1986 Porsche 911 broken head studs and 915 transmission grinding

This customer loved taking his 1986 911 coupe to Watkins Glen International Speedway. Over time the car developed an oil leak on the passenger side of the engine. He was able to ignore it for a while but then his transmissions second gear started grinding to the point where he couldn’t drive it anymore. He found us and brought the car in for an evaluation.

We removed the valve covers and found three broken cylinder head studs. The dilivar® head studs are known for fracturing over time. Since the car had 140k miles on the original engine the customer decided it would be a good time to perform a complete rebuild.

After splitting the engine case we found the connecting rod and crankshaft main bearings in good shape for the mileage but definitely ready for replacement. We sent out all of his parts to the machine shop to have them checked and polished. This engine received a full cleaning to remove the years of grease and grime.

The valve guides in the cylinder heads on this car were so worn that you could literally grab a valve and shake it around. We have a custom phosphorus bronze valve guide that we like to install. They are a tighter fit and last much longer than the original manganese bronze guides. Then we reassemble the engine with new timing components, connecting rod and main bearings, piston rings and all seals. We pressure test and clean every oil cooler before install to prevent headaches from occurring. We also change out all of the oil cooler hoses because they like to split after reassembly due to their age.

Next we opened up the transmission to find the cause of the grinding. It turned out that at some point the clutch pressure plate had broken and was causing the clutch to never fully disengage. Each time the customer was shifting the input shaft on the 915 transmission was still engaged. The synchronizers of the transmission were only able to keep up with the spinning shaft for so long before they wore away. Next the gears themselves from grinding each other ended up filling the transmission with metal filings. Those metal filings made their way through the transmission bearings and destroyed every one inside this transmission. This transmission required a complete rebuild and two new gear sets. After the rebuild we filled it with Swepco 201 transmission oil. We have found that this makes for the smoothest shifts in these older 915 transmissions.

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