Here is the 1982 Motobecane Parts Price List, provided by the Motobecane Club de France. It has specifications for most hardware items.
Below are the nuts and bolts that appear in the 1974-81 US model 40 and 50 parts manuals. They are listed in order of increasing size. This helps with identification and error checking.
sect part# price Nuts .
00 01556 $0.00 nut M3-0.5 x 3.0 hex6 (D3/50x6x3)
31 01430 $0.60 nut M4-0.7 x 3.0 hex7
12 13339 $1.00 nut M5-0.8 x 3.5 round knurled
15 01485 $0.40 nut M5-0.8 x 4.0 hex8
10 00878 $3.00 nut M5-0.8 x 12 hex8
32 01511 $1.50 nut M5-0.8 x ?? flange-lock
15 01436 $0.55 nut M5-0.8 x 6.0 tall nylock
34 00079 $1.00 nut M5-0.9 x 4.0 hex8
35 00679a $2-1 nut M6-1.0 x 4.0 hex 10
02 00010 $0.30 nut M6-1.0 x 4.5 hex10 (5.0)
05 01176 $0.90 nut M6-1.0 x 5.5 nylock
02 01526 $1.20 nut M6-1.0 x 6.0 flange
00 00378 $0.00 nut M7-1.0 x 3.0 hex14
02 00011 $0.50 nut M7-1.0 x 6.0 hex10
37 00198 $2.50 nut M8-1.0 x 4.5 hex13
20 14147 $2-$1 nut M8-1.0 x 6.5 rounded? pulley
02 17523 $2.50 nut M8-1.0 x 7 nylock motor mount
00 13258 $0.80 nut M8-1.25×6.5 hex13
29 16555 $3.50 nut M10-1.0 nylock swingarm
00 15179 $0.00 nut M10-?? x ? rear wheel
00 13217 $0.00 nut M38-1.5 x 20
sect part# price Bolts .
44 20674 $0.80 bolt only M5-0.8 x 8 hex8 horn
34b 14267 $1.50 bolt only M5-0.9 x 10 hex8
42 16714 $1.20 bolt only M6-1.0 x 9 hex10 pointed
35b 00046 $1.00 bolt+nut M6-1.0 x 12 hex10
34 00679 $1.00 bolt+nut M6-1.0 x 15 hex10 (16)
23 00968 $1.00 bolt only M6-1.0 x 20 hex10
?? 00687 $0.00 bolt only M6-1.0 x 27 hex10 from price list
02 00050 $0.80 bolt only M6-1.0 x 33 hex10 less threads
01 14936 $1.00 bolt only M7-1.0 x 33 hex10
01 00366b$1.50 bolt only M7-1.0 x 40 hex10
01 15276 $1.50 bolt+nut M7-1.25×45 hex10 flange-head
01 00366 $1.40 bolt only M7-1.0 x 48 hex10
09 01189 $1.90 bolt+nut M7-1.0 x 53 hex10 exhaust
37 15874 $3.00 bolt+nut M8-1.0 x 37 hex13
37 20369 $2.00 bolt+nut M8-1.0 x 45 hex13
37 17023 $0.00 bolt+nut M8-1.0 x 39 hex13
39 15875 $1.70 bolt+nut M8-1.0 x 57 hex17
02 19886 $6.00 bolt only M8-1.0×140 engine mount
02 20453 none axle only M8-1.0×160 for up-exhaust
39 22710 $0.00 bolt only M8-1.25x? Traveler seat clamp
29 15872 $10-7 bolt+nut M10-1×135 swing arm
sect part# price Carriage Bolts .
23 15629 $2.50 bolt+nut M5-0.9 x 19 hex8
05 23018 $1.50 bolt only M6-1.0 x 15
28 14991 $2.00 bolt+nut M8-1.0 x 45 hex13
28 14991 none bolt+nut M8-1.0 x 45 hex13
sect part# price Fender brace Bolts .
34 21167 $7-5 brace bolt M6 x 11 (20 total)
34 21167l $8-4 brace bolt M6 x 14 (23 total)
34 20100 $7-4 brace bolt M6 x 17 (26 total)
sect part# price Screws .
