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Cam Timing Sprocket Bolt keeps coming loose


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Help!  I'm in the final stages of rebuilding a 72' 240Z.  

The Cam timing sprocket bolt has now come loose for the second time.  The first time I pulled the head and had two valves replaced.  (Also got a good bit of hazing about proper torque...)  Replaced everything and torqued the cam bolt to 105 ft/lbs.  Two short test drives, everything just fine.  First longer drive, she stopped running about two miles out and when towed home, the cam bolt has loosened again.  (Cam stopped spinning, pinged the top of several pistons, now need to remove the head and get valves inspected/replaced...  again)

 

Any thoughts on why this keeps happening?  I don't want to continue fixing the same thing if something else is the root cause.  (Definition of insanity...)

 

72' 240Z

Original engine and E88 head

Original intake and SU carbs (rebuilt by Z-Therapy)

Aftermarket electric fuel pump (with flow gage to check and adjust the flow)

Aftermarket exhaust 3/2 exhaust header

280X Non-Turbo Distributor

Original AT (Rebuilt locally)

 

Thanks,

Chris

 

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zbloke, great idea, but the threads in the cam as so trashed, I'm pretty sure it won't tighten now if I tried...  However, The sprocket wouldn't "wiggle" after it was torqued down, not that the tolerances we are discussing could be felt by hand...   Thanks!

Edited by pete6583
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If the threads in the end of the cam are trashed I personally wouldn't want to reuse it or even attempt to Helicoil it either

 

Same for the bolt, if its threads are anything like the threads in the end of the cam again I personally wouldn't reuse 

 

As for an underlying issue causing the problem, do you know any history of the engine prior to the bolt coming loose? was it a running engine when it came to you? have you completely rebuilt it or part rebuilt it? left it untouched and just got it running?

 

 

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The engine was running when I got the car.  Completely disassembled (except the head).  Machinist said cylinders still had factory cross-hatching, and head looked recently rebuilt.  Rebuilt engine with new bearings, rings, seal, gaskets, hoses, timing chain kit, ss bolts.  I have no other info on the car or the engine.  

 

I'm waffling on new cam or have the machinist helicoil.  He's got almost 50 years experience and thinks it will be just fine...   New cam means new lifters and springs as well to do it right...

 

Thanks for your thoughts and the information!

 

r, Chris

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2 minutes ago, pete6583 said:

The engine was running when I got the car.  Completely disassembled (except the head).  Machinist said cylinders still had factory cross-hatching, and head looked recently rebuilt.  Rebuilt engine with new bearings, rings, seal, gaskets, hoses, timing chain kit, ss bolts.  I have no other info on the car or the engine.  

 

I'm waffling on new cam or have the machinist helicoil.  He's got almost 50 years experience and thinks it will be just fine...   New cam means new lifters and springs as well to do it right...

 

Thanks for your thoughts and the information!

 

r, Chris

 

I might be massively wide of the mark here but, the bit I've highlighted in red combined with the issue you're having would lead me to believe the engine had this bolt coming loose problem before coming into your possession or if the head had recently been worked whoever had done it had overlooked something, the result being the bolt coming loose 

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I had the cam gear spin on my L28 once. In that case, the bolt did not seem to have loosened appreciably, but it was the dowel pin that wiggled out far enough for the gear to spin on the end of the cam. I ended up swapping the cam to a different head, and shimmed behind the pin to prevent it moving again. This might be a different problem from yours though, as it sounds like things really came apart on you.

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On 7/20/2019 at 6:14 PM, pete6583 said:

I'm waffling on new cam or have the machinist helicoil.  He's got almost 50 years experience and thinks it will be just fine...

Well if your machinist really is experienced then it might be worth it, he probably won't be using a Helicoil, more likely he would use a Time-sert, get your machinist to re seat the dowel pin as @Canadianz mentions above, if it wiggles out you're in for bent valves.

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  • 1 month later...

Time-sert as grannyknot says. Make sure you get a insert that goes as deep as possible but still allows the expander to be used. The inserts come in many different lengths. I would be looking at bolt length and proper washer as mentioned above. Put the bolt in with no washer and make sure it goes in deep enough that when the washer is added the bolt definitely does not bottom. 

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