I used a deep well socket, 10mm I think, to get mine down.

Flat side up I think? It's been awhile.

Hi, I need some help.  I switched tire size and needed to put back the 20 tooth red speedometer pinion gear that was accurate for the original sized tires.  I've done this before, but this time I decided to buy new inner and outer o-rings.  The outer was fine to replace, but I knew I was in trouble when I had a heck of a time removing the inner.  I should have left that alone!

Here's my problem.  I couldn't insert the inner o-ring no matter what I tried.  The inner o-ring is metal covered by a rubber material.  It is not flexible.  I thought it had to go down the sleeve to where, it seems, there is a indentation.  Maybe I'm wrong about the placement??  Maybe it just lays in there "loose"??

Then I bought a new pinion gear with sleeve for a 280ZX, but although almost identical, it was 3/32" (.24cm) smaller in diameter so I couldn't use it.

Questions:  1) How to insert the inner o-ring.  Which side is up (toward the sleeve threading.  2) Why was the new pinion smaller in diameter?  Could it be for a 4 speed automatic?  I have a 3 speed automatic.

Thanks for any help.  I'm feeling pretty dumb now.

Pictures show top down (from threaded end) the pinion sleeve.

Thanks.  I'll give that a try as soon as I get the new o-rings.  11mm fit for me!  

Thanks again for the help.  I attempted inserting the new o-ring without success.  But I found the problem while extracting the mangled new o-ring.  The problem I was having was a piece of the original o-ring was still in the pinion sleeve unbeknownst to me.  When I extracted the new o-ring a piece of the original came out.  Now I was able to easily insert a new o-ring. 

Left picture is 1st new o-ring that got mangled trying to insert over a piece of the original o-ring that remained in the sleeve (right picture).

A lot of grief for an over looked piece of garbage!  The advice led me to the solution.  Thanks again for taking the time to help.

John B from Mount Clemens, MI

P.S. Anybody out there have a headlight washer pump for a 1982 Datsun 280ZX THAT DOESN'T LEAK?  I bought a couple that were "leak tested", but still leaked so I'm still searching.  I've tried all the usual places eBay including parted out, Z store, Z car source, ZParts, etc.