That is exactly how mine fit, too. I had to do the same, thing: Used a crowbar to pry it out and thought I'd break the ear off, then had to use a crowbar to pry it back in, fully lubed up. My old O-ring also had a nice, crisp square profile from being crushed in there so long.

I don't think the super tight fit is particularly necessary, but you do want a good seal as that O-ring keeps the oil in the pony clutch and drive gear compartment and out of your master clutch. It isn't under pressure, but the bottom of the O-ring is submerged in the pony clutch oil when that compartment is filled to the proper level. It could seep.

Of course, you're the guy who wants to make your master clutch into an oil clutch, so maybe you can get by without any O-ring at all!

I figured the engineers knew what they were doing with that thing and designed that O-ring and groove the way they did for a reason. Kind of like the fuel filter tower O-rings that always seal until they don't, but always seal up again when replaced with the proper O-ring.

Pete.

that o ring is a very tight fit in my experience as well, I thought i missed abolt as it didn't want to budge when i first attempted to remove the pinion clutch housing.

When i reassembled, i had the hole and the housing very clean by using a wire wheel on a hand grinder to remove any rust and paint. i then used copious amounts of never seize to lube the o ring and the housing to prevent rust formation in the future. it was very tight to re-install, patience and persistance paid off. i was worried about damaging the o-ring but there weren't any sharp edges so it went together well and didn't leak.

the original o ring does come out looking like a square ring from being in place for so many years, i to was surprised when the new one i ordered from cat came as a regular round o-ring.

We've been through this before....
2A-3676 is 6.99mm dia., 85.090mm I.D., 99.070mm O.D.
Option #1
8H2046 (same as a dash 340 trade size) 5.33mm dia., 85.090mm I.D., 95.75 O.D.
Option #2
Cat # 095-1554, 5.7mm dia., 84.60 mm I.D., 96.0mm O.D.

No reason that o-ring has to be so tight.

I just received a used pinion assy for my Cat 12 with d318 engine. Also has an old square o-ring. This post was helpful answered my question. I am pretty sure i have the same pinion. Still waiting for a dang parts book from e-bay

The 8T has a different pinion housing but uses the same o-ring seal.

Get yourself a bucket of "Assemblee Goo" It's a transmission assemble lube.

Stuff is amazing for orings. I use it at work. I've struggled with some orings. Tried grease, oil, wd40, brake caliper lube. They all suck, compared to that Assemblee Goo.

www.ebay.com/itm/Transmission-Rebuild-As...oo-Gel-/321844744184

A bucket will last you a life time.