We have seen cylinderhead cracks in all models of M96 engines used in 986 Boxsters and 996 911’s, up to and including model year 2001, but the 3.4 996 cylinderheads are far and away the worst offenders.
Some of the cracks are very large and easily spotted, others are less obvious, and some are down right microscopic. Click here to see a video of one of these tiny cracks discovered during pressure testing. Even if no cracks are visible the only sure-fire way to determine if a head is cracked is by pressure testing, as was the case with the head in the video. It’s crack was not visible to the naked eye, but showed up when subjected to 100psi air pressure.
— Every crack has been in one of five areas beneath the follower housing.Never in a combustion chamber or deck area.
Early 3.4 heads and some 3.2 heads had just two (instead of 3) coolant expansion plugs beneath the lifter housing. These heads have two locations that are subject to cracks.
—I have rarely see a head that has more than one crack, but over time among the two & three plug heads I have observed 5 specific crack locations Each crack has it’s own unique fault line and a crack in any given position will follow pretty much the same path from head to head.
—Some radiate from one of the coolant expansion plugs over toward the nearest spark plug well.
-Others migrate from a valve guide bore across the machined spring well and end at the nearest spark plug well.
–Most cracks occur on the outer cylinder positions, but on occasion the center cylinder produce a crack that runs from a spark plug well across an exhaust spring well and end in a valve guide bore.
—Occasionally the plug well to spring well crack will continue from the spring well on to the nearest head bolt bore. These are the worst examples, but we can repair them too.
–Most of the cracks that gravitate toward a valve guide bore head toward an exhaust guide. However, the 3.4 heads that have just 2 coolant expansion plugs beneath the lifter housing will sometimes crack from a spark plug well to an intake guide bore. I have not yet seen this on a 3 expansion plug head.
With over 25 years of cast aluminum crack repair to draw upon I have developed routine repairs for every type of crack that we see with these heads. The type that run into a guide bore are much more involved than the type that don’t and are therefore more expensive to repair.
Crack repair requires a very specific series of processes to end up with a reliable repair. And the cracks that run through the valve guide area are the most demanding. Even shops that say they can repair cracks will often shy away from these challenging repairs, or worse do a poor “repair” that will get through the warranty period but fail down the road.
Our repairs aren’t just a patch, they actually strengthen the casting. Every crack that we have seen with these heads has been to one of several specific areas I have identified as being skimpy in aluminum. When excavating the crack for welding we have a specific pattern that we cut in one of our mills to expose the entire weak area, not just the crack. As a result the finished repair has more material in the cracked area than it did previously.
This is done using advanced welding techniques that involve the use of our custom built casting oven. Pre & post heating temp and cycle times are very precisely controlled. Duration of the welding process is closely monitored to ensure the casting is not overheated. We adhere to minimum weld cycle times divided by a specific period of time in the oven between weld cycles. This technique prevents the welding operation from introducing new stresses to the casting. When the weld work is finished and the post-cool cycle is complete the casting stresses and weakened areas will be long gone. What’s left is a much stronger casting.
Next up a pressure test. Once we have confirmed the integrity of the weld the necessary machine operations are performed, after which the head is pressure tested once more for added confidence in the finished repair. We have yet to have a failure with a single repair. Many have been put t o use on high performanc ena race en Some have been put to use on high-performance rebuilt engines and have many hard miles and in some cases laps on them.
Some before and after pics.
Hoffman Automotive Machine (706)769-3783 
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