The impact force required to buck a monel rivet is higher than the force required for an aluminum rivet – the washer may be used for backup to prevent damaging or dimpling the 2024 aluminum sub-structure on impact.

Thank you mrbliaison for your answer. If this practice is OK, do you think long term secondary effects might occured (corrosion, etc.) because of the two dissimilar materials ?

There shouldn’t be a problem between the stainless steel shroud and the aluminum sub-structure since the aluminum is probably anodized and primed.  The rivet, however, should still be installed wet with a high-temperature sealant since the aluminum and monel rivet are quite dissimilar.

Thank you mrbliaison for your time and your help.

mikeone ..
Being well acquinted with the component you describe ..
The Backup washer is standard practice because on re-assembly the original hole is usually oversized and the washer not only provides "tighter hole" but also assists the technician from "dumping" the rivet. The washer provides additional "clamp-up". For those of you who wonder why the next size fasteners is not used - edge distance is usually a problem (and the fastener usually swells up sufficiently to take up the gap)- and since these components get rebuilt on  a regular basis - fastener upsize is not recommended unless absolutely necessary.

Sealant is not critical because 1/ airtight seal is not required here. 2/ Dissimilar metal corrosion not a major factor in this area. 3/ High Temp sealant is VERY viscous and prevents satisfactory riveting.

Cracking & Heat distortion of the "barrel" are the major concerns. Sealing would not provide any additional benefit in this application (only handicap the technician in accomplishing his task)

Thank you edmeister for clarifications.