Early 4.2 Filter Head Drain

The early 4.2 filter head, Tecalemit type FA2119, was used in all S1 4.2 production and about half of S2 production. You might think it barely merits mention, but there is a design peculiarity which has gone without comment to this point. It regards the drain plug. The plug is installed on the bottom of the casting . Removing it allows the oil trapped in the canister to dribble down the side of the oil pan. A customer stripped the drain plug thread in the casting, and asked whether a new, larger thread could be cut in the outer end of the casting. The answer was no, here's why.

The "plug" is actually a 3/8"-24 bolt, 1.25" long. Here's the tricky part: the bore is only threaded at the deep end, and only for about 1/4". The near end of the bore is actually 7/16", larger than the bolt, and it wouldn't seal without the copper washer. Let's look at this more closely.

Oil enters the filter around the outside, passes through the filter and exits through the center. In order to drain the entire filter, the drain bolt passes through both the inlet and outlet areas, but it is only threaded into the 1/4" between the two. The lower portion of the bore is actually wider than the bolt, and it has a "window" into the outlet area to allow it to drain. So if you were to replace the stock bolt with a larger bolt that only threads into the lower part of the bore, inlet and outlet would be directly connected through the threaded section. At the best of times, the only thing separating dirty inlet oil from clean outlet oil is a few turns of screw thread. Without that, the flows would intermingle.  So if you strip the thread as my customer did, you will need to find a long tap and repair the threads at the far end of bore. So take care not to strip the threads on your filter head.  If it was easy, it wouldn't be a Jaguar.

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