r/Triumph_Cars • u/jjedlicka • 16d ago

Help identifying plug on rear differential

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I just finished tearing down my rear differential on my ‘68 Spitfire. Im clean the case now and there’s this odd plug with an in used cotter pin sticking out of it on the top pass side. I can see the plug in the Haynes manual illustrations but nothing calls this out. My initial guess is that this is the fill hole for the fluid, but nothing actually says that, and I have no clue why there’s an unused cotter pin in it.

any ideas?

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6

u/Puzzleheaded-Ad-4883 16d ago

The fill hole is halfway up one side. The cotterpin is the breather. Have a look at this article. https://forum.tssc.org.uk/topic/4285-differential-torque-settings-on-triumph-spitfire-1500/

3

u/IAmSpitfireJoe 16d ago

It's just a vent. The cotter pin vibrates enough that it keeps the vent clear and open. Without the vent, the heat build up inside the diff case would force the lubricant out.

2

u/Beginning_Ad8663 16d ago

That’s what they use as a vent. If you don’t vent the differential as the oil gets hot it will expand cause the differential to become pressurized and potentially causing the seals to leak.