Engine Stud Material

[Porsche_monkey]

I pulled three threads when assembling my 2.7 7R. I have had the case helicoiled (everywhere) but in order to repair the pulled studs I had to deepen the three holes slightly.

Now three studs are a bit short. Can I buy metric studs of equivalent material? Can I use grade 8.8 metric socket head cap screws? I do not want to compromise the thread engagement in the engine, nor on the top (nut) end.

Help.

[911pcars]

I would suggest having the head stud hole welded (filled), then rethread for an insert (not Helicoil). This is probably not the cheapest alternative, but the closest to the right way to repair it. In addition, Helicoil inserts are not the recommended repair solution for this area.

Sherwood Lee
http://members.rennlist.org/911pcars

[Wayne 962]

Hmmm, you need to read my book before you go any further (http://www.101projects.com).

It seems you are going down the wrong path. First of all, helicoils certainly won't hold up - you'll get pulled studs probably within 100-200 miles of driving the car. It's a waste of time. You need to install case-savers in there.

In addition, the book indicates the proper mounting height for the studs - they *do not* bottom out in the case, but instead are set at a specific height.

Get the book - well worth it when you consider that there are a lot of mistakes to be made...

-Wayne

[Porsche_monkey]

I have the book. These are not the cylinder studs, they are studs within the case. ie the ones that are recommended for helicoiling.

Should I post a picture?

[jgparker]

I pulled 4 case studs on my last rebuild too. I'm assuming you are talking about case perimeter studs, or chain-housing studs, not cylinder head studs, right? I used time-serts, not Heli-coils. The time-serts did deepen the holes slightly in some cases, but I used red lock-tight and set the studs to the stock height, rather than bottoming them out.

I think old Mag cases are just really bad for pulled studs. The worst part is when you pull a perimeter stud just as you are finishing bolting your case together, forcing you to pull it all apart again. Not much you can do about it.

[Porsche_monkey]

Bump. Anyone??

[dtw]

You can use grade 8 or 8.8 studs which can be ordered from just about anywhere. It is not recommended that studs be replaced with other types of fasteners such as bolts or screws. Bad for the case, you'll pull threads all day long.

[Porsche_monkey]

Sorry, I'm giving info piecemeal here and that is making this harder...

I pulled the two studs on the main rear bearing, they are M10x180mm, I also pulled the stud above the inner oil pump bearing, M8x130mm. So I need M10x200 and M8x150 replacements.

I know the shorter perimeter studs are available, but these studs are longer than 'normal', and therefore I can't find new ones anywhere.

I don't see why an 8.8 stud is any different than a 8.8 socket head cap screw.

Can anybody help me salvage my engine case?

jgparker: Yes, you need to re-seal, buy more loctite and replace all the O-rings, thats why I hope this will work next time around. On the other hand, eventually I will have enough practice that the 45 minute time limit is easy for me.

[dtw]

Someone can explain this better than me, but the way I understand it, bolts into the case place the force on the threads in the case, which it was not designed to withstand. Studs are threaded into the case, and then when a mating component (trans, head, engine mount, what have you) is placed onto the stud and sandwiched in between the case and the fastening nut, the torquing force is placed on the stud, not the case.

As mentioned previously, if you properly insert the pulled holes and then install the original stud to the correct depth with red loctite, you should be fine. You don't have to bottom the stud out in the hole. I would recommend ditching the helicoils in favor of time serts.

[Porsche_monkey]

Originally posted by dtw
Someone can explain this better than me, but the way I understand it, bolts into the case place the force on the threads in the case, which it was not designed to withstand. Studs are threaded into the case, and then when a mating component (trans, head, engine mount, what have you) is placed onto the stud and sandwiched in between the case and the fastening nut, the torquing force is placed on the stud, not the case.

Either way the threads see the same load ....

I had to go slightly deeper due to the pulled threads. That makes my stud length iffy.

I'm trying heli-coils in two places, if they pull I will 'up-size' to case-savers.

[dtw]

Nut onto a stud: load is place onto the threads on the exposed end of the stud. Ideally there is no torsional force on the threads already installed into the case.

Bolt into a case: all load is placed on the magnesium threads. Very Bad Thing.

Think of the heads studs. There is no load applied to the head studs when they are installed into the case, nor are they even threaded all the way in. They are frozen in place with red loctite. When the heads are bolted down, the torsional force is on the stud, not the case. There is force applied to the case parallel with the stud, but not torsionally. It's the torsional force that strips the threads in most cases. I hope some of this makes sense.

[dtw]

More info here: Stripped threads in mag case

[Porsche_monkey]

Actually, I do have a case-saver in place where I am planning to use a bolt. So it should be fine to replace the stud. I know it's 'butchered', but it is a 2.7 after all.

I will live with a little bit less engagement on the main rear bearing studs and loctite them prior to assembly.

With my fingers crossed I will re-assemble the case....

[Wayne 962]

Right - the studs are the correct way to go for reasons that DTW gave...

-Wayne

[85Gold]

In the past, and not on P cars which I just got my first one, I have made studs out of bolts by cutting off the head with a hacksaw and then threading the end with the proper die. Voila a new stud.

Peter

[jgparker]

On the bottom rear of some later mag cases, Porsche used a nut and bolt in place of one of the studs and nuts as a case perimeter fastener. If it is good enough for the factory, I think it would be good enough for me in a pinch. (if you pull one of the studs, that is)

I agree that you should not thread a bolt into the case. This will most certainly pull due to the weakness of the mag threads.

[Porsche_monkey]

85gold: I believe all studs are made with a rolled thread, not a cut thread making them far less likely to snap under tension. This would be especially true on the longer through bolts and the cylinder bolts, but not on the case perimeter.

jg: the nut was sort of the last resort in the back of my mind.