Washers

A hardened washer gives a nut or bolt head a flat, hard and consistent surface to turn on. That matters in an engine joint, because the friction under the nut decides how much of the torque becomes clamp load. ARP washers come in inch and metric sizes, chamfered or plain, in black-oxide chrome moly, stainless steel or cad-plated steel.

Turned straight onto an aluminium casting, a nut can dig in and gall, so the friction varies from one fastener to the next and the clamp load varies with it. The soft surface also settles under load, a process called embedment, and the joint loses preload as it does. A hardened washer spreads the load over a wider area, keeps the friction even and protects the casting.

Washers are listed by size, type and finish:

• Inside diameter: matched to the thread, such as 1/4, 5/16, 7/16 or 1/2 in, or M6, M10 and M12.
• Outside diameter: from compact sizes such as .550 in to wide ones such as 1.30 or 1.55 in. A wider washer spreads the load further, but it must sit flat within any counterbore or spot face.
• Thickness: for example .063, .120 or .250 in, or 3.0 mm. A thicker washer resists dishing under a high clamp load.
• Chamfer: a chamfer on the inside edge clears the radius under a bolt head. Washers listed with no chamfer have a square inner edge.
• Insert washers: a short spigot, listed by its outside diameter and depth, locates in the hole and keeps the washer centred.
• Finish and packs: black oxide on 8740 chrome moly, stainless steel or cad plating. Singles, pairs and packs of 10 are listed, and some are sold as con rod washers.

Fit chamfered washers with the chamfer towards the bolt head or nut. Turned the other way under a bolt, the washer’s sharp inner edge bears on the radius beneath the head, and the bolt cannot seat properly. Lubricate the face the nut turns on, as the torque figure assumes, and keep the side against the casting clean and dry so the washer stays put.

Check that the spot face is flat and free of paint and casting flash before fitting. Use one washer per fastener unless the instructions say otherwise, and replace any washer that is dished, scored or galled.