Head stud double washer
 

In the process of putting my 911sc engine back together, one side done, but on the other side I can't get one of the head studs in far enough. All 12 bottom ones replaced with steel.
All of the others have gone in fine, but even after running a cleaning tap down, the stud is around 3-4mm further out than the others. I can't see or clean anything out of the hole. I can't see an option other than stripping the engine completly and taking it to be machined (i really don't want to do this :( ). Although not ideal, would it be OK to just put two washers on the stud before I put the barrel nut on, so there is still enough room to torque the barrel nut. Without the two washers, i'm guessing there will too much thread to get the barrel nut to fit properly.

Thanks.

Surely you need to find why you can't install the studs correctly.

I would be really unhappy with the method that you are suggesting as I don't think two wrongs can ever give a great result.

Westy - if you have a mag case, I'd really advise against it. The case end is the weak spot there.

If an aluminum case, maybe that extra 3/4mm of thread won't really matter. But I bet that is part of what Chris (an engineer who really knows about the strengths of things) is wondering about.

Plus what I, too, am thinking - the studs you removed were all the same length, and all installed at the factory height of 135mm above the case. So why won't this one insert deeper? When you measure the depth of the hole, is it the same as all the rest? Does your chasing tap go in the same depth as in the other holes (shouldn't be difficult for you to pull a couple of the studs for checking). And is your chasing tap a bottoming tap, so it will go in full depth, unlike all taps for through and through holes, which have a significant taper so they can get started more easily?

I'm sure you have inspected the stud threads, and one would not expect there to be a defect on them, but you could try swapping studs.

Working around this is sort of like replacing a blown fuse without figureing out what caused the first one to blow - trouble lurks.

If you overcome fears of not haveing enough thread engagement (and for steel on steel, you are at 100% with thread engagement depth = thread diameter - I don't know how that works on the rather weaker aluminum), an extra washer would do. Or use a flange nut - a better mousetrap than the barrel nut any day of the week. What is important is the clamping force generated by stretching the stud, and the torque spec is a way of achieving that stretch. As long as the fasteners are strong enough, how you get there isn't critical. Extra fastener material will slightly reduce cooling air flow, but hard to imagine doing this on one stud would produce a measureable effect.

Westy:

I am not sure if I am going to have as much credibility as the other two who have chimed in here, but I have to suggest more investigation is done to have caused this sort of issue. From the sounds of things, you would be adding a band-aid that will have to be addressed in the future. I would not add two washers to fix this issue - that is not the intended design. Sorry to sound like a debbi-downer...

I know your all right ;) was just getting excited about getting it all back together and back in the car, all gone well so far. The tap is tappered at the bottom, so I will look for a different one, tried two different studs and they both get very tight with 4mm to go. They might go in with more for, but don't want to cause more problems. I would imaging with some antisieze or oil it would go in, but then I don't think I could it out well enough to get the loctite to work.

Thanks for the sanity check, I would only be worrying about it and checking the stud every 500 miles :D

Like many others here I used an M10 bolt with slots cut into it to run down the threads and pick up any rubbish which had worked its way into the head stud threads.

If you use a tap you may remove material leaving the threads a loose fit.

andy

I too believe this is the best way to clean threads... from experience on engines other than Porsche.

My experience is that there is sometimes there is old threadlock in there. Sometimes it's a blob that you can chip out with a jewelers screwdriver. Other times it's more or less smeared around so that won't work. I've never found anything to dissolve it. I haven't tried it, just an idea, but maybe you could heat the stud and/or hole and power your way thru it. Good luck.

All - Westy said he ran a cleaning tap down. That suggests he knows the difference between that kind of tap, and the regular cutting taps which we are often told by the professional engine builders we ought not to use for cleaning, as it will cut too much when used to clean things up.

I think BRP is pointing the way.

Yes, ran a proper cleaning tap down, and tried an old stud with four slots cut in. Must be something stuck at the bottom of the thread. This was the only stud I had to heat to remove, so maybe some loctite melted into a lump. WIll try to pick it out.