Relocating Oil tank into Fuel tank

B

BrapNewb

New Member
Hey,


I am looking to relocate the oil tank of a '78 CB750F into the rear portion of the fuel tank. I read this once as an off handed comment in the SOHC forum and have heard this done on choppers.


Thinking that I can take a band saw down the rear portion, replicate original tank assembly in rear portion, have a area sectioned and welded back on. It sounds easy enough right, haha. Trying to do lots more research before diving in.


Haven't really found any threads attempting this. Any helpful info? Do's and Don'ts? Thoughts?


Thanks!
 
I remember there was some talk about this.....the only detail that stuck out for me was the concern of the oil getting so hot near gasoline.
 
Ok, after some more reading I don't think autoignition of the fuel would be a problem. I looked up the MSDS on several fuels and they all have an autoignition around 500F. Also, if I built two bulkheads to create an air gap between the two tanks then it would buffer the heat transfer, if needed.


I've read that the oil max temp will max out around 275F (http://forums.sohc4.net/index.php?topic=38253.0) after long and hard riding. And, the oil averages 95F during the summer. Can you guys confirm these numbers on oil temps?


If, these are about right then how would these temps affect the paint on the tank?
 
Colorado Norton Works did that (stashed the batt, too) on a Commando a few years back and I've been curious about it ever since. Made for a clean looking scoot. Will be keeping an eye on this for sure.

 
I've always wanted to do this for someone--not having a CB750 and all. How I designed it in my head was to cut the bottom off the tank. Just use a flap disk to carefully sand the welds down. Once the upper and lower is split, the job should actually be quite easy.

What I would do is weld a square 'box' to the bottom of the fuel tank. That way, you can run the oil lines up from the bottom. Once this is welded and tested for leaks--leaks would be VERY BAD--you should also apply some pressure. Will with a water and dish soap mix and pump it up a few PSI. I say this because the oil COULD gain pressure as it heats.

On the top of the 'box', that's where you put the filler tube--just a 1" or so tube. Then, cut a hole in the top half and let the tube pass through. Weld the top to the bottom as well as a filler bung/cap. Make sure the cap can vent.

As far as heating up the fuel, I personally would not worry so much about it. The fuel will help cool it. If you're really worried, you could always run an oil cooler. It's a cool project that you should have a go with.

--Cheers, Chris
 
I don't think it would hurt the paint but you wouldn't be able to ride the bike very far at a time until you had to stop for hours to let it cool down. It might not be too bad in the winter, might be nice actually, but in the summer you won't be able to ride the thing.
 
maybe put the oil in the front away from your legs, and use an oil cooler mounted to the front frame legs. I like the idea of mounting all of the things to the frame and then having the "tank" be just a cover for the oil tank and fuel tank, this also may help with heat dispersion, and you could make the oil tank portion out of aluminum.
 
Theres been a few bikes that have done something similar. Sorry but Im being lazy right now and cant remember where I saw them. Heat really is the big issue. You would be superheating your fuel. Not hot enough to ignite, but you dont want hot fuel from a performace standpoint. And, as stated above, you dont want to be touching that section of the tank.
Another issue, is cooling of the oil. There would be almost no real potent airflow reaching that section the tank for cooling, and with such short feed and return lines, theres no cooling happening there either. The oil really needs to be able to dissipate heat to assist in keeping engine temps down.
Another idea would be to make a nice aluminum tank that could be mounted under the motor. Or even down near the swingarm pivot.
 
I had a T100 many years ago with a GRP tank with two compartments. Fuel in the front and oil to keep the family jewels warm. being fiberglass heat was not an issue even in summer and I rode that thing for hours on end.

Steel is a different proposition, so I'd insert an oil tank from below so that it's surrounded by fuel or air inside the fuel tank.

It needs to be larger than stock to get rid of heat.
 
Just a side note, many old Harley's had split fuel tanks where one side held gas and the other side was the oil tank.
 
Sounds do-able but would sure make for a lot more fuel stops to fill up with gas. I assume it would cut the miles per tank in half, at least............
 
Do you think the gas tank is thin enough to still provide enough productive heat transfer if, the oil tank was placed at the front of the tank?


Kong-- I have been looking at those big twin tanks for choppers and like the concept. Naturally, the tank would still be in one piece but with the front right or left section as an oil tank like the Benjie's Killsaki tank.


sakigas2.JPG.jpg



But, I would still want the tank to look stock, except for twin fuel/oil caps.


VonYinzer-- What if I increased the capacity to about 3.5 quarts? Pair this with an almost doubled length of the oil lines. I could add an oil cooling system too? Also, if I was thinking about creating a double bulkhead to create an air gap to decrease thermal conductivity, any thoughts?


Thanks for all the comments! I am going to sketch up some examples and I will post them to further help explain.
 
Alright,


I have been talking with several fabricators and have lots of interest but it will cost quite a bit. So, I need to sort out a few issues before I think about dumping what little money I have.


Would this cause vapor lock? If so, wouldn't the vent tube help eliminate this problem?


Thoughts?
 
Yeah, of course you'd have to vent it. You'd also have to fill it, and hence would likely vent from the filler cap, no?


--Chris
 
When its made, buy a small junk motocross tank and cut the bung/cap off. Weld it on. Its a small spin on cap with a vent tube that you can run down to an out of the way place.
 
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