I just got out of the garage, everything is fixed and the car is running.  First the fan was an important clue but the problem was actually with the crank pulley.  It looks like the pulley separated into two pieces and the round part of the pulley was sort of spinning with the rest of the pulley most of the time.  When the broken pulley got out of alignment it would smack around against the other side of the pulley and the timing cover, creating the sound at random intervals.  I pulled the pulley off of my other Z and the problem is resolved.  The first photo is what it looked like on the car, you can barely tell its broken.  The second photo is what it looked like after it was pulled, two totally separate pieces

Edited December 31, 20159 yr by Kurbycar32

10 minutes ago, Kurbycar32 said:

I just got out of the garage, everything is fixed and the car is running.  First the fan was an important clue but the problem was actually with the crank pulley.  It looks like the pulley separated into two pieces and the round part of the pulley was sort of spinning with the rest of the pulley most of the time.  When the broken pulley got out of alignment it would smack around against the other side of the pulley and the timing cover, creating the sound at random intervals.  I pulled the pulley off of my other Z and the problem is resolved.  The first photo is what it looked like on the car, you can barely tell its broken.  The second photo is what it looked like after it was pulled, two totally separate pieces

 

Hopefully no damage was done.  A bad damper can break the crankshaft.  Send the broken crank damper to Dale Manufacturing (Google it) and they will rebuild it better than new.  Do not install a used damper.  They are 40+ years old and will fail like the last one.

Edited December 31, 20159 yr by Jeff G 78

I don't understand the new forum software.  I couldn't get my reply outside the quote box.

Tom Monroe says to stand on them to check the rubber before installing.  He "had" a three row, now it's a double. 

I took that third one off mine and stood on it, seems okay so I'm using it.  Maybe I need to get a heavier person to double check? 

i have one of these on my rebuilt L28 it's a nice part - i had the engine internals & clutch & flywheel all balanced to 1/2 gram and it revs really smoothly all the way up to 7K.

 

http://www.ebay.com/itm/Datsun-70-83-240Z-260Z-280Z-280ZX-L28-Crankshaft-Harmonic-Balancer-Damper-044-/331724815985?hash=item4d3c56fe71:g:s1UAAMXQyY1TVtVX&vxp=mtr

looks like you're not using the extra pulleys anyways, so for $90 why bother with a rebuild?

Just that the Dale rebuilds are known to be the best.  Knowing what happens when they fail, I'd rather have them rebuild a OE damper with the best bonding agents.  I believe they still charge about $110 so the cost is similar.  They come with a lifetime warranty too.  I've had dampers fail on my race car and it's not pretty.

Both cars have AC in their future so im going to need that second row.  Ill send the split one out for repair, thanks for the tip

So I'm assuming the Rebello motor was supplied without a pulley? 

So it was not balanced with the rest of the assembly?

 

22 minutes ago, madkaw said:

So I'm assuming the Rebello motor was supplied without a pulley? 

So it was not balanced with the rest of the assembly?

 

Correct.  I supplied the pulley with my core motor.  Frankly I thought it was just a pulley and not a harmonic damper, otherwise I would have taken care of it during the motor build

I guess I am surprised that it would even have been used by Rebello if it hadn't been rebuilt. My builder refused to balance my assembly without me rebuilding the damper. He looked at the rubber and the age and said it was a no go.

Glad you figured it out.

Another member asked about harmonic damper failures in non-racing applications so I figured i would post an update.  First I sent the pulley out to Dale mfg and it came back as good as new.  Website here:  http://hbrepair.com/harmonicbalancer_018.htm Second I didn't realize at the time but the pulley bouncing around down there knocked the point off of my timing indicator so I had to replace that as well

Glad you got it fixed. That is certainly one to keep an eye on.