With a further increase in O.D., thickness and bearing area, the Mil-Spec Extra-Duty (Extra Heavy) helical spring spilt lock washer absorbs exceptionally high torques and retains tension under heavy loads. WCL also refers to these as Extra Heavy (EH).

We also have Extra-Duty (Extra Heavy) Helical Spring Spilt Lockwashers.

Material
C/S = Carbon Steel
304 SS = Type 304 Stainless Steel
316 SS = Type 316 Stainless Steel
420 SS = Type 420 Stainless Steel
PHO = Phosphor Bronze
KM = K-Monel

Ordering Example: MS35340-42 is a #8 Extra Duty, Carbon Steel, with a Mechanical Zinc & Yellow Dichromate finish.
#1/4 EH C/S L/W MZ1 is a 1/4 Extra Duty Carbon Steel Lockwasher with a Mechanical Zinc finish.

Finish
PLN = Plain
CD1 = Cadmium & Clear Dichromate
CD2 = Cadmium & Yellow Dichromate
CD3 = Cadmium & Black Chromate
MZ1= Mechanical Zinc & Clear Dichromate
MZ2= Mechanical Zinc & Yellow Dichromate
ZN1 = Zinc & Clear Dichromate
ZN2 = Zinc & Yellow Dichromate
BKO = Black Oxide
PHO = Phosphate
HDG = Hot Dip Galvanized
MG = Mechanical Galvanized
lw11
Nominal
Washer
Size
Inside Diameter
A
Min.
Inside Diameter
A
Max.
Outside
Diameter
B
Width
C
Thickness
D
MS 35340 Dash Numbers1
Steel Finish
MZ2 or CD2
  2    .088    .094    .208    .053    .027  39
  4    .114    .120    .253    .062    .034  40
  6    .141    .148    .314    .079    .045  41
  8    .167    .174    .375    .096    .057  42
  10    .193    .200    .434    .112    .068  43
  1/4    .252    .260    .533    .132    .084  44
  5/16    .314    .322    .619    .143    .108  45
  3/8    .377    .385    .738    .170    .123  46
  7/16    .440    .450    .836    .186    .143  47
  1/2    .502    .512    .935    .204    .162  48
  9/16    .564    .574    1.035    .223    .182  49
  5/8    .628    .641    1.151    .242    .202  50
  3/4    .753    .766    1.355    .279    .241  51
  7/8    .878    .894    1.571    .322    .285  52
  1    1.003    1.024    1.794    .366    .330  53
  1-1/8    1.129    1.153    2.013    .411    .375  54
  1-1/4    1.254    1.280    2.222    .452    .417  55
  1-3/8    1.397    1.408    2.429    .491    .458  56
  1-1/2    1.504    1.534    2.627    .526    .496  57
1 – MS51416 has only 3 dash numbers which are in steel (C/S) Zinc2 (ZN2)