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

pjo046 · 06-30-2013, 08:14 AM

So, I am in the middle of my engine rebuild. I am taking everything slowly, I read the workshop manuals thoroughly, and I follow the torque recommendations using two different click type torque wrenches (2-24Nm and 20-210Nm). I clean the threaded holes in the block with a tap, and use the opposite type of tool (Not sure of the english word) on the bolts. I turn the torque wrench slowly and steadily when torquing down bolts.

Still I have managed to start pulling the threads in some bolt holes.

First there was a M6 bolt on the lower balance shaft cover, as described in this forum thread:
Question on torque values (Engine rebuild)

Some days later, I found out I forgot to install the support bracket which goes under bolts 11, 12, 13 and 14... The one seen on the picture below, supporting the balance shaft bearing/seal housing/power steering bracket.

(Sample picture)

I undid those bolts, installed the bracket, and put the bolts back in. First tightened them in the correct order to 15Nm, and then started tightening them to 20Nm. But then bolts 11 and 14 started to turn too much(torque feeling not increasing when turning the wrench). I immediately stopped tightening them, so the threads have probably just started deforming, they are not totally shot. I left them in there figuring they hold somewhere between 15 and 20Nm clamping force as is.

And wouldn't you believe, one more bolt hole has started stripping. This last one is on the waterpump. It's the one which is all the way to the right when looking at the waterpump, i.e. the hole on the right of this sample picture from arnworx (Note the irony of the picture text..).

I first tightened all these small su..ers by hand, then torqued them to 4Nm, then 8Nm. (2 steps in an effort to prevent stripping, to easier feel when/if things go wrong). So this last bolt started spinning probably somewhere between 6Nm and 8Nm.

Is it possible that I can get away with leaving this as is? I know the "correct" answer is to get hold of helicoils and fix this properly. But then I might risk spinning the as of now good threads, and by the time I am finished I have helicoiled half the block.... I am thinking as there is some clamping force present (The threads aren't totally wasted), it might perform ok. What I guess I am looking for is to know if anyone else here has left bolts like this in place with success? I was thinking I should go out in the garage tonight and see just how much torque the bad bolts hold as of now, or rather see if they hold say three fourths of the required value (Don't want them to spin further, so I'll stop at such a torque level if they don't start moving by then).

On a side note, I will be using Evans waterless cooling fluid on the engine. This results in a lower pressure in the cooling system as well as eliminating corrosion, so it should help preventing a leak in the area supported by the weak fastener.

For the rest of the build, I will stop at 90% of the actual torque spec when tightening the rest of the low torque bolts (8 to 20Nm bolts, except the oil pan gasket bolts) to avoid further stripping. I will clean the bolts and holes with acetone and let dry before assembly, to assure no oil/grease being present as thread lubricant.

Other tips or tricks I should know of? I know I must seem like a moron, managing to stripping this many threads. I am really frustrated about this.

mytrplseven · mytrplseven's Garage

This sure sounds like your torque wrench is of the wrong values. Outside of that, I'd recommend using anti-seize on the threads when threading back into the aluminum block. This also applies to the spark plugs. I've never had a bolt strip out or pull on aluminum because my two torque wrenches are well known and respected manufacturers. I believe helicoil is in your future. You sure don't want critical bolts to fail later.

Jrboulder · 06-30-2013, 10:05 AM

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

Jrboulder · 06-30-2013, 10:07 AM

The same torque with a lubricant will increase bolt stretch and preload.

pjo046 · 06-30-2013, 10:52 AM

That is my understanding as well.

Anywho, I went out in the garage and checked. The bad water pump bolt (Or bolt hole more precise) holds 4Nm while spec is 8Nm. At 5Nm it starts moving.

For the balance shaft M8 bolts, number 11 holds 15Nm. Starts moving on 16Nm. While number 14 held 16Nm. Didn't push it further than that.

The low value torque wrench was new one week ago.. It has not been dropped, it is zeroed out after each use, and so on.

I find this strange as well. Surely the torque values stated by Porsche must have some margin of error? Wouldn't think bolts rated at 20Nm would start ripping threads at 25Nm for example..

As for wether the bolts in question thus far are critical or not, surely the one bolt on the water pump is not critical. Should it snap, it may cause a coolant leak. Or it may cause it as it is already as it isn't clamping more than 50% on the gasket compared to the other fasteners.

As for the bolts on the balance shaft cover: Two bolts have insufficient clamping force (with one quite possibly very close to spec) whereas two have the proper clamping force. This end of the balance shaft cover is also held in place via one bolt in front of the power steering mount going into the front of the block. Furthermore, both the balance shaft belt and the power steering belt have forces pulling inwards towards the block, not outwards (Which would be a problem if the bolts should fail). I am mostly worried about oil leaks from the balance shaft cover, and of a coolant leak at the water pump. Is my reasoning at fault?

pjo046 · 06-30-2013, 11:07 AM

Of course I understand that no one here can give me a guarantee that I can keep this as is and finish putting the engine together.

I assume taking out the bolts that has started to spin, and try to fix the bolt hole threads with a tap and also clean the bolt, is of no use? If anyone have done this with success, do tell about it.

