Author Topic: 6.9 oil tank hoses and related issues  (Read 178 times)

raueda1

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6.9 oil tank hoses and related issues
« on: 09 September 2019, 06:10 PM »
In the course of my ongoing mega-project these hoses will be replaced.  My intent is to replace the hoses and reuse fittings. Fittings are in good shape.  The question is hose size.  There are 3 hoses from the sump to the tank.  I measured the hoses very carefully on 2 different cars and came up with:
    large     = 20mm ID, 30mm OD
    medium = 16mm ID, 26mm OD
    small     = 13mm ID, 23mm OD

It seems that the closest replacement hoses exactly match the OD but all have 2mm larger ID.  For example, the respective hoses from Belmetric.com https://www.belmetric.com/smooth-high-pressure-oem-c-14_139/
are 22/30.5 ID/OD, 18/26 ID/OD and 15/23 ID/OD.  In all cases the ID is 2mm bigger than what I measured.  Interestingly, these exact same size hoses appear on various air-cooled Porsche sites for the old dry sump 911's.  I greatly doubt that this is purely coincidence.

It's also worth noting that the OD of the tapered, threaded fitting tube corresponds exactly with the replacement hose ID within about 0.5mm.  This means that the fitting compression is coming from the outer aluminum sleeve, not the tapered tube.  I'm very confident of my measurements which were repeated 10 times to 0.02mm and averaged.   I'd also expect some swelling from long term hot oil exposure, so it's entirely possible (or probable IMO) that the ID difference between the new hoses and the old is from oil swell.  Even NBR rubber swells a little - though it's a little suspicious that it's the same 2mm in all cases.   

So, the question:  do I go ahead and use these replacement hoses despite the 2mm ID difference?  I'm pretty convinced that the replacements identified are fine to use.  However, given some of my recent blunders, I've developed a sense of self doubt. :o   What do you guys think? 

[And a few sidebar comments: 
First, these fittings are an absolute bitch to get off.  I doubt it could be done without removing the oil tank.  I also cut the hoses with a hacksaw (at which point you're committed, not turning back!).  The trick was to use a torch and get the nut very VERY hot. 

Second, I noted that in several cases that the hose and outer sleeve could be easily unscrewed together by hand, right off the tapered tube.  The threads in the sleeve are apparently deep enough to keep the hose from blowing off.  However, it also means that the compression seal is pretty much gone because of rubber compression set.   You can't tell any of this by looking and there's nothing to tighten up if there's a leak.  By their design the fittings are as tight as they'll even be when they're assembled.  If anybody is running around on original oil hoses it would be prudent to change them before something bad happens.

Finally, if you've never had the oil tank out of the car it's probably worth doing.  The shape allows road dirt and debris to collect on a depression on the tank behind the wheel well.  You can't see it, reach it or easily hose it out.  Mine had a large deposit of dirt that had obviously held moisture.  The steel underneath the dirt was very pitted, too deep to grind off.  It's clear that sooner of later the tank would leak.  Fortunately I had a better tank as a backup.]
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

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Re: 6.9 oil tank hoses and related issues
« Reply #1 on: 09 September 2019, 06:19 PM »
Dave yes indeed I can second you points on removing the tank for a look-see.  IMHO not the best design and location, but I guess that was really the only place to mount that big-ass tank!  It really does get caked in grime and the rust devil has a field day.

Also those nuts are probably the hardest thing to get loose on a 6.9, only after the front hydro ball joints :o
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

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Re: 6.9 oil tank hoses and related issues
« Reply #2 on: 09 September 2019, 07:24 PM »
Dave yes indeed I can second you points on removing the tank for a look-see.  IMHO not the best design and location, but I guess that was really the only place to mount that big-ass tank!  It really does get caked in grime and the rust devil has a field day.

Also those nuts are probably the hardest thing to get loose on a 6.9, only after the front hydro ball joints :o
LOL, I just needed to gin up my courage to heat until the hoses started smouldering.  Hoses were trashed anyway but it just seemed so....  extreme.  But a blowtorch is your friend. 
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Randys01

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Re: 6.9 oil tank hoses and related issues
« Reply #3 on: 10 September 2019, 05:40 AM »
 I don't see an issue with the larger ID hose. In broad principle terms, the oil pump's performance is to some degree compromised by frictional losses on the suction side. The greater the bore, all other things being qual, frictional losses will be less.  On the scavenge side the pump would prefer to be discharging into a 44 gallon drum, so again, a small increase in bore makes for a gnat's whisker of less back pressure.
The oil pressure being delivered to the engine assumes a pump design that cannot pump more than it  sucks. It was designed with a positive supply margin and now it has a bit more. The return or scavenge is only returning  oil that has done its thing and fallen into the sump. This side of things may well be engineered to move volume but not necessarily at any great pressure.

raueda1

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Re: 6.9 oil tank hoses and related issues
« Reply #4 on: 10 September 2019, 08:39 AM »
All good, thanks gentlemen.  Hoses were ordered.  1 meter is minimum order which is well over 2x what's needed.  If anybody wants to do this delightful project and needs hose please PM me and we can make arrangements.  Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0