An 8-inch lug-style butterfly valve installed between ASME B16.5 Class 150 raised-face flanges is bolted with 3/4-10 UNC fasteners, 8 tapped holes per flange face, 16 cap screws per joint, and a 2-1/4 inch minimum cap-screw length with A193 B7 as the default stud/bolt material grade [S1][S4].
Stud length on the same 8-inch line shifts to roughly 6-1/4 to 6-3/4 inch when a wafer body is substituted, because the through-bolts span the combined flange and body face instead of threading into a lug [S1][S2]. For the lug variant specifically, the bolt sits inside a tapped hole, so its length is the engagement depth of the lug plus the companion flange thickness, not the full face-to-face dimension of the joint.
Why the lug pattern doubles the fastener count versus wafer
A lug body has threaded holes machined through each side of the valve, so each flange must be bolted independently to the lug instead of being clamped through the valve body with one long stud [S1][S3]. For an 8-inch Class 150 flange, ASME B16.5 specifies 8 bolt holes on a 11.75 inch bolt circle at 0.875 inch hole diameter, and the lug body replicates that pattern on both faces, yielding 16 fasteners per joint, not 8 [S1][S4]. A wafer body on the same 8-inch line uses 8 through-bolts because the bolt passes once through both flanges and the wafer body without engaging a thread inside the valve [S5][S9].
Cap-screw length for the 8-inch lug at Class 150 is 2-1/4 inch across every published chart reviewed, while the wafer counterpart on the same 8-inch line ranges from 6 inch to 6-3/4 inch depending on whether a heavy-hex machine bolt or a stud is used [S1][S2][S5]. That gap, roughly 4 inches, is the most common spec error in the field: a maintenance crew pulls an 8-inch wafer stud off the shelf and tries to use it on a lug valve, leaving 3 to 4 inches of shank protruding past the nut and no thread engagement at all.
ASME B16.5 Class 150 dimensions that drive the 8-inch callout
Per ASME B16.5, an 8-inch Class 150 raised-face flange carries 8 bolt holes on a 11.75 inch bolt circle with 0.875 inch through-hole diameter, and the typical flange thickness for a Class 150 weld neck is 1.125 inch, with a 1.06 inch raised face height [S2][S4]. Lug-body valve face-to-face on the 8-inch line is normally 2.5 inch for short-series lugs and 3.0 inch for long-series, which is what fixes the 2-1/4 inch cap-screw length: the screw must clear the 1.06 inch raised face plus a 1.0 to 1.2 inch tap depth in the lug and still leave 1 to 3 threads exposed past the nut [S1][S3].
Bolt thread on the 8-inch Class 150 lug is 3/4-10 UNC ( Unified Coarse, Class 2A external / 2B internal per ASME B1.1), and the hex nut is heavy hex per ASME B18.2.2; the matched heavy hex head of the cap screw is per ASME B18.2.1 [S2][S5]. Going finer, 3/4-16 UNF, is not permitted by ASME B16.5 hole geometry and will not pass the 0.875 inch clearance hole; this is the second most common field error after length mismatch [S2].
Material grade and where to deviate from A193 B7
A193 B7 (chromium-molybdenum alloy steel, quenched and tempered, 125 ksi minimum tensile) is the default stud/bolt grade for 8-inch lug service at Class 150 and is the grade that every chart reviewed in this guide specifies unless the user overrides it [S2][S4][S5]. For low-temperature service below -46 C, A320 L7 is the standard sub-zero substitute and is the more common cold-service swap on 8-inch chemical and LNG lines [S3].
For sour service, A193 B7M (the restricted-hardness variant of B7, 235 HB maximum) is required, per NACE MR0175, when H2S partial pressure exceeds 0.0003 MPa and the pH is below 4.5, a frequent condition on refinery hydrocracker and amine-unit 8-inch lug valve retrofits. Stainless A193 B8 Class 1 (304) or B8M Class 1 (316) is occasionally called out on 8-inch water-treatment lug valves where external atmospheric corrosion dominates the failure mode, but neither stainless grade is approved for sour service without the more restrictive B8M Class 2 cold-worked condition, which is rarely stocked for 3/4 inch [S3].
Washer, nut, and torque considerations specific to 8-inch lugs
Milwaukee Valve's bolt-length tables explicitly exclude any washer thickness; if F436 hardened washers are used, the stud or cap-screw length must be increased by the washer thickness, typically 0.09 to 0.13 inch per F436, and 0.25 inch for F436 extra-thick washers on 3/4 inch nominal [S2]. On the 8-inch lug line, that adds 0.09 to 0.25 inch to the published 2-1/4 inch length, and rounded up to the next 1/4 inch, this gives 2-1/2 inch. Lubricant choice on the threads moves required torque by 20 to 40 percent versus dry, so published Class 150 torque values for 3/4 inch B7 cap screws, roughly 200 to 300 ft-lb depending on gasket, only apply with a documented bolt-nut friction coefficient between 0.10 and 0.15 [S3].
Cross-tightening pattern on a 16-bolt lug joint must follow a four-pass star sequence (4 bolts at 30 percent, then 70 percent, then 100 percent, then a final 100 percent circular pass), not a side-to-side pattern; an 8-hole flange only needs three passes, but the 16-bolt lug joint on the 8-inch line is the smallest line that requires the extra pass [S1][S3]. Cap screws on lug bodies must also be short enough that opposing screws do not contact each other inside the lug pocket, a failure mode that cracks the lug body when the valve is over-torqued [S5].
Lug versus wafer: a side-by-side callout for the 8-inch line
The 8-inch lug and the 8-inch wafer share the same flange (ASME B16.5 Class 150, 8 holes, 3/4-10 UNC, 11.75 inch bolt circle) but diverge on fastener count, length, and engagement [S1][S4][S5]. Lug valves, fixed independently on each side, take 16 cap screws at 2-1/4 inch each, screwed into the lug tap; wafer valves use 8 through-bolts or studs at 6 to 6-3/4 inch each, depending on whether a heavy-hex machine bolt or a stud bolt with two heavy hex nuts is selected.
The key trade-off: a lug valve can be used as a dead-end service valve (one flange unbolted under full pressure), while a wafer cannot, so the extra 8 fasteners on the lug are not a redundancy choice, they are structural, since each cap screw carries the full line pressure on one flange face [S3][S9]. For more on how lug geometry interacts with seat materials in butterfly valve selection, the lug body normally pairs with a replaceable seat whereas wafer bodies more often use a molded-in liner; this drives the 1.0 to 1.2 inch tap depth and indirectly the 2-1/4 inch cap-screw length on the 8-inch line. Engineers who spec stud-and-two-nut arrangements on 8-inch lugs should note that 3/4 inch studs for tapped lugs are non-standard; the lug thread is designed for cap-screw engagement, not through-bolting [S1][S5].
Common 8-inch lug field failures and the spec fixes
Three failure modes recur on 8-inch lug installs. First, mismatched length, with 6-inch wafer studs installed on lug valves, leading to zero thread engagement and an immediate leak on pressurization; the fix is a 2-1/4 to 2-1/2 inch cap screw per published charts [S1][S2][S5]. Second, B7 substituted for B7M in sour service, leading to sulfide stress cracking within weeks; the fix is a hard check of the process H2S partial pressure against the 0.0003 MPa NACE MR0175 threshold, not just a visual on the bolt head stamp [S3].
Third, mixed threads, with a 3/4-10 UNC cap screw forced into a metric M20 lug thread, a common error on European-built valves installed in North American plants. ASME B16.5 and EN 1092-1 bolt patterns for DN200 (8-inch) are not interchangeable: EN 1092-1 PN16 uses M20 studs on an 8-hole pattern with a different bolt circle (200 mm) and the 8-inch ASME Class 150 bolt circle is 11.75 inch (298.5 mm); a Class 150 lug body will not bolt up to a PN16 flange without a transition ring [S3][S4]. For related fluid-system bolting decisions in adjacent equipment, the chemical processing solenoid valve selection reference covers the seal and body-side fastener trade-offs that follow once a downstream solenoid is added to the same line.
Track the next update by checking whether your 8-inch lug supplier has published a current revision of the bolt chart (Milwaukee Valve's IBFV tables and Southwest Valve's ZSPEC guides are both revised periodically), and confirm whether your plant's 8-inch lugs have been re-classified under any new ASME B16.5 errata in the 2025-2026 revision window. A 1-2 inch physical measurement of the lug tap depth on a sample valve, plus a thread gauge check of 3/4-10 UNC, is the fastest on-site verification.
The underlying component specifications are covered under stud welder, and construction machinery and equipment.