-4%). If it does not specify the maximum margin of error, don't rely on it.

If, despite following the above recommendations, you start pulling threads:

- If you stop fast enough so that no visible damage has occured to the threads(I.e. no aluminium shavings/threads loosening, severly deformed threads, or similar), I would try cleaning the bolt hole threads once more, and install a longer bolt as per AKCJ's recommendation. Test fit the bolt first to make sure it is not too long / too short, and adjust the bolt length untill it is just right.

- If that fails, or if the threads are already too damaged, use Helicoils, Timeserts or similar products. My experience with this is that drilling in aluminium can be difficult, and it is easy to break the drill bits because the drill bit clings/sticks to the aluminium and then snaps. That's why I recommend trying AKCJ's method first. If you do have to drill in the aluminium block/other engine part, use lots of cutting oil, high speed on the drill bit (As opposed to what you would do on regular steel), and drill small steps at the time (Take out the drill bit often and clean away the aluminium shavings to prevent clogging).

Unless anyone disagrees with the recommendations, perhaps this one post could be made a sticky at the top of this forum section? And then people could add other engine build tips and tricks to the sticky thread? My experience with gathering information for my build has been that lots of searching has been needed, conflicting advice have been given in different threads etc, so putting all of the information in one thread should provide usefull!

EDIT:
I would recommend buying new bolts before starting the rebuild, as many of the bolts are corroded, have slightly damaged threads etc. Unfortunately I didn't think of this before I started putting my own engine together. New bolts can be had for cheap. Just make sure you buy good quality bolts with high enough grade marking. Stainless steel bolts would be a good upgrade.
Here is some usefull information on bolt properties:
http://www.specbolt.com/technical.html

[porschetub]

Originally Posted by jrboulder

don't use antisieze or any other lubricant on any bolt that isn't specified for, especially on a brittle aluminum block.

x2