PART NO.  SPL.PAGE  DESCRIPTION                                                           
17H7734   16.K.3    Caliper -R/H -(front brakes, 1600 and 1600-MK-II Std.model)
17H7735   16.K.3    Caliper -L/H -(front brakes, 1600 and 1600-MK-II Std.model)

17H7873   16.K.3    Bolt -bridge -(caliper, front brakes, 1600 and 1600-MK-II std.model)

This is the 17H7873 bolt (2 required for each caliper) used to hold together two halves of the Lockheed disc brake caliper. The Workshop Manual says do not split the caliper for service. But it also talks about what to do if it does have to be split, and replacing these bolts is part of putting it back together.

Initially it looks like a 3/8-20-UNF x 2-3/4 Hex Head Bolt with 3/4" hex. But it has a special drawing number, for a number of reasons. The most obvious is the thread length, which is longer than normal for a standard bolt. Thread length measures 1-9/16" minimum, but the original parts seem to vary up to 1-3/4" (where standard thread length is only 1-1/4").

Most of these pictures of the bolts in black oxide finish are for OEM New Old Stock parts. They appear to be "billet" parts machined from hex bar stock, possibly automatic screw machine parts, machined all over including top and bottom surfaces of the head, and separated from the bar stock with a lathe cut-off tool. This is commonly done when the specification requires high quality steel and/or high precision finish. The point is, this is not a common hot-forged bolt, and not rolled threads. Think high strength and precision. If you were looking to buy such bolts from common stock, look for at least grade-8 bolts, and do get the thread length right. For instance, McMaster-Carr does not list a bolt with this sprcial thread length. You might have to buy a standard bolt and re-cut the thread forgreater length.

Also crank them up to the proper torque so they cannot stretch or wiggle in operation. SAE torque spec for this bolt would be 90 lb-ft wet, or 120 lb-ft dry. Get out the big boys' tools. This is probably why they have extra length thread, for low-stress grip in soft cast iron with high pull tension.

Additionally there is a dab of Glyptal type sealant/adhesive on the threads. Lockheed must have been serious about being sure these bolts could not work loose in service, as they also used a locktab on each bolt, and always advise not to disassemble the caliper unless it must be necessary. They are also specified as single use bolts, but that may only be due to the adhesive on the threads. If you do ever R&R these bolts, you should use high temperature thread lock adhesive (400-dF) with reassembly.

Photo above right shows the variable thread length on the original factory supplied bolts. Below left shows the machined flat head with no markings. Photo below right shows the thickness of the caliper half with the clearance through hole being 1-5/16". This would be maximum length for the plain unthreaded shank below the hex head.