Must do machine work for your Magnesium Crankcase
Magnesium Crankcase line bore
In the factory shop manual Porsche says to line bore the crankcase to correct for any misalignment. To line-bore a case, the two halves of the case are bolted together and torqued. A fixture is attached to the front of the case and a cutter is run through the crankcase bore. There are two drawbacks to using this method to restore the crankcase bore alignment.
1. Though this will restore the crankcase main bores alignment however now you will need to use oversized bearings to make up for the increase in tunnel diameter. When the cars were new bearing availability was not an issue. However, today sourcing an oversize bearing has become difficult and very expensive.
2. Though the main bore has been corrected nothing else in the case has been corrected for misalignment. This can result in oil leaks and other rotational issues related to the intermediate shaft.
Magnesium Crankcase close and hone
A better method to correct for crankshaft bore misalignment is to close and hone the crankcase back to standard. This is done by machining both halves of the crankcase mating surface. This action reduces the main crankshaft bore diameter. The main crankshaft bore, and intermediate shaft bores are then re-machined back to the standard diameter. This method of repair is much better than line boring by itself for the following reasons
1. The engines sealing surfaces and all line bores are now machined back into alignment. This makes for a much better case assembly and much less chance of leaks developing due to wrapped mating surfaces.
2. The case will still be able to use standard size bearings. Standard bearings are still readily available and are considerably cheaper than oversize bearings.
3. Will usually clean up any case porosity issues that have occurred over time.
Points to consider when you close & hone the case
There is a couple of drawbacks that must be taken into account. When you change any dimension it will need to be made up for. In this case, to maintain compression ratios and other critical engine dimensions a spacer will need to be installed. The spacer goes under the barrel to correct for the change in crankcase height. This is not a major issue, but should not be overlooked. Typically, the machine shop will stamp the size of the spacer required above cylinder 2 or 5, letting any future builders know that the case has been machined. Some machine shops will supply a shim for under the barrel to make up this difference.
Also when you close and hone a magnesium case you should be aware of the parts that cross the two halves. That would be the oil breather vent, intermediate shaft cover, and sump cover. We show you how to deal with these parts under case assembly. But generally, they will have to be relieved to fit. This means oversizing the holes in the covers. Never force the parts on, remember you have removed material from the overall size of your magnesium crankcase.
Crankcase cylinder surface machining
The crankcase cylinder mounting surfaces are another area that is subject to wear and distortion. A poor seal here will result in an engine oil leak. We recommended as part of a magnesium crankcase machine service to surface these as well.
As with any machine operation that removes material from a design dimension, that material will have to be made up for with the use of a thicker barrel shim. You may stack shims at this point to make up all the needed differences.
Sump Plate surface machining
Corrosion can be a problem for magnesium cases. If you have a lot of corrosion on the sump plate sealing surface, you should add this to your machine work. You should make the decision to do this before you send the case out. That way you can remove the studs beforehand and have a better idea of the full costs involved with the machine work on your magnesium case.
Oil bypass modification
Another must-do modification to your magnesium engine case while it is apart, is the oil by-pass mod. This is a service you can add to the right machine shop. Take a look in your case half to see if it is updated or not.
In 1976 Porsche made a change to the way the oil pressure control circuit worked. In this change the oil by-pass bore that originally would dump engine oil into the crankcase when the pressure valve was open was re-directed. Instead, oil was directed back into the oil pump intake pipe. This was done because of a change in the engine oil pump design. The change resulted in a scavenge pump that could no longer keep up with the volume of oil that could be by-passed into the case.
When using an engine case built prior to 1976 and using an oil pump larger than the originally designed pump, the bypass modification should be done. If it is not done it would be possible to overfill the crankcase on full-pressure bypass. This could result in mechanical failures. Failures are due to cylinders’ hydraulic locking or at the very least excessive tailpipe smoke due to oil in the cylinders. If using the standard oil pump, it is optional to do the modification.
Crankcase Head Stud Thread Repair
One of the weak points of the Magnesium crankcase is the cylinder head stud threads. These threads are prone to failure allowing the head stud to pull out of the case. The fix for this problem is to install a “Time-Sert” or case saver. The Time-Sert or case saver is a solid piece of cast iron and is much stronger than the original threads in the crankcase.
