Depressing Early 260z bumper struts

[heyitsrama]

Howdy,

I’m trying to flush mount a rear early 260z bumper to a early 260z. From other threads I have read that you can drill out a leak hole for the oil, the push the shock inward and weld it into place.

I drilled a hole in the strut l, drained a significant amount of oil out of the strut, but the strut does not stay depressed, it seems like it keeps popping outward. I beat on it with a dead blow hammer, I can see that it moves inward then comes right back out. Is there a trick to this that I’m missing? 

[SteveJ]

Drill a hole at least 6mm while the shock is compressed , tap it, and run an M6 bolt into it. Just a suggestion. I removed those boat anchors when I swapped over to a 240Z rear bumper and used some mounts designed to affix 240Z bumpers to 280Zs.

[heyitsrama]

@SteveJ I need up fabricating my own brackets but I messed up because I copied the body side mount from the 240z, I did not realize that the holes are offset on the 260z/280z body.

I suppose I can remove the struts out of the car, but am not looking forward to dropping the tank. 
 

I can make mounts for the 260z body but the issue is that the mount position on the bumper is further out than the 240z, there might be some awkward bracket that I can make, but it seems like keeping the struts is easier to do.....

are the struts super heavy?

what’s the point of threading in the m6 bolt? Should it be in the face of the strut? Or along the shaft?

 I was able to shove a pin into the passenger side bracket and appeared to pop through something that make it able to push down, no luck on the driver side.

73 240z on the top,

early 260z on the bottom.

Edited November 22, 2020 by heyitsrama

[SteveJ]

The thought for threading the bolt into the housing and the strut would be to hold it in place.

I don't remember how much the struts weighed, but it was not insubstantial. 

I bought the bumper brackets from Datsun Spirit, and those brackets allowed me to mount the 240Z bumper at the correct height.

[Captain Obvious]

The bumper shocks are high pressure mono-tube struts. If you want to know what that really means, or how they work, or why it matters in this case, let me know and I'll go into the details, but the summary is...

Carefully remove the little screw in the middle of the bumper mounting flange. It's holding back significant pressure. Mostly gas, but there might be some oil in there too. Wear eye protection and don't let anything fly. I suggest covering it with a rag once you get the screw cracked loose.

After that, the shock should compress and stay wherever you put it. (Until acted upon by another outside force of course.)

[Captain Obvious]

Actually if you already drilled a hole and significant oil came out, it really shouldn't matter. But try taking that screw out first and see what happens

 

[heyitsrama]

11 minutes ago, Captain Obvious said:

Actually if you already drilled a hole and significant oil came out, it really shouldn't matter. But try taking that screw out first and see what happens

 

On the driver side I drilled 2 holes where you are holding the tube in the second image, I also drilled out the screw hole on the face but am still not able to get the strut to collapse.... If I try to pull the strut out is it going to be a pain to get it reinstalled? Perhaps I need a deeper punch to break through whatever membrane/seal is back there. 
 

I was thinking of removing it and trying it on the shop press, seems to be more accessible. 

[Captain Obvious]

There is only one significant hurdle to getting the shock out of the bracket and that's the nut on the back side. It's not hard to get to, or hard to turn, but the problem is that there is nothing (except a little friction) holding the shock shaft from spinning. But once you get that nut off, it's not difficult to take the shock our or put it back in.

So if you clean up the threads good and put some penetrating oil on it and then hit it with an impact wrench, it should come right off. If you put a ratchet on it, I give it 50/50 that the shock shaft will spin inside the body. Here's the nut:

And if the shaft does spin, about the only thing you can do at that point is cut the nut off with a cutting disk. Big ol' PITA:

I did two of them, one came off easy with a ratchet and the other did not. After my (lack of) success, I helped another Z guy do the same and told him to skip the ratchet and use an impact gun. His both wizzed off with no trouble at all.

[Captain Obvious]

Oh, and I mentioned above that the nuts are easy to get to... That's with the whole thing off the car. I don't remember how difficult it is when the whole assy is installed. Sorry.

[Captain Obvious]

Sorry, one last thingc... I drilled up here. And you can see from the shock shaft that mine stayed compressed no problem:

And after cutting the shock open and analyzing the guts, I left a note for myself if I ever have to do it again:

 

[Captain Obvious]

OK, really really last one this time. 

My theory is that your drill hole is too close to the bumper mounting flange and because of the way the mono-tube shock works you're trying to force oil through a tiny orifice and it just takes time.  I think you have two choices:

1) Drill another hole further back.

2) Push the shock in and hold it there for enough time for the pressure to bleed off.

This is a pic with the shock cut open. If you look closely down into the tube, you'll see a very small hole in the center of the piece at the bottom. I'm thinking you've got a mix of gas and oil in the shock tube and when you push the shock in, it compresses the gas, but not the oil. If you compress it and wait long enough, that pressure should eventually equalize?

Shock cut open showing the orifice that separates the two chambers of the mono-tube design:

[heyitsrama]

The deepest spot I got my drill bit is 3” i, I’ll drop the mount and try it tomorrow, I tried fitting my hands back there and it feels like there is a plate covering access to the nut.

thanks for the thorough explanation @Captain Obvious I appreciate it 🙂