I installed an MFactory LSD, a Synchrotech carbon synchro kit as well as various OEM bearings and any items that were not included in the kit in my 93 Si hatch with a D16Z6. I referenced the service manual and several videos. The installation went smooth as well as the first test drive where I went through all the gears keeping it under 50mph. However, on the highway it pops out of 5th between 55 and 65mph. The transmission was not making any noises or grinding and shifted smoothly. I drained the fluid and checked to make sure the snap ring was on the counter shaft bearing which it was. I pulled the transmission again and opened it up and there is no sign of an issue. The car has new Hasport motor mounts, energy suspension bushings on the shift rods and a K-tuned shifter. I’ve had no prior issues with the transmission which has less than 150k miles.
The first thought was that 5th gear may be bad so I am attaching some pictures. To me it looks fine, but maybe there is an expert out there that can verify. At this point I am stumped, the only other thing I can think of is to take it to a tranny shop and have them look for any issue. If anyone knows of anything else I should be checking, I would appreciate their input.