|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. Visual inspection is not adequate for detecting leaking cylinder head cracks, as some are no larger than a pin head at the surface, as was the case with the head in the video. We had no idea where the leak was till we hit it with 60psi air pressure. Yet when we milled away the tiny leak point, we found a fair size crack beneath the surface. It is very common for small surface cracks to be atop a much larger crack, especially in the floor of the oil galley, where the leak in the video happened to be.
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 and intensive pneumatic peening of the weld is completed the casting stresses in that area 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 work it’s CNC machined with proprietary custom carbide tooling, then pressure tested again for added confidence in the finished repair. We have yet to have a single failure of a repaired head, and we know that some have been stressed hard on high-performance and even competition engines.
It’s worth noting that as of 11-23-16 we have repaired 3 heads that went into service and later cracked in a different location. At least one of them was a dedicated track/race car. That’s 3 out of nearly 125 heads, so the odds are pretty favorable. In all three cases the new failure point was repaired and the heads put back in service. We have no practical way to predict these future failures and therefore can not guarantee that the repaired casting will not crack elsewhere at a later time. We can only guarantee our work, and that the head passed multiple pressure tests before leaving our shop.
The large crack in the photos above is the most common crack we see with these heads. A lot of people think it’s a death sentence. And for good reason. I liken this repair to open heart surgery. It requires very specific skills, tools, knowledge and techique, in this case a major excavation of material and extensive welding and cold peening followed by a series of complex, precision machine-operations. The overwhelming majority of shops out there are not equipped to perform this surgery, but it’s routine for us. Our patients go on to live full throttle lives.