29 21126 none screw only M3? for tank plate
29 21093 $2.00 screw only M3-0.5 x 11 for emblem
42 20550 $1.00 screw+nut M4-0.7 x 10? phil
29 20060 none screw only M4-0.75×10? slot fork cover
31 15356 $3.50 screw only M4-0.75×14 sunk slot
00 13245 $0.00 screw only M5-0.8 x 12
29 17424 $7.00 screw only M5-0.9 x 12 wide slot
19 17425 $6.00 screw only M5-0.9 x 10 wide slot
36 20105 $5-3 screw only M6-1.0 x 11 phil-wide
42 22588 $6-4 screw only M6-1.0 x 15 phil-wide
00 14311? $1.00 screw only M6-1.0 x 19 phil-wide
00 00000 $1.00 screw only M6-1.0 x 23 phil-wide
42 53435 $1.00 screw only M6-1.0 x 25 phil-wide
sect part# price Washers .
03 14987 $4.00 washer flat ∅4? for decomp
31 01088 $0.30 washer flat ∅4
34b 00052 $0.20 washer flat ∅5-10
23 00874 $0.40 washer lock ∅5 spring
34b 01171 $0.50 washer lock ∅5 inner teeth
07 01545 $0.70 washer flat ∅6-14 x 1
34 00020 $0.20 washer flat ∅6-12
35 00797 $0.50 washer lock ∅6-11 outer teeth
02 01172 $0.50 washer lock ∅6-14 inner teeth
23 00004 $1.00 washer flat ∅6 thin
07 01525 $0.30 washer ??? ∅6
07 01113 $0.70 washer flat ∅7-20 x 1
02 16541 $0.20 washer flat ∅8
29 01422 $1.00 washer ??? ∅8
28 18738 $1.00 washer lock ∅8-?? inner teeth
02 01527 $0.50 washer lock ∅7-16 inner teeth
18 14750 none washer shim ∅15-?? x ??
18 14931 $5.00 washer shim ∅15-21 x 0.1 mm
18 14932 $5.00 washer shim ∅15-21 x 0.25
18 14933 $5.00 washer shim ∅15-21 x 0.45
18 14562 $6.00 washer shim ∅15-21.7 x 0.5 use 14562
18 14562 $7.00 washer shim ∅15-21.7 x 0.8
18 14588 $7.00 washer shim ∅15.5-31 x 0.85
21 15998 $7.00 washer shim ∅16-26 x 1.0
21 14863 none washer shim ∅16-30 x 0.1
21 14865 $7.00 washer shim ∅16-30 x 0.3
21 15001 $7.00 washer shim ∅16-30 x 0.75
21 14153 $7.00 washer shim ∅16-27 x 2.0
06 01285 $4.00 washer shim ∅19-24 x 0.1
06 01286 $4.00 washer shim ∅19-24 x 0.2
06 01287 $4.00 washer shim ∅19-24 x 0.3
06 01288 $4.00 washer shim ∅19-24 x 0.5
07 23780 $8.00 washer alum ∅30-33 x 1
19 16542 none washer wave for start shoe
18 21946 none washer wave for start shoe
sect part# price Spacers
23 16689 none spacer 6-8 x 11.5
08 19891 $8.00 spacer 7-12 x 30
37 20690 $2.50 spacer 8-16 x 3.5
37 17024 $3.00 spacer 8-16 x 5
37 22674 $2.00 spacer 8-16 x 7.5
00 23024 $3.00 spacer 8-12 x 10 Model 7
02 21014 $3.00 step s. 8-12-17 x 10.5
02 20534 $4.00 spacer 8-12 x 18
02 20534 $4.00 spacer 8-14 x 18
02 20790 $6.00 plastic 8-11 x 23
21 20738 $8.00 spacer 16-20 x 13
sect part# price Plugs and grommets
35 16948 $0.00 plug ooo 12-07-10 x 8 40,50 R fen. F ∅7
35 20094 $0.00 plug ooo 11-07-09 x 9 for 50 R fen. R ∅7
29 10533 $0.00 plug ooo 00-00-00 x 0 for 50 frame ∅8?
35 17160 $4.00 grommet 08-10-15 x 9 for 40,50 fen. ∅10
35b00000 $2.00 grommet 09-11-17 x 7 for 40 fen side ∅11
28 20389 $2.50 grommet 05-08-12 x 5 40,50 frame ∅8?
14 04326 $0.00 grommet 02-00-00 x 0 40,50 stator