SU Oil Consumption

[Captain Obvious]

Yes, your understanding and description are correct. The tube is blind ended at the bottom and the only opening into that cavity is by way of the hole at the top. So barring defect or damage, the only way into or out of that reservoir is from above.

I can picture three ways to get oil out of that blind cavity.

1) Evaporation - Which should not be happening to 20W motor oil

2) Check valve not passing fluid fast enough on the downstroke and lifting oil up and over the top of the walls - Which you tested

3) A crack or other passageway near the bottom of the tube that shouldn't be there - Very unlikely to be identical on both carbs

I have seen motor oil "migrate" from one location to another, but it happens very slowly. If you take a metal soup can and fill it halfway with oil and put it on the shelf for a year, I swear that the outside of the can will be oily. Your issue is happening much too quickly for that though.

I must be missing something...

[Jetaway]

I must be missing something...

More likely that I haven't done some test exactly right. Thanks for your help Captain! If I ever figure out why or why I thought they were using oil but really weren't, I'll post it here.

Chris

[Captain Obvious]

Yes, if you reach resolution, I'd love to hear it.

I've still been thinking about it, and I took a good look at the damper tube under bright light and magnification. I didn't see anything that would contradict what we've already covered. The cavity is dead ended at the bottom, and what's happening to you should only be able to occurr through the top.

What I was thinking I might find was that the damper tube wasn't one piece, but instead was made from multiple parts pressed together at the factory. My thinking was that any seams could potentially allow for a low end leak. I didn't design the thing, but for ease of manufacturing, I would have done it with multiple pieces. Much easier than creating a highly polished, deep, blind ended hole with a square face at the bottom.

I would have started with tubing, already highly polished ID and OD, and pressed a plug into the bottom end that had been machined to accept the needle.

However even under light and mag, I found no evidence of that. It's possible to have been designed in such a way that there are no visible seams, but I wouldn't know for sure unless I destructively disassembled one. I've got a parts carb, so I may do just that, but it won't be for at least a week. If there are multiple parts, it would open up a whole new family of explanations and solutions.

BTW - This won't help any, but here's the other thread I was thinking about where there was a guy saying that he kept losing damper oil. As far as I know, he never reached resolution either:

http://www.classiczcars.com/forums/thread43034.html

[Jetaway]

May have to chalk it up to mysteries of the universe, but I'll persist.

Next time I need to add oil, I won't, and I'll peer down into the chamber to see what I can see.

A few people have posted that there shouldn't be any oil consumption. Which would be fine by me, but the FSM states that the "damper oil should be checked approximately every" 3,000 miles or every 3 months. That certainly implies that there is some path or mechanism by which the oil is depleted. But that's it. Nothing about what constitutes excessive or normal oil usage or what to investigate if consumption is high as far as I can tell.

Chris

[Captain Obvious]

You're right about universal mysteries. I'm not quite ready to give up on this one though. Not until I cut one open. :ogre:

I've seen that same note in the FSM about checking the damper oil, and I agree about the implication. I'm thinking that it's just prudent to check it every now and again. Shouldn't have to add any, but still prudent just to check?

Perhaps oils in the past were a little more volatile than they are now? Maybe there's a tiny bit of evaporation when hot, and maybe that evaporation used to be more prevalent in the past with less sophisticated additives? Just musing ideas...

[Bruce Palmer]

My oh my, some of these threads do drag on....... Near as I can tell y'all have reinvented the damper oil wheel about three times. Just saying.

Keep going if you think there is at least unresolved issue out there.

[Jetaway]

My oh my, some of these threads do drag on....... Near as I can tell y'all have reinvented the damper oil wheel about three times. Just saying.

Keep going if you think there is at least unresolved issue out there.

Maybe I missed it, but the only definitive "cause" for high carb oil consumption I recall reading was overfilling. No, that is not the cause in my case as I have not and am not overfilling the carbs with oil.

Chris

[Bruce Palmer]

It all depends on your definition of high oil consumption. Filling to the top of the inner tube is not over filling like when guys fill to the top of the threads..... that's over fiilling.....

If you fill to the top of the inner tube today, put the dipstick back in and drive the car, take the dipstick out tomorrow, you can refill the inner tube to the top again. You can do that day after day after day because the puck will displace it's volume of oil every time you reinstall the dipstick. Beyond that, I dare say, you can do that once every six months. How do I know?

This is one of those "over the top" conversations and by that I mean the only place for the oil to go is "over the top" of the inner tube...... Now that's funny I don't care who ya are......

[Captain Obvious]

I took apart a reservoir tube from a flat top and verified that it is made up of multiple pieces. Unfortunately, I don't have a round top piston that I'm willing to autopsy, so I cannot definitively say that the round tops are multiple pieces as well, but after seeing how they did the flat tops, I bet that the round tops are multiple pieces too.

I can now theorize possible fluid loss through the seams between multiple pieces plugging the bottom of the reservoir tube.

So, if you are in fact losing oil and it's not your imagination, that may be where it's going?

Here's some pics of what I did. Flat top piston on left, round top on right:

Bottom of flat top. You can clearly see the seam line between multiple pieces:

Bottom of round top. There is no seam, but the diameter is smaller than the OD of the tube. In fact, it is the same as the ID:

A few seconds on the arbor press, and the flat top tube is out and apart:

Anyone have a beyond help round top piston that they want to donate to the cause?

[Jetaway]

Thank you, Captain Obvious. At last a possible explanation.

I haven't posted recently simply because I haven't driven the Z much. Got out for a couple hours on Friday and now have put on 600 miles since my last post and oil fill. Yes, it is low and needs filling.

Bruce -- I'm sure you know how a Z runs without oil in its carbs and how much smoother and more powerfully it runs out with oil. Well, I'm also aware of the difference and while I haven't tried overfilling, letting the oil return to 'full,' and overfilling again, sincerely doubt that it has a noticeable effect on engine performance.

Chris

[Bruce Palmer]

Okay, help me out here. You fill the inner tube with ATF or whatever your juice of choice is and you insert the puck on the end of the dipstick. The puck then displaces it's volume of the liquid of choice returning the oil in the tube to the top right? You then go drive and come back and check the level again and it's down a touch, right? My question is this: As long as the puck is submerged in the oil a ways down the tube, what more needs to be accomplished for the shock absorbing to happen. It's puck, oil and sidewall clearance to the tube. As long as the puck is in oil when the piston is down that's all you need to be concerend with. If you want to keep filling the tube to the top then the engine will keep sucking some fraction off the top of the tube lowering the level. All the work is taking place down in the oil at puck level.

Now once that level gets down to where that minimal vacuum up in the top end of the dome won't suck any more out I'll bet you could drive for six months without adding any oil. Want to know how I know? My wagon is daily driven and I add some ATf about twice a year and I think it's more head games than necessary.

[Captain Obvious]

Bruce, Let me see if I can help.

What's going on here is that he fills the dampers up to (and not above) the fill-to line scribed into the stalk, drives around for just a couple hundred miles and then checks the levels again only to find that the level has dropped significantly below the fill-to line.

He has stated that he is not overfilling the tubes to capacity and then displacing fluid up and over the top when he puts the stalks in.

What makes the situation interesting enough to warrant two pages of forum is that he is not overfilling the dampers and then wondering where the oil went... He's filling them to the correct level and then wondering where the oil went.