Case Tightening Sequence

earossi · 04-16-2014, 06:06 PM

When mating the case halves of a 911 motor, I have found conflicting advice on the sequence of securing the fasteners that hold the two halves together. Some sources specify that after applying case sealant and mating the two halves, one should first install and torque up the long case through bolts. Once torqued, you then move onto all the perimeter bolting. Both sets of fasteners are to be cross torqued in a sequence that is listed in many documents.

Another source stated that the case perimeter bolts should be the first ones torqued up followed by the case through bolts.

So, which of the two sequences is the correct one and why? And, lastly has anyone joined the case halves and suffered an issue that can be traced to using the wrong bolting sequence?

earossi · 04-16-2014, 06:10 PM

On a 993, what is a reasonable amount of time for a Porsche shop to remove and replace the powertrain? Is there a standard? Or, is there a large variation in the time required?

HawgRyder · HawgRyder's Garage

If I don't have a factory sequence to work from...I always do the inside to outside pattern.
So...for a case...it would be case through bolts (in 2 or 3 stages) and check the crank for binding at each stage.
Then...the perimeter bolts...from the middle of the case to the front and back.
Any binding felt will need checking...but be advised...the crank might get tight...then loose as you approach the set torque.
Bob

tocobill · 04-17-2014, 08:58 AM

The 993 has a sequence. I just did it to check my main journal clearances. Its in the porsche manual ... page 88 of 1481 ... 50nm for thru bolts. That is what I would follow as bad advice is well just bad advice. Chances are the tech is following the manual as well. Thru bolts are first then the case perimeter bolts. Buy new perimeter bolts. I got mine that are the oval torque for very little online.

earossi · 04-17-2014, 09:34 AM

Right. My real question is what are the consequences of doing the perimeter bolting first and then the through bolting? Both phases done per the cross bolting specs, of course.

HawgRyder · HawgRyder's Garage

think of it this way...if the perimeter bolts were tightened first...and they kept the case from flatening out...when you tighten the through bolts...you put pressure on the center of the case...so in an extreme case...the center of the case might break.
You would be trying to crush the "pucker" out of the case and the perimeter bolts would fight you.
On cars with straight six engines...the center of the head is done first...so that the head will end up flat...if not...the head might not seal in the middle 2 cylinders...whereas the end ones would be OK.
Metal "squirms" ... or flexes...you must allow it to find it's own settling position.
Also...if you do the perimeter bolts first...how do you know the through bolts have really clamped the crank?
The bearing halves might still have a gap in them.
Bob

tocobill · 04-17-2014, 04:23 PM

Bob beat me to it. Think of all the extra beef in the case that supports the crank. You would want to have that tight first and not twisted as its what keeps everything else supports and where it needs to be. Think of all the machine work and time that is put in to make sure the bore align of the journals is true and both side are a machine match. The case perimeter is just to seal. If you were to do the outer bolts and something where to get tweaked you would then tighten the thru bolts and most likely stress or crack the internal structure that isnt designed to flex like the perimeter.

earossi · 04-17-2014, 07:03 PM

Good explanation. But, if the case were closed up in the reverse fashion (i.e. Perimeter first and then through bolting), does the case need to be split again before rebolting in the proposed sequence? Or, can one simply relax all bolting allowing the case to relax before re torquing everything in the recommended sequence? This may be wishful thinking since the case seal may fail as the case relaxes. Comments?

BURN-BROS · 04-18-2014, 10:17 AM

Case bolts first, then perimeter.

We did a back to back test and found torquing the case thru bolts first yielded less resistance when turning the crank(no pistons) . The test was done on a mag motor.

HawgRyder · HawgRyder's Garage

The problem with loosening the bolts and then doing it correctly is that the sealant will have set.
Now you need to split the case...clean the old sealant...and re-apply new before proceding.
Bob