Cam nut backs off?
 

Yes you read it right!!

A little background....

The engine is a 3.8 Turbo with custom ground cams from a reputable supplier, Target hp is between 700-1000HP. The engine have 12 injectors, custom throttle bodies, pauter rods, AASCO springs, Ti retainers, basically no expense spared

This engine has managed to turn the right side cam nut OFF, three times!!!!

The results are bent valves.

After the first failure the engine was rebuilt using a check and balance method i.e. each step was checked by another person

Second failure 2 people checked the assembly!!

We have been running this engine at .7 bar and short shifting to break it in and analyze the data and its failed 3 times

Any insight?

Is this 993 style? The kind that uses only the torque of the bolt to keep it in place?

If so, then I'm guessing that the racing valve springs and aggressive cams might be causing a great deal more friction to overcome than the Factory torque spec can hold.

Not using the bolt, using the big ole nut

O.K., but what is the base motor? Does it use the locating pin like the early-993 and 964 and older, or does it use the late-993 style that uses holders on the back side of the cam towers to time the cams, using only friction to hold the cam to the sprocket?

I believe the later 993 style uses the bolt only, so I guess maybe you had some early SC cam cores reground, and that is why you have the big nut instead of the bolt? That may be the problem. Did you torque the nut to 110 foot pounds? If you torqued it to the bolt-type spec, this could be it.

Switch to the later style camshafts. I have never heard of this happening repeatedly, but it certainly seems that the bolt method of attaching the cam sprocket is better for higher HP engines. If you think about the geometry of the setup, it becomes obvious why (larger outer nut is easier to spin). I assume the pin fell out too?

-Wayne

This was a common problem on the 993RS engines which used the same essentially friction method of maintaining cam timing. I back dated mine to the 964 & '95 993 pin style. You will need new cams, sprockets and hardware for this backgrade.

FVD sell this kit for~$185
http://www.brombacher.de/catalog/tun...vd10550000.jpg

or you can put together your own from stock 964 parts.

The base for this engine is a 3.6, The cams are hardwelded regrinds.

We use the pin/spring washer/big ole nut to secure the sprocket to the cam (ala the pic Bill posted).

The torque on the nut has been verified and hands on tested by 3 people now.

I think the threads on the reground cams are effed up..

we got new nuts for this engine LOL

Wayne, I thought about the geometry and moi things, but the cam spins at half the crank speed..this engine makes so much power you dont need to wind it over 7400 (we think) so the cam is spinning in 911S territory nothing ou tof the ordinary.

Red loctite, and a spot of TIG are next

Weird!

Well, you answered all my questions, and nothing makes sense, except what you concluded. Poor threads, or possibly improper depth of threads, resulting in the nut bottoming out on the cam instead of the sprocket.

That's about all that's left, and would kind of explain why it's only happening on one side.

Let us know what you find when it's resolved. This will definitely help me in the future.

We examined the camshaft, and the threads looked fine. we measured and compared some cams regarding the"grip" of the cam nut, and found everything to be within reason.

This time we timed the cams and when the number we wanted was achieved we used red loctite on the cam nut.

We took the car to Watkins Glen to shake down.

3 days of track time

starting at .7 bar and progressing to 1.6 bar

The car flies ( it wants to, we need more downforce and not a BS AIR wing)

140 something at the top of the esses,176 going into the bus stop ...... 2:00 laps

Still we dont know why the cam nut backed off in the first place

:mad:

Wish I could report why it happened, but we dont know. But we have prevented it from happening again using red loctite.

Its going to be a pain in the balls next time we refresh this engine!!!

I'm suprised that on a project like this the cams were not previously timed.

A bit of locktite on all critical fasteners never hurts either.

They timed the cams again, because you lose the precise timing every time you loosen the big nut. There is still some movement even with the pin in place.

What I meant is that the timing should be checked any time the nut is loosened.

Your post made it sound like you didn't time the cam any of the other times you worked on the valvetrain.

Hmmmmm yes the cams had been timed previous to the nut backing off, and the cams are timed prior to securing the cam nut.....

The issue of this post is the cam nut backing off... Typically this connection is not a red loctite application.
Red loctite did fix the problem though..

I think that I'd find a safety wire pro/aircraft mechanic.. and tell him any hard numbers.. and get his reaction.. till you have a better idea..........Ron

Ron, Billy and I did some head scratching on this one, red loctite worked. Just got home from 3 days at Watkins Glen. The engine is amazing..176 or so entering the bus stop.

I dont know if the cam nut is a candidate for safety wire.

Nah, loctite is the poor mans non-aerospace equivalent of safety wire.

If the problem re-appears....you might try the old Harley method.
On harley sprockets...there is a series of tapped holes to put a screw or small bolt into.
The idea being...tighten the big nut...then install the small screw/bolt into the first tapped hole that would prevent the big nut from backing off.
I have even put a small washer under the small screw to bind the big nut better.
Bob

Before I would recommend the harley method, making a locking tab out of the appropriate sheet metal would be a much better route.

The holes in the cam gear would make this an easy task, but if I was a gambling man I think the red loctite will be more than enough.