This problem of intake valve seats dropping has struck quite a few owners over the past few years. We first became aware of it with track cars back around 2014. Shortly after, we started seeing seats actually fall out in our ultrasonic parts washer (heated to just 165*) and our casting oven (heated to 210*) during valve guide installs. Lucky for all involved it happened in the shop and not on the road or track. As you can imagine the results can be catastrophic to the engine and the wallet.
As a result of these issues, three years ago we began compulsory replacement of all 3.4l 987 (not to be confused with the 996 3.4) and 3.6l 996/997 intake valve seats when we service these heads. In June of 2018 we expanded that to include exhaust seats after seeing several failures. We will not rebuild these heads without total replacement of all valve guides and valve seats.
Here’s a picture of a 3.6 head that dropped an exhaust seat.
It’s worth pointing out that the mechanic working on the car made a bad situation worse by pulling a spark plug to diagnose what caused the engine to lock up. The sparkplug was damaged in the failure and when the mechanic pulled it out it mucked up the plug threads. We recommend not pulling the plugs to scope a cylinder on any locked up engine or engine that has clearly suffered a major failure. We can mill out the offending plug in the shop. These heads do not have a lot of meat between the sparkplug and the intake seats for an insert so we always attempt to save the factory threads.
We recently had several intake seats drop in the shop on a pair of 3.4 987 heads. We always take measurements when this happened and this time around we decided to snap some pics and share the results. Here’s what we found: Interference fits of a mere .003″. And the thing is, that .003″ was on the tight side of what we usually see. In the past we have measured interference fits as small as .0015″, making those heads ticking time-bombs.
We’re pretty sure these heads did not leave the factory with seats this loose. They wouldn’t have made it this long. It’s my opinion that these seats are loosening over time as a result of a perfect storm of valve-train conditions that leads to major valve bounce on the seats. This bounce issue is compounded by the severe valve train operation of the aggressive intake lobe profile of the VarioCamPlus system which, by itself, is quite capable of inducing valve bounce. The compounding comes from the aggressive valve action inducing strong wave actions in the dual non-interference parallel valve springs. This wave action transfers through the retainer into the valve leading to lateral and spiralling valve motion during valve events, contributing to the severe intake guide wear so common with these heads. This guide wear also promotes valve bounce as the valve lands out of square with the seat and settles spirally. This has a wallowing effect on the aluminum counterbore that the seat is pressed into and loosens it over time.
Valve bounce is a serious problem, but accompanied with worn valve guides it can be quite destructive and is normally very hard on valve seats, leading to valve seat wear in older engines. In the case of modern powdered metal valve seats, as in these heads, wear is not an issue. These seats flat do not wear. They may break, like the seat in the above picture, but they do not wear. All the shock of harsh landings related to the aggressive cam profile and valve bounce and spiral settling is transferred through the extremely hard valve seats into the aluminum. This shock relaxes the bore, leading to the loss of interference fit. The rest of the story is told in failed parts.
We machine new seat counterbores for a more shock proof seat with excellent wear qualities. Here’s a pic of a finished counterbore.
We have never seen this issue of dropped valve seats with any other model of M96/97 cylinderheads, though we also insist on replacing the intake seats in 3.8l heads due to their similarity to the 3.6l heads.