Why don't you want to use a flat washer under the head of your NAS1351 bolt?
There are good reasons for using flat washers under the head of a bolt. The washer helps distribute the bolt flange contact load on the mating part surface, and this can be an issue with the limited flange area of a socket head bolt like NAS1351. A washer with an ID chamfer will prevent edge loading at the bolt head/shank fillet. If the bolt is torqued at installation, the washer will prevent damage to the part surface material around the hole and its corrosion protection such as primer, plating or anodize.
I always use washers, but I have a case where I’m boxed in with a certain bolt pattern I can’t change and positioning that is putting the hole right next to a boss. There is no room for a washer. I know I would have to worry about galling, but this is a one time installation. Also I know I would have to worry about hole size and contact area of the head of the bolt and someone is doing the FEM analysis on it. Part its bolting is A286, so no surface coating. Thanks
IF galling between similar alloys [IE: CRES-on-CRES, CRES-on HRA] is an issue, may install the cap-bolts 'lightly wet' with lubricating oil or antiseize compound [threads and underside of head], with/without washers.
NOTE.
Lubricating oil must be compatible with all materials and rated for the expected heat exposure. IF the mating component contains hydraulic oil or lubricating oil etc, then use that particular oil as lubricant.
NOTE.
IF installed with antiseize compound, make sure the compound is rated for the operating temperature of the component.
NOTE.
May need to adjust installation torque value if/as requird (torque values adjusted for 'torque-turn bolt head with lubricant' VS typical values/charts for 'torque-turn NUT dry [no Lube]').
Regards, Wil Taylor
Trust Me! I'm an engineer!
Trust - But Verify!
We believe to be true what we prefer to be true.
For those who believe, no proof is required; for those who cannot believe, no proof is possible.
Unfortunately, in science what You 'believe' is irrelevant û "Orion"
Muessel... NAS620 washers [anti-galling CRES] would be a good-call as noted by 49again.
I sometimes get 'creative' [=desperate] with standard hardware and allow edges of 'simple parts' [such as washers] to be altered by machining or filing [flattened/chamfered/radiused/etc] minimum required to attain clearance in spotfaces, adjacent-to radii, adjacent-to-each-other, etc. IF this is the case, then suggest You callout a high strength CRES washer, such as an NAS1149Exxxx* and specify how to 'trim it' for clearance [assuming this is even possible] [no re-finish required].
NOTE.
Even the NAS620 [anti-galling CRES] could be altered slightly [chamfer/radius edge, etc] if needed.
Regards, Wil Taylor
Trust Me! I'm an engineer!
Trust - But Verify!
We believe to be true what we prefer to be true.
For those who believe, no proof is required; for those who cannot believe, no proof is possible.
Unfortunately, in science what You 'believe' is irrelevant û "Orion"
Here is an aircraft quality high-strength A286 structural washer designed specifically for use with socket head bolts like NAS1351. The washer OD is the same as the NAS1351 bolt head diameter, so it should work for your application. It is also available with an ID c'sink to prevent interference with the larger head fillets used on high strength tension bolts.
