Don't use lock washers of the split type! These are the ring shaped washers which are split and bent into a spiral.
Before some one sets fire to my shorts, please consider that automotive manufacturers stopped using them for normal production over 20 years ago because they found them to be unreliable. These washers function by placing a tension load onto the fastener which intends to keep the fastener from loosening by keeping the threads loaded. They also are intended to dig into the head of the fastener and the work surface.
Problems, most grade 5 (SAE bolts) and 8.8 (metric) are under a tensile load which far exceeds the tensile loading provided by the split washer. This means that the washer makes no contribution to loading the fastener.
In addition, Grade 5 and 8.8 bolts are too hard for the split type washer to penetrate sufficiently to get a good grip. This also makes these washers unnecessary.
Another problem is that the tensile loading of the fasteners (Grade 5 and above and 8.8 and above) is often sufficient to break the washer which then can work out of place removing all load from the fastener. This is not good!
The finisher is that these washers dig a piece out of the soft aluminum every time they are used.
Use a good quality flat washer. Put the flat washer onto the bolt or cap screw in the correct direction (put the rounded edge side of the washer toward the head) so that the washer does not dig into the fillet under the bolt head and possibly initiate a head failure.
Tighten the fastener correctly and use a locking device such as safety wire, cotter pin, lock nut or Loctite if desired.
Forget lock washers unless there is some unusual requirement.
While on the subject.....make sure that you are using a hardened flat washer rather than a mild steel one as is the case with typical hardware store washers which are intended for use with Grade 2 or below SAE fasteners (or metric equivalent). If you use a soft washer and then tension (tighten) the fastener to specs all will seem well but over time the soft washer will give under the pressure and will press thinner. This will remove the tension from the fastener and so the clamping force will be lost. With the loss of tension and clamping force goes head gaskets, breakage of bolts under reciprocating loads and so on. Bad idea!
Another one commonly not understood is the issue of locking nuts. A locking nut which has a deformed end (one end is pressed out of round) may be rated for use with high strength fasteners (SAE Grade 8 or metric12.8 and above).
A nylock nut with a rod of nylon (or other plastic sorts stuff) which protrudes through a radial hole drilled into the threaded area of the nut) may be also suitable for high strength fasteners. A nylock nut with a ring (washer) of nylon secured in one end of the nut is only suitable for low strength bolts (cap screws) because a hardened nut (for high strength) cannot be crimped over the washer. Easy to tell!