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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: CreativeNotice on November 05, 2011, 07:30:33 pm

Title: thm400 speedo o-ring replacement
Post by: CreativeNotice on November 05, 2011, 07:30:33 pm

So my 82 c20 thm400 had some serious leakage. I've replaced the pan gasket (and the filter for good measure) and that took care of the major leak but there's still a little bit dripping from the speedo cable. From reading about it online I'm thinking I should replace the o-ring, but are there any other parts I should go ahead and replace while I'm in there?

Title: Re: thm400 speedo o-ring replacement
Post by: Engineer on November 05, 2011, 10:30:13 pm

IIRC, there is more than one o-ring on the speedo adapter. You may consider replacing the tail shaft seal while you are back there.

Title: Re: thm400 speedo o-ring replacement
Post by: CreativeNotice on November 06, 2011, 01:28:41 pm

So I've pulled it apart and there's a main o-ring around the diameter of the pinion housing that slides into the tranny. Then there's a gasket on the interior where the speedo cable connects. The pinion shaft goes there there and there is a small spring surrounding the gasket which causes it to seal against the pinion shaft.

If I can find the parts, I'm going to replace the o-ring, pinion and pinion shaft gasket w/ spring.