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Go to solution Solved by TobyB,

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I always thought it was a reference mark for final factory assemblies that were properly torqued. 

Agreed, yellow paint applied at the junction of a fastener to its mating surface indicates the fastener has been torqued to final spec. It is normally lacquer, which is very brittle when dry and cracks easily under load. Any cracks in the paint that show upon later inspection indicate the fastener has begun to to loosen, a good quick check when doing a "nut-and-bolt" inspection on a track car.


I use yellow nail polish on critical fasteners, in particular the CV joint bolts. A more expensive option is official "anti-tamper" paint or "anti-sabotage lacquer" as used in the aircraft industry.


--Fred '69 & '74tii

Edited by FB73tii


'74tii (Colorado) track car

'69ti (Black/Red/Yellow) rolling resto track car

'73tii (Fjord....RIP)

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If you're talking about the factory marks,


the ones I've seen aren't  lacquer, it's more of a quick swipe with enamel on a cheap brush.


I always figured that it was proof that an inspector had checked fitment and attachment-

it's like a dog peeing on a hydrant more than anything- 'I was here.'


And it seems to be kind of random, although I never cleaned up parts cars before shredding them.



"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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As a few others have posted, the yellow paint mark is often used during the assembly process. For Fasteners requiring critical torquing, assembler would torque per spec and immediately mark fastener with paint as proof fastener was correctly torqued.

I've worked in assembly operations where a paint brush or paint marker is attached to the torque wrench (often duct taped) to make the torque and mark process easier for the assembler.

BTW - type of paint usually isn't critical, just needed mark to last long enough for final inspection.

Andy W.

'72 Tii & '74 Tii

'88 M3 & '91 318is


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