Walnut shells...
As I recall, the walnut shells were used to clean the gearbox internal surfaces and oil ways and blasting out any residual shell particles using high pressure air/nitrogen had worked a treat for many years.
That is until Health and Safety got involved and called into question the safety case of using high pressure blasting as a maintenance procedure, so the gas pressure used to purge the casing and oil ways was reduced, the outcome of which was that residual shell fragments remained lodged in oil ways and this was not picked up prior to MGB rebuild.
Loss of torque...
I have lived several lifetimes since my Chinook days' at Odipus, but as I recall, at the time Boeing issued an alert bulletin to check the FWD MGB Spiral Bevel Ring Gear retaining bolts for loss of torque loading every 25 flights hours.
As noted, it was a long time ago, but I believe the engineering concern at the time was the minimum safe edge distance from the centre of the attachment bolt holes to the edge of the ring gear, as such it was not considered safe to open up the holes and install larger diameter bolts.
A pain in the butt as it was, Crab Air was happy to wait for Boeing to identify a fix that that they were happy to have installed on MoD gearboxes, but not so for the NS Commercial World. As such an engineering solution was expedited between Boeing, the FAA and UK CAA that resulted in tapered bolts being installed that could accept higher torque loadings, but clearly the modification process produced its own problems.