leaky fiberglass tank

Not open for further replies.


I found the source of the peeling paint on my new bike's (71 750 roadster) tank. There's a crack about an inch long at the very front where the tank wraps around the steering head. Gas flows pretty freely thru it when it sloshes up there. Can this be repaired or am I looking at a new tank? If it can be repaired does anyone have a recommendation on where to send it? Don't think I want to try repairing it myself.

Oh, and if I do have to replace the tank - will a steel one fit my bike?

It's repairable, how much work it would be depends on where exactly it is, all for cosmetic reasons. If the crack is at all on visible areas of the tank you'll have to deal with getting a smooth surface to paint on again. If it's all underneath then you can be a bit sloppy. In order to repair it right you end up grinding out a bit of the glass and cutting a "V" into it right where the crack it. (without widening the crack at the bottom of the "V"). Then you lay fiberglass resin and sheet's of fiberglass "patch" over the ground out area. Finish it all off with a thick coat of the resin and you can sand it smooth. **Note this is a HIGHLY simplified synopsis of what goes on, but since your not doing it yourself I wont bore anyone with the details. "BONDO" makes a fiberglass repair kit that's not bad if you do decide to try it yourself.

Any shop that does fiberglass work could help you with it if they're willing. Try a Marina or any boat repair shop, they'll have a good glass man/woman on hand. After it's repaired I would definatly coat the interior, the new area of glass that could come in contact with the fuel will most likely not react well with modern gas.

Or just replace it with the steel one, it'll fit.
fiberglass tanks


Getting into a Commando new to you can be a headache with problems not resolve by the previous owner. My '72 roadst has a fiberglass tank that I've repaired many times.

It's unfortunate someone painted your fiberglass tank. Originally the color was in the gel coat and gas leaks can't hurt it. The biggest cost for you will be having the tank repainted. In that case it might be less expensive to find a steel tank replacement that has good paint.

When there's a leak it's always at the joint between the upper shell and the lower pan which is what you have. I use a Dremel grinder with a round burr. Cut into the resin along the crack and go in about a 1/4 inch. Try not to cut into the upper shell if you can avoid it. Rather than a "V" shape cut, you want a groove that has some undercut that will lock in the new resin. Glass reinforcement isn't necessary if this is just a hair line crack. Mix up some polyester fiberglass resin with the hardener and fill the groove you made. Stand the tank so that the resin doesn't flow out. Go over all the joints while your at it and seal everything. When all is cured, I suggest getting the inside sealed. Most of the sealers on the market such as POR are not very satisfactory for a glass tank. I had mine done by a local Norton nut that has some very toxic stuff that works great (always the case).
That was me posting above (forgot to log in). Hmm, sounds like maybe I could patch it myself. It's not in a visible area so I don't have to worry about the patch looking good :) One of the sidepanels has a small split that I was going to try fixing myself. Since I'll have plenty of leftover resin, I might as well try the tank repair also.

Some of the paint peeled off the top of the tank but I could probably touch that up with a spray can and avoid paying hundreds of dollars for a full repaint (for now). Of course, if I run across a decent steel tank, the glass tank might be going on ebay!

David, what about the gas that's saturated the crack? Do I have to clean that out somehow?

PO maintenance issues - definitely a problem with the bike. The other big problem I have right now is the lighting; nothing works except the high beam flasher and the taillight. Everything else, including the brake lights, is dead. But I have a wiring diagram and a VOM so I can start digging into that. Not safe to ride in traffic without brake lights so that's my number one priority right now. This leak is important too. I don't feel comfortable riding it with that much gas leaking out. It's just a hairline crack but is far more than a "pinhole" as PO described it.

I guess that's how vintage bikes go. Always more problems than you think at first, always costs more time and money than you thought it would. Sure is a beautiful bike though!

Any crack, hairline or not should be reinforced. While resin will seal the hole there will always be a structural weak point in the tank, as the strength in fiberglass comes from the fibers and not the resin. Resin itself is a fairly brittle material and if a situation occurs where the tank is stuck it is far more prone to crack again if it has not been re-enforced. It may be more work, but safety comes before simplicity in my book.
Glass tanks

I agree with David on the shape of the chanel to be cut into the hair line crack. This only needs to be one eighth deep if that, but the undercut should keep this type of repair in place for your life time. More important is the ends of the crack must be found and it is good to go alittle beyond the ends and drill small one eighth holes that can be filled in during the pach stage. If gravity is not being kind to you holding the resin in place durring curring, a piece of masking tape that loosly covers the crack and is tightly pressed at the outer edges will hold the liquid in place. When mixing resin with hardner be precise if it goes off too quick it will be weak and to little it will be too slow, may take a week to harden. This repair shoudn't scare anyone. When you are done you need to line the tank with the only garranteed product on the market for fiberglass tanks. See www.hirschauto.com this product when done right will take longer to apply then the repairs because of drying times between cleaning and coats. But it will make your tank melt down free. If the new gas we have gets to your tank it will slowly melt doing great harm to your engine.norbsa
lighting failure

Hi Debbie,

I think we have the tank repair covered.

Regarding the lighting, with the high beam flasher and the tail light working it could be just a burned out bulb situation. The circuits on the Commando are fairly simple. The brown with green stripe powers all the running lights. If not bulbs look for shorts in that circuit, but first make sure that lead is connected at the ignition switch. Sometimes the female spade connector is loose and falls off.
Lighting is in better shape than I had feared. The low beam on the headlight was burned out, neither brake switch worked, and the high/low beam switch only works in the low beam setting. Front brake switch started working after I cleaned the spade lugs. Rear switch needs to be replaced. The horn doesn't work (what a surprise) but I'm not too concerned about that. Everything else works! So just a couple of cheap parts to replace :)



The left hand switch cluster should be the one with high/low headlight. This can be dismantled and the contacts cleaned, but be very careful not to lose small screws, springs and steel ball. Keep the push buttons and lever facing down removing the other plