Group standard TL 188

Issue 2016-05
Class. No.: 50231; 61100

Descriptors: fastener, nuts, nut, bolts, bolt, welding beads, weld spatter, damage protection, screw, internal thread,
external thread, coating adhesion, paint residue, PFA, Perfluoroalkoxy alkane, FEP, Fluorinated
ethylene propylene

Mechanical Fasteners
Protection from Weld Spatter and Paint Residue for Internal and External
Threads

Previous issues
TL 188: 2000-03, 2007-10

Changes
The following changes have been made to TL 188: 2007-10:
– Standard completely revised
– All-around FEP coating included
– Section 3.3 "Analysis of the coating material" included
– Technical responsibility updated

1 Scope
This standard specifies the requirements for all-around coating of parts with internal and external
threads, bolts made of steel, and holes, wherein this coating is applied for the purpose of prevent‐
ing faulty installation on assembly lines due to
– Residue of primer coats applied using the electro-coating method
– Weld spatter from the welding of parts with internal or external threads, bolts, and holes.

It applies only to a limited extent for use of threaded parts with special requirements, such as
– Ambient temperatures > 90 °C (due to influence on the coefficient of friction)
– Waxed components
– Components made of magnesium
– Thread dimensions M6 and M8 (due to critical pitch angle, see Volkswagen standard
VW 01129)

Always use the latest version of this standard.

This electronically generated standard is authentic and valid without signature. Page 1 of 4
The English translation is believed to be accurate. In case of discrepancies, the German version is alone authoritative and controlling.

Technical responsibility The Standards department

EKDV/3 Matthias Jesser Tel.: +49 5361 9 48810 EKDD/2 Ute Hager-Süß EKDD
EFAH/4 Harald Möbus Tel.: +49 5361 9 39663 Tel.: +49 5361 9 49035 Thomas Behrens

All rights reserved. No part of this document may be provided to third parties or reproduced without the prior consent of one of the Volkswagen Group’s Standards departments.
© Volkswagen Aktiengesellschaft VWNORM-2015-07d
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TL 188: 2016-05

– Electrical connections
– B/screws with adhesive bonding and clamping coating.
NOTE 1: With these preconditions, nuts or bolts/screws coated with coating for protection from
weld spatter and paint residue (BSSL) may be used only after testing by the appropriate depart‐
ment, because the coefficients of friction change with temperature. As a precautionary measure,
RIPP bolts may be used in combination with BSSL-coated nuts to avoid low head coefficients of
friction. Alternatively, if the results of evaluations are negative, e.g., pierce nuts and blind rivet nuts,
as well as pierce screws and blind rivet screws, may be used.

2 Designation
The all-around coating for protection from weld spatter and paint residue (BSSL) is designated as
follows:
Coated as per TL 188 – BSSL
A particular color can be specified for the all-around coating, e.g.:
Coated as per TL 188 – BSSL-red
A particular material can be specified, e.g., PFA:
Coated as per TL 188 – BSSL-PFA

3 Requirements

3.1 Basic requirements

Approval of first supply and changes as per Volkswagen standard VW 01155
Avoidance of hazardous substances as per VW 91101.
The coating operator must document the quality of the coated parts by means of statistical process
control (SPC).
All materials or parts required contractually or by order must correspond to the approved samples
in all aspects.
No changes, neither in the composition of the material, nor in the manufacture, are permissible
without prior notification and approval of the persons responsible for this standard. Only compo‐
nents that are true to gage size will be used as test nuts or screws/bolts.
Compliance with thread coefficients of friction in the range of 0.07 to 0.13 is mandatory. The testing
must be performed as per VW 01129.
NOTE 2: The temperature of the part to be coated is for a short time (≤ 15 s) maximum 400 °C
during the deposition of the all-around coating and must be taken into account for the component
geometry. The corrosion resistance is reduced as a result.

3.2 Coating material

Permissible coating materials are
– PFA (perfluoroalkoxy alkane) uncolored 1)
– FEP (fluorinated ethylene propylene) uncolored 1)
1) Pigment coloring as per agreement with the responsible persons; for a designation example, see section 2.
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TL 188: 2016-05

3.3 Analysis of the coating material

3.3.1 Verification of permissible coating materials

The permissible materials as per section 3.2 are stored in the Volkswagen Polymer Laboratory
(GQL-P/4, Brieffach 1437) as fingerprints that can be examined there if necessary.

3.3.2 Length, area, and thickness of coating

The coating must be applied to the contact surfaces of internal threads of nuts, threaded plates,
and holes and to the contact surfaces of external threads and bolts. The coating must have a uni‐
form, continuous appearance and must have no drips, streaks, or flaked-off spots.
Due to different machining equipment, the thickness of the coating on the thread crest and/or on
the thread root might be different. However, the screw-in or screw-on running torque must be fulfil‐
led (see table 1).
If not otherwise specified in the drawing, the coating layer and the coating area must comply with
the following restrictions:
The coating on parts with external threads must extend to the thread runout for parts with a shank
and over the entire thread length for parts that are threaded almost to the head. Coating must not
adhere to the head and the adjacent or load-bearing surfaces of bolts, the hexagonal surfaces of
nuts and their adjacent or load-bearing surfaces, the welding rings, spots, or projections, or faces
and any head surface of welded screws, bolts, and nuts.
The coating must completely cover the entire thread up to the thread inlet on the side of the nuts to
be welded in order to ensure the function.

3.3.3 Torque
At least 10 randomly selected parts are needed for testing one batch.
The test criteria are the permissible screw-in or screw-on running torques (table 1) and the meas‐
ured thread coefficients of friction as per VW 01129.
For the testing, washers of type HL or type HH equivalent to DIN EN ISO 16047 are used as load-
bearing surfaces, together with phosphated and oiled nuts and screws/bolts with matching
strength. The devices under test must be tested for trueness to gage. The values of the screw-in or
screw-on running torques must correspond to the specifications in table 1.
In case of deviating screws/bolts or nut surfaces that are not true to gage size, the screw-in or
screw-on running torques must be no more than 0.001 × d3 higher than those specified in table 1.
In case of quality problems, the results with true-to-gage-size phosphated and oiled surfaces apply.

Table 1 – Screw-in or screw-on running torques of pierce nuts and weld nuts, as well
as screws and bolts
Initial screw-in or screw-on running torque
Thread dimensions
(Nm)
M6 ≤ 0.5
M8 ≤ 1.0
M10 ≤ 2.0
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TL 188: 2016-05

Initial screw-in or screw-on running torque

Thread dimensions
(Nm)
M12 a), M12 × 1.5 ≤ 3.5
M14 × 1.5 ≤ 5.0
a) Restricted use (for further information, see VW 60361)

For the release of a new product, in addition to the torque and coefficient of friction testing, a mate‐
rial analysis must also be performed and documented.

4 Applicable documents
The following documents cited in this standard are necessary to its application.
Some of the cited documents are translations from the German original. The translations of Ger‐
man terms in such documents may differ from those used in this standard, resulting in terminologi‐
cal inconsistency.
Standards whose titles are given in German may be available only in German. Editions in other
languages may be available from the institution issuing the standard.

VW 01129 Limit Values for Coefficients of Friction; Mechanical Fasteners with Met‐
ric ISO Threads
VW 01155 Vehicle Parts; Approval of First Supply and Changes
VW 60361 Mechanical Joining Elements; Parts Reduction; General Guidelines
VW 91101 Environmental Standard for Vehicles; Vehicle Parts, Materials, Operat‐
ing Fluids; Avoidance of Hazardous Substances
DIN EN ISO 16047 Fasteners - Torque/clamp force testing