911 CV joints and bolts

[Walko]

We all talk about keeping our 911's in good running order but how often do people repalce the CV joint bolts.

I know most mechanics reuse the bolts but I question whether this is a safe practice especially if we are using the cars for spirited driving.

If we can reuse the bolts what sort of life should we put on them.

Also at what torque should they be installed?

Michael

[RWebb]

you want to search out a long but high value thread by rececale

pay particular attention to comments by Grady Clay & Jim Sims

also buy yourself a Bentley manual

torque depends on which bolts you have - since you do not ID your car no one can say

you can reuse the bolts until you drop dead as long as they don't drop dead
So:
you are first going to carefully clean all - ALL - dirt grase or oil off each bolt - your wife's dishwashing machine is perfect for this
Then:
- inspect the wrenching feature carefully - that is the 6 point/hexagon indented in the center - be sure it is not gouged out in any way
if in doubt take a sharp close up pic and post it - find a friend with a macro or micro lens

- inspect the threads carefully - they should be perfect

discard any failures

ALWAYS use new washers under the bolts - NEVER reuse

the moon plates should be clean and not too beat up - reuse them

Next, spray the bolts, moon plates and internal threads in the CV joints with brake cleaner. Obviously, do not get any inside the ball bearing working parts of the CV joints - it will ruin your nice new grease.

pay esp. attn. to cleaning procedures for the CV joint itself. Use Q-tips and then Q-tips with brake cleaner to clean out the female threads.

I often put a bolt in loosely and then pull it back out to ensure there is no grease on it - if there is I repeat the cleaning.

[Walko]

Thanks for your comments about the cleaning of the bolts.

The reason I have bought up this topic is that I have had two failures and after discussion with the engineers at work they have advised that the high tensile bolts used to hold the CV joints to the gearbox flange and also the outer drive flange should be a one use bolt the same as a rod bolt in the engine.

My bolts have failed in shear not tensile so I am inclied to believe that teh bolts must have a life and hence why I am now trying to figure out what other parts may have a defined life once they have been in and out a few times.

I will have a look at the other post.

Porsche does not list a moon plate or washer for the 77 with M8 bolts but does list them for the turbo and sporto which have M10.

The car in question is a 77 3.0 Carrera.

Michael

[Jim Sims]

911 CV joint bolts are not "stretch" bolts like connecting rod bolts. The CV joint bolts when tightened properly are not yielded. Provided the wrenching features in the heads and the threads are in good condition the bolts may be reused. Shear failures in the bolts likely indicate the bolts were in operation loose. Proper tightening and appliances like Schnoor lock washers (use new Schnoor lock washers each time), moon plates and drilled heads with lock wire can help prevent loosened bolts. Checking/re-tightening the bolts after a couple hours or a 100 miles of operation is a wise precaution. Correct torque for the M8 bolts is 32 ft-lbs and for the M10 bolts it is 60 ft-lbs. Insure the bolt and flange threads are clean and free of grease. I use new S8 Schnoor washers and reuse the M8 bolts on my '76 911 CV joints. I have had the CV joints off and apart several times in the last eight years - never had a bolt loosening problem with them.

[RWebb]

"high tensile bolts ... should be a one use bolt the same as a rod bolt in the engine."

- Nah.

- something is wrong if you are getting repeated failures. Post a pic...

- I advocate Moon Plates for everyone! It will be a fundamental platform in my Presidential Campaign, which will start shortly after Paris Hilton's.

[Nine9six]

If memory serves me correctly, the M8's get 30 ft lbs, and the M10's get 60 ft lbs.
Its my opinion that the moon plates can be reused as long as they are in decent condition. However, new schnoor washers should be installed each time.

+1 on Mr Webbs instructions.

[RWebb]

ah - I see Jim beat me to it...

I'll add that the only time I have ever had a loose CV bolt (2 cars and about 14 years I guess) was when I cheated and re-used a Schnorr (or is Schnoor?) washer.

[Jim Sims]

Yes, it's spelled Schnorr; I must be feeling Dutch tonight.

[Eagledriver]

The smaller size CV joints like on your car are more likely to have this happen than the bigger Turbo or 86 Carrera units. If your car is stock there is no reason the stock bolts shouldn't work. I'd follow the precautions listed above and check the torque every few months until you are satisfied they are not getting loose.

-Andy

[aston@ultrasw.c]

What the 'kin hell is a "Schnorr" washer.

I just installed new axles. They came with bolts, the car had moon plates, schnothing else.

[Jim Sims]

A "Schnorr" washer is a hardened steel, ribbed or serrated Belleville-type spring washer. It is used for fastener locking purposes.

http://www.schnorr.com/safetywashers.html

[RWebb]

definition stolen....
Acronyms, Abbreviations, Technical Terms & Jargon

[Grady Clay]

Michael,

Here is the ’mother of all CV joint information’ thread.
While it is very long, you will be well served reading (and studying) it in its entirety.

Reconstructing Constant Velocity (CV) Joints
Reconstructing Constant Velocity (CV) Joints

Take the time to read it before going farther.


Does your ’77 Carrera 3.0 have 108 mm CV joints with M10 bolts (8 mm hex) or the smaller 923 type 100 mm CV joints with M8 bolts (6 mm hex)?

Clearly, if the bolts are failing in shear mode, they are being run loose. This is where the clamping force is insufficient and the CV joint can slip on the flange.

Another possibility is some issues of the Bentley manual list in error the torque on M8 bolts as 63 ft-lbs (the correct torque for the M8 is 33 ft-lbs). If the wrenching feature doesn’t fail first, the bolt can yield and become ‘stretch thinner’ (Jim knows the term). I can see this drastically reducing the clamping force and resulting in shear failure.


EDIT: add

If you have the M8 bolts, I would change them every time. Always use new Schnorr washers regardless of bolt size. If you don’t have ‘moon’ washers, get some – they are readily available from salvage. Be aware to bolt length, discussed in the above linked thread.

If I had a 911 with 100 mm CV joints, I would seriously consider back-dating or up-dating to the 108 mm CV joints. You need axle assemblies, stub axles and transmission axle flanges. The only issue is with late SC and some Carrera (915) that have ‘fine splines’ for the transmission axle flanges.


A trick for assembling CV joints on the flange without getting grease everywhere: Make three headless bolts with a screwdriver slot. Use them as guide pins to locate the CV joint on the flange and then install the other bolts. Remove the guide bolts one-at-a-time and install the correct bolt with new Schnorr washer.


Best,
Grady

[Walko]

To every one thank you for your input.

I had planned on upgrading to the later CV (M10) when i rebuild the gearbox and install the LSD.

I will keep an eye on the bolts for now and obatin some new schnorr washers. However I note our host does not list them for the M8 only the M10.

Thanks

Michael