To install a Time-Sert the original threads are drilled out. Next, an oversized thread is cut into the case. The Time-Sert is then screwed into the case using the special installing tool. Once the insert has bottomed out, the installer tool continues to turn through the insert. As the installer is a tapered thread it expands the insert out against the case. This results in a much better bond with the case as the insert is not just relying on the threads to retain it.
General thread repair on magnesium cases
Time-Sert Thread repair in a magnesium case
Repairing threads in old parts is one of the most common repairs that need to be performed. There are several different ways of repairing threads, though some are better than others. The thread repair that we prefer is the Time-Sert method.
Just like for the head studs, we like Time-Serts for any other threaded hole. This method involves drilling out the old threads and or broken bolts. Then tapping a new oversize thread to accept the insert. As the Time-Sert is screwed into the hole, the installer tool expands the insert against the hole. This gives the insert a much better bond than just the threads alone.
Helicoil thread repair in a magnesium case
The other main type of thread repair is installing a Helicoil type insert. This type of thread repair involves the same drilling out of the original threads and tapping a new oversize thread into the hole. However, instead of installing a solid metal insert it is a stainless-steel wire. The wire runs into the threads. There are several issues with this type of repair
1. The thread repair relies solely on the threads only, and can pull out or fail easier than a time-sert repair
2. The insert can be miss-aligned causing the threads to be unevenly spaced. This means the bolt will not screw in.
3. The insert can cause the fastener to become stuck in the hole, due to it unwinding.
For thread repair, you need to be correctly centered and located over the hole when setting up the drill. Doing a thread repair while the engine is out and apart is usually much easier to do. This is because being able to mount individual parts into the mill or a drill press is much easier than drilling out a broken stud in the car.
When dealing with broken fasteners it can be much harder than dealing with just a stripped thread. Usually when a bolt breaks the break is not clean and uniform. Instead, the surfaces are often jagged and hard to drill into while maintaining a good center.
Cutting off the broken bolt is best when the part is out of the vehicle. This is because an end mill, unlike a drill, will cut an uneven surface without being deflected off-center. The issue with using an end mill bit in the vehicle is because it is not self-centering it cannot be used in handheld drills.
Normally your magnesium case will be at the machine shop for some length of time. Depending on the machine shop’s backlog, your machine work could take from 4 weeks to 3 months on average. We have waited up to 6 months!
Use this time to do your component restorations. Get the carburetors, or your mechanical fuel injection manifolds started. Work on the heads. Rebuild or service your ignition distributor. Measure your pistons and cylinders, rebuild your chain tensioners. The list goes on…
There is so much more to do before you get your magnesium case back.
We have a full course on the 911 Air-Cooled Cylinder heads
When you get your Magnesium case back from the machine shop
When you get your magnesium engine case back from the machine shop, remember it is not clean enough for re-assembly. You will need to fully clean the case. If you had the machine shop do the head stud thread update, clean all the threads out. If you are doing the head stud work yourself, you will be cleaning the case many times.
We start the rebuilding when the casework has been finished. Check out installing the crankshaft in this lesson
At this stage do not forget to take the case half measurement. This measurement will be used to calculate your compression ratio.
6 thoughts on “Bringing your Porsche Magnesium Engine Case Back to life.”
You have really leaned forward in the foxhole to educate and train the 911 community. With Kurt as a Subject Matter Expert (SME), the explanation and videos are the best on the internet.
Keep up the educational lessons, it helps me understand my car and motor far better than ever before.
Col Scott Kirklighter
I appreciate the comment, Scott. We have lots more lessons in the works.
Do you have any images or video of the close and hone process, particularly the close part? I would love to see how it is done.
Hi Jason, they have to machine the case surfaces first to make the main bearings too small. Then the big boring bar has to pass through while the case is together to bore them back to standard. It takes bigger machines than we have here. Next time we go to Ollies we will ask if we can film the process. I could not find any Porsche specific case close and resize!
Can the case halves be machined flat on a large knee mill, 11×58 or do they use something else?
The case halves have to be cut in one pass. So the tooling needs to be big enough to cover the whole area. Speeds and feeds have to be considered too.
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