Here’s the casting in question on all of the 87-88 13bt engines (as well as the nonturbos).

The 89-91 castings are supposed to be larger and/or reinforced, however this is NOT always the case. IN fact, I’ve taken apart dozens of US spec and jspec 89-91 13bt blocks and only about half of them have the reinforcements pictured below. There seems to be no real pattern to which engines did and did not get the reinforced back iron.

Here’s the 89-91 casting that’s most common. Note the triangular chunk that serves as a reinforcement between the dowelpin casting and the oil filter pedestal, while the casting itself is the same size.

Here’s another variation of the 89-91 that I’ve found quite rarely. Again I cannot find any pattern as to what engines were built with this casting. Note that it not only has the triangular reinforcement, but the casting itself is almost twice as thick as the previous one. This is the same casting size that the FD plate uses, however the FD plate cannot be used in a 13bt.

Here’s what a broken 87-8 casting looks like. Again, this usually occurs as a result of overboost/running lean/pinging.

The dowelpin usually stands perpendicular to the plate…

And this illustrates how much play is now in the dowelpin area…

Usually, from what I’ve seen, the engine will twist enough that the rotor tips will actually slap the rotorhousing surface. This can harm apex seals, but if the engine’s been recently rebuilt then the (hopefully) upgraded seals that were used may be in good condition, as is the case with most of these failures I see. Usualy the seals will bind somewhat in their grooves because the soft rotor material actually distorts inward on the seal a bit…this is easily repaired with a file most of the time, but severe slap could cause one or both rotors or rotorhousings to be damaged beyond reuse.

In these pics the shiny metal area shows you where the rotor rubbed the rotorhousing several times, removing the brownish heat discoloration and carbon normally seen on rotor faces.