Problem with case bolts + viton o-rings

[DCred911SC]

Hi,

Doing my first engine rebuild and have noticed that when using viton o-rings on the case through bolts, the washer on the bolt head side does not fully seat against the case spot face when fully torqued. The o-ring seems to be preventing it from fully seating, and I can get a very thin feeler gauge in between.

Looking at where the o-ring locates in between the washer and the case, there is nowhere for it to compress into other than a small chamfer in the washer, and a slightly larger counterbore in the case, but I don't see how it can be fully compressed into there. I would think that the softer blue o-rings would be pushed into there, but not the harder viton.

Has anyone else noticed this, and is it OK ?

Any advice appreciated

Thanks

David
911SC 1982

[MatthewBrum]

Are you sure the o-rings are made of viton and not something harder. The o-rings should be soft and compress easily. Are you sure the feeler gauge is going completely under the washer?

[DCred911SC]

Hi Matthew,

The o-rings are 8mm ID x 2mm section, definitely Viton and are 70 - 75 Shore D (which I believe is normal for Viton O-rings). I can get the feeler gauge part way in until it touches what I assume is the squashed O-ring.
The opposite side, with the dome nut, seems OK because this has a double chamfer for the o-ring to go into (one on the washer, and one on the case).

Doesn't seem right to me that is the way it is supposed to work

[356RS]

Hi Dave, Did you check to surface area on the case to make sure there was no rough spots? The seating area for the o-ring and special washer is often cleaned up by the machine shop when work is being done on the case to make sure the seating area is flat and smooth. All the o-rings are the same size for many 911's 356's and 914 cases and also use the same special washer. I did think there was any chamfer on the engine case, only on the washer. Did you use the o-rings that came with the gasket kit? If not that could be the problem.

[DCred911SC]

Hi Mark,

I haven't used o-rings which came with the gasket set (it was a Victor Reinz set with blue o-rings), as consensus of opinion is that the blue ones are useless and just tear when tightening the bolt. Viton o-rings seem to be recommended by the forum, so those are what I've used.
With regard to the chamfers, only one half of the case has a machined chamfer the same as that on the washer, the bolt head side only as a counterbore and very small chamfer.
I'm now thinking about using the blue o-rings on the bolt head side, and the viton on the nut side. My thoughts are that if I can keep the bolt head stationary while tightening, the soft blue ring should be OK, and the tougher viton one on the thread side shouldn't tear as easily.

[356RS]

Like Matthew said, the o-rings should be soft and compress easily. I was trying to locate some of these o-rings in my gasket kits cabinet but I guess I'm out of the kits. I was going to measure them to see how thick they are. You said 2mm, that sounds OK. Just hope there soft enough.

[mistertate]

Hopefully this isn't hijacking but what do you guys think about Wayne Dempsey's suggestion in the 911 Engine book to use a silicone (RTV) sealer in addition to the o-rings? I always thought that was a little strange as I was taught to not seal an o-ring because it is supposed to be able to shift around a little bit to swell and seal itself.
But it seems like both methods work for this application

[EBS_Don]

Hey John!

I really don't recommend using RTV at all. If the o-rings are lightly lubricated with oil or Dow Corning 111 they should install easily and seal nicely. I've used the green o-rings on 911's and 914's and the washers should sit flat with the case when torqued down. I've seen some pretty rough mag cases that need a little touch up on that surface to be flat but an SC case would be unusual.

Don

[sabeo.m]

Originally Posted by EBS_Don

Hey John!

I really don't recommend using RTV at all. If the o-rings are lightly lubricated with oil or Dow Corning 111 they should install easily and seal nicely. I've used the green o-rings on 911's and 914's and the washers should sit flat with the case when torqued down. I've seen some pretty rough mag cases that need a little touch up on that surface to be flat but an SC case would be unusual.

Don

Don What's up, glad to see you here! Thanks again for all of your help, the engine is running great!! John, DC 111 worked great for me, I didn't use RTV on my rebuild...

regards,

Sabeo Mederos.

[mistertate]

Originally Posted by EBS_Don

Hey John!

I really don't recommend using RTV at all. If the o-rings are lightly lubricated with oil or Dow Corning 111 they should install easily and seal nicely.

Don

What up Don, all the 944S rebuild parts you sold me are spinning around in a happy engine.
I use oil too. I like to use the CMDlube Extreme Pressure lube if I'm really worried about it. I've noticed a few cases where the o-rings were pinched by the washers upon factory installation. I also don't notice a lot of leaky case bolts. Considering the variety of methods in this mentioned in this thread and lack of significant problems I guess this isn't that critical of an area?

[Flat6pac]

nothing spectacular, just put some grease on each one so when you torque you don't tear them.
Grease makes the return tubes install easily too

Bruce