irobertson

What I have learned about Viton O-rings and case through bolt holes.
I tripped over this little issue unknowingly, and I haven't seen much info relating to it, so I figured I'd document my experience & solution for the next guy who is doing the same job.

I have an '88 Carrera with the original engine, which I'm currently putting back together. I mated the case halves a couple of months ago and I recently noticed the Viton O-rings were squishing out between the washers and the case. Oh oh!!
After doing some research and getting some concise feedback from Henry Schmidt on the issue, it became clear what my problem was.

There is a chamfer on the through bolt holes in the case on the 4, 5 & 6 side, but not on the 1, 2 & 3 side. Turns out this is a problem for the O-rings, and they get pinched between the washer and the case because they don't have enough room. None of the rings on the chamfered side had any signs of being pinched or crushed. Only the ones on the non-chamfered side.

The solution was to remove each through bolt one at a time, chamfer the holes on other side of the case, and reinstall the bolts with new o-rings.
Here are some pics of the job.

Pic 1 - The pinched/crushed O-ring.
Pic 2 - The factory chamfered hole.
Pic 3 - The factory non-chamfered hole.
Pic 4 - Blocking the hole with a shop towel plug pushed in from the other side.
Pic 5 - Adding some grease to catch the filings. (Once done, I pushed the plug the rest of the way through, which carried any remaining filings along with it.)
Pic 6 - After cutting the new chamfer. (I did this using the tool by hand. I found I had more control and could keep it from chattering as well.)
Pic 7 - The gap the o-ring creates on the non-chamfered side.
Pic 8 - The gap the o-ring creates on the factory chamfered side.

I used a feeler gauge to set the gap to around .040 for the first one I did, and the rest were easy to duplicate by eye.
Tool was a 3/4" 90 degree countersink.

Tomorrow.... Continue assembly, woo!!

Amber Gramps

Why does the head of the bolt not sit flush against the case?

I'm an idiot that is just waking up. This is showing the pre-compression space the O-ring has prior to it being tightened down.

theiceman

ian thats amazing work. have never heard of this. wonder why the factory didnt do both sides the same ?

Johnm1

Really interesting. I had not heard of this. Man, you are braver than me.

Johnm1

Ian, looks like I have to do this as well. Right side has chamfer. Left does not.

Johnm1

Ian, did you say you just held the bit and turned by hand?
How did you keep it square in hole? (Maybe a dumb question)

irobertson

I welded a 1/4" extension onto the counter sink, and then put it onto a nut driver handle. My case already had studs, so I just used those to keep it steady as I turned it. Light pressure and a fairly quick turn yielded better results than heavier pressure and slow turning. While there is something to be said for being perfectly square, I wasn't overly concerned about it being off slightly. After all, the rings are going to be squeezed into whatever gap is there. I was more concerned about ending up with a smooth (chatter free) surface.

Disclaimer;
I should mention, I have yet to put my motor back in, so I have no idea how effective this will be at preventing leaks.
Enter at your own risk, blah blah blah.

Johnm1

Thanks, Ian. This will be real helpful.

Ed Hughes

I could swear all my holes were chamfered.

Johnm1

Just wanted to say thanks, Ian. If you had not posted this I'd probably have to take the motor down again. You probably saved me another tear down.

Performed basically your operation. Ollie's did the chamfer on the left side, but I felt there was a lot of burrs and the chamfer was very thin. So I did your trick.