Restoring a 1973 BMW R75/5 Motorcycle

January 1, 2007

It’s a gas

Filed under: 1973, airhead, BMW, motorcycle, R75/5, restore — Penforhire @ 4:04 pm

Cleaning all the little carburetor bits is a fairly dull task. No pictures to share I’m afraid and I’m nowhere near done. But I can tell you that steel wool is my friend.

Today I also took apart some of the remaining gas tank components and they are worthy of a few pics.

The gas cap is held onto the tank with a press-fit pin very similar to the carb’s float levels. This is the underside of my cap and it looks like the sealing surface could use replacement.

1697-gas-cap-underside.jpg

The hang-up is the center piece holding the sealing disk (seems like a stacked pair of rubber disks) in place is like a rivet. I can’t tell exactly how to remove it. Clymer and Haynes have nothing to say on the subject or I missed it. I do recall Joe holding up a brand new rivet in a plastic baggie and saying something about having to drill out the old one. I’m waiting for a call back from him to confirm the technique before doing something that irreversible. Some light prying confirms it is not about to just pop off.

Here are the emblems and their gaskets from the sides of the tank.

1698-tank-emblems.jpg

They are expensive-looking hefty ceramic-and-metal pieces. I recall dire warnings about making sure the tank curvature is correct and using the gaskets, otherwise cracking and crying will ensue. I’m not sure what the best way to polish up the emblems is yet. They have a slight yellow tinge.

Here are the rubber side panels, with one flipped over to show the waffle-pattern on the backside.

dscn-tank-panels.jpg

These are just mounted with glue so I heated the tank up in the sun and gently (well, gently for me) peeled them off. They are still flexible and might be renewable if I can get some plasticizer into the outer surfaces. Otherwise, IIRC, replacements cost $20-something each and I’m trying to avoid any expense that is not a cosmetic necessity. I’m staring at enough expenses as it is.

Here is the naked tank. Note this is not the “toaster” tank (metal side-panels) that seems to be more collectible now. What I read is the toaster was not so popular at the time because it is a smaller tank, 4+ gallons instead of 6+ gallons. Note my precision. What am I, Wikipedia?

1700-naked-tank.jpg

I didn’t mention it before but, after pulling off the fuel hoses all you have to do to remove the tank is unscrew two wing-nuts at the rear (under the seat) and pull the tank back and up, off a rubber bumper-thingie in front. I haven’t decided if I need to replace that rubber mount or the rubber under the wing-nuts. Their condition is okay now but the effort to get back to them after I reassemble everything means I really should replace the soft bits.

Here is a close-up of the tank’s filler neck.

1702-tank-filler-corrosion.jpg

Yep, we’ve got crud. The factory lines the tank with a red-colored coating but that breaks down over time. I couldn’t get a good focus inside to show you but, yep again, the coating is going away and there were red bits captured in the carb strainers. This brings me to a variety of restoration methods people have used.

I read about putting a load of nails in the tank and shaking it like you’re playing the tambourine at a dance marathon. That doesn’t sound too appealing. Too much work. I also read about putting some lengths of chain in the tank, wrapping the tank in some linens, and tumbling it in a clothes dryer (no heat). Now THAT sounds like a winner. I do still see a couple of issues.

I emptied the tank a couple of days ago and set it in the sun a few times but I still smell gas. Just how smart would it be to tumble gas fumes in a dryer? I’m thinking I could rinse the tank with alcohol and that will evaporate more thoroughly. Any thoughts? Issue number two is what happens when my wife notices me tumbing a gas tank in the dryer with some of her favorite towels? It doesn’t matter which linens I pick, they are likely to be her favorites. I’ll work on a solution.

Lastly, I have to decide on what internal coating to seal the tank with. There are at least three available systems: Kreem, POR-15, and KBS. Those are just the ones I found in a quick search. I have no idea which is better than the others (I’ve heard of Kreem before, and I don’t just mean Eric Clapton). I call them “systems” because they come with multiple fluids to prepare the tank surface before the final coating. That final coating has to be something like an epoxy to withstand gasoline and thermal stresses. The tank should be sealed before any external painting because the sealing chemicals can damage the outside finish very easily.

I’m not entirely sure how to close the filler neck and two petcock openings while I slosh chemicals around inside. You can’t use the actual petcocks because they’ll get gummed up by the sealer if it gets in there. I’m thinking aluminum foil backed up with saran wrap, held on with rubber bands. Any better ideas?

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2 Comments »

  1. To get the old seal off the fuel cap, just cut it off, and you can slip a new on on without any fuss. No need to take apart the cap.
    The only thing that will get the inside of your tank clean is muriatic acid. You can buy a gallon for about $5. Use it full strength and slosh it around. It will get all the old red liner out as well as any rust. It may take a few hours. Rinse with water, then you need to coat the inside of the tank with a surface prep, (Metal Ready from POR-15 company works the best). Now, you need to re-line the tank with whatever you decide to use. DO NOT use KREEM. POR-15 sells a very good silver colored liner, and there are other epoxies available.
    You can buy little rubber stoppers for pennies at the hardware store to seal off the petcock opening. You don’t need to seal off the cap opening, just slosh the muriatic acid around so it gets up around the opening. It needs to vent. WHen you seal the tank you can use the rubber stoppers, and place a couple pieces of duct tape over cap opening when you slosh the epoxy around. Feel free to contact me if you have any other questions. Good luck.

    Comment by Ashley Histand — January 18, 2007 @ 6:26 pm

  2. I’ll agree with Ashley in that you do not want to use Kreem. We’re using the KBS Gold Standard tank sealer in our shop exclusively now though. They have complete do-it-yourself kits, or just buy the larger sizes of the individual preps and sealers as we do, it’s more economical. There are other products such as Redkote, but they don’t hold up in extreme temperature conditions. Just follow the instructions and don’t skip the preps, as inside the tank, you’ll rely on the chemicals to do the trick before the sealer. Remember one man’s sealing is another man’s nightmare.

    Comment by Mike Walsh — February 4, 2007 @ 9:12 am


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