69gaugeman

I have started accumulating and preping the 84 for a rebuild. I like to change all the bolts to stainless hex or if it looks good to socket head cap screws. I have run into an interesting situation with the top cover bolts. The WSM shows two different grade bolts (8.8 and 12.9) the ones on my transmission are 12.9 and I was wondering why the difference. The manual also does not show when to use either one. Why the extra strength and torque? The standard stainless screw is aprox. 8.8 in strength. Can anyone enlighten me on this?

The transmission is from an 82 and is being used to to convert the 84.

Thanks,
Rod

GlenL

Do you mean the twelve m6x22mm bolts? The PET shows just one P/N for those. Those will be fine with 8.8 bolts. The 12.9 (or US grade 8) are needed for high-tension applications and also high-risk uses.

Curious: you want to dress up the top of your tranny?

69gaugeman

It's a long story. I just like to have every thing stainless and or SHCS while I am there. It's sort of like polishing the casting seams on the topside of the rear crossmember when you will never see it either. I just wouldn't feel right knowing that I could do it and didn't.

fst951

Most stainless steels are not strong enough to do what you are asking.

Hope you aren't too far along.....

Good luck.

69gaugeman

A4 stainless bolts are about the same as 8.8. I was wondering why they put 12.9 bolts on the cover. I know that the cover is part of the structural integrity of the case but they show both grades of bolts on the WSM, only the low grade in PET and no explaination anywhere. My transmission has the 12.9 bolts in it now. Not too far along as I am doing a ground up and just starting to get things cleaned up in prep of putting back together.

Carl Fausett

...no they are not. I have dropped a number of posts about the use of stainless as an automotive fastener... don't do it. I would not use stainless accept for something completly cosmetic. They lack shear strength, and are miserable in torsion.

Test it yourself (do not take my word for it) - clamp up a stainless M6 or M8 fastener in your vise and put a socket on it and twist it off. Now do same with a 8.8 fastener. The difference can be felt. If you use an old beam-type torque wrench you can measure the torque and watch where it fails.

One common misconception is that the 12.9 fasteners are stronger than the 8.8 fasteners. That depends on the KIND of strength you are looking for. The 8.8 is actually tougher, it will yield and twist more before failing.... the 12.9 is more brittle. Stronger in tension, weaker in torsion.

This is analagous n SAE between Grage 5 and Grade 8 fasteners. In many applications, Grade 5 is preferred because it'll take some bending and twists and not fail. A Grade 8 fastener is stronger on a dead pull, but because it is brittle in other directions, not always the best choice.

As to why the difference between the years of top bolt fasteners by Porsche on the 928 transaxle.... I'd suggest go with the last recommendation Porsche made. Sounds like they learned something from failures in the field.

69gaugeman

Carl,

I aggree that the stainless is not the same as the steel screw. If the screw is excessivly over engineered for the application then a stainless screw would be fine. Now when (oops did I say when) I build a track car I will not use any stainless screws. My concern was why they spec'd different bolts and what effect it would have to change (are the screws over engineered?).

Thanks for your insight. As a mechanical engineer in the automotive industry I understand many of the differences you have spoken about in this and other posts.

Have you experienced any transmission failures that could be atributed to the cover?

Also the cover screws are only 2x diameter engagement into the case material which is significantly weaker than the bolt material.

Rod

Carl Fausett

No, but I have not seen any where near as many transmissions as 928 International for example... maybe we can get them to chime in here. I like your point about the depth being only 2X dia of screw....

I do know that Porsche was having "synchro failures" that were actually case expansion problems causing the synchros to seize up. They redesigned the transaxle and strengthened the size of the ribs around the outside of the case. I wonder if this is related... like another thing they did to better control the expansion of the case.