Other than that I am mostly interested in knowing wether others have had bolts starting to spin but done nothing about it, and still avoided leaks. Or if you ended up with leaks. I would also want good arguments for why I can't leave this as is but need to helicoil.

Some valuable information regarding correct torquing methods:
How to Use a Torque Wrench - How to Tighten Lug Nuts - Popular Mechanics

Gawernator · Gawernator's Garage

Some of those small bolts don't have a very large margin of error at all

jeffrsmith · jeffrsmith's Garage

Balance shaft covers are known leakers, I would not put it back together without having similar torque on all the cover bolts. Where you are right now, it is much easier to take it apart and fix it correctly. It may take you a little time now, but if you put it back in the car like that and discover it is leaking, it will take you a whole lot more time and will not be nearly as convenient. Same issue with that water pump bolt, don't chance it, although the torque spec is not very high, the sealing relies on even torque all around the housing.

You might put it back together and have no problems at all, I think you would be lucky. If you do proceed without properly fixing the threads, I would not want to think of how many nervous drives you will have expecting a leak to develop if it is not fixed correctly.

Having said all this - doesn't the saying go something like "if it doesn't leak it isn't a Porsche?"

pjo046 · 06-30-2013, 01:47 PM

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.

So I need to buy a new wrench tomorrow, and check where the oil pan bolts are at now. The oil pan gasket is slightly bulging some places, but this is normal?

Tomorrow I am also buying helicoils and fixing the broken bolts. Hopefully it isn't too dificult. But what should I do with the upper balance shaft cover, as this is then clearly overtightened? No bolts stripped, but still. Do I need to back down on the bolts and tighten them all over with a new wrench? If so, will it compromise the Loctite 574 seal?

I f.....g hate the store where I bought that no good torque wrench!.... Aargh..

944fanatic · 06-30-2013, 02:32 PM

I haven't read this whole thread, but i'm sorry this happened, I know it can be very frustrating, just don't let it get the best of you.

VirginiaF1 · VirginiaF1's Garage

Quote:

Originally Posted by pjo046

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

I f.....g hate the store where I bought that no good torque wrench!.... Aargh..

They have Harbor Freight stores in Norway?

pjo046 · 06-30-2013, 11:09 PM

No, but they have something similar..

Today I'm buying a $150 wrench.

pjo046 · 07-01-2013, 12:24 AM

So the main question now; Can I leave the oil pump and upper balance shaft cover as is? it is obviously somewhat overtorqued. but all bolts to the same amount, and none of them has stripped. the loctite 574 has cured by now. Should I back off the bolts one at a time and retighten with a new wrench? Or can that distort things/cause leaks?

jeffrsmith · jeffrsmith's Garage

If it were me, and I had been able to torque all the bolts to similar values, although over-torqued, I would leave them.

If I understand the idea behind using torque values it is to ensure even clamping pressure without overstretching and damaging the fastener. You have even clamping pressure at the moment, if you have indeed over-torqued and attempt to take the fasteners out you may damage/further damage the threads in the block.

CHICKS · 07-01-2013, 05:23 AM

That oil pan gasket will make it seem like the bolts you first tightened weren't even touched. That thing is thick and squishy. What I find suspect is in your first post you said you were were using a tap and die to clean threads. Those are thread cutters, not cleaners. You may have weakened the threads while cutting. I also find torque wrenches not as accurate at the lower end, or higher end of their ranges. I might use a beam style torque wrench when I get to putting my motor back together.

pjo046 · 07-01-2013, 05:25 AM

Think I'll follow your suggestion and leave them as they are. Only thing is on the oil pump, 2 bolts will need to come out when mounting the plastic cover after the water pump is fixed.. But maybe leave the rest on, and retorque the 2 that has to come off to spec (10Nm). Or I can check how much torque is required to move them, before I take them off. And tighten them to the same level.

pjo046 · 07-01-2013, 05:30 AM

Quote:

Originally Posted by CHICKS

That oil pan gasket will make it seem like the bolts you first tightened weren't even touched. That thing is thick and squishy. What I find suspect is in your first post you said you were were using a tap and die to clean threads. Those are thread cutters, not cleaners. You may have weakened the threads while cutting. I also find torque wrenches not as accurate at the lower end, or higher end of their ranges. I might use a beam style torque wrench when I get to putting my motor back together.

I have been thinking about that issue myself.. Although I was carefull with the tap, it may have contributed to my problems..
I need to get hold of a set of thread cleaners instead. But what's done is done. I returned the 2-24Nm torque wrench today, and am going to buy a higher priced one later today at a different store. That one comes with a calibration paper, and is guaranteed to have a maximum error of 3%. The other one I had, didn't have any advertising as to maximum margin of error at all (Which is clearly no sign of quality).

CHICKS · 07-01-2013, 05:40 AM

I use harbor freight and tekton el cheapo click type wrenches that are +-4% with no issues to speak of. Good luck with the rest of the build!

pjo046 · 07-01-2013, 05:50 AM

Thank you. I will commence the work tonight. First I'll check what torque my oil pan gasket is torqued with as of now (2Nm according to the returned wrench), and report back.

Will also let you know how the helicoiling is working out. :-)

mytrplseven · mytrplseven's Garage

I honestly wasn't aware that anti-seize was on aluminum was an issue. Glad you guys pointed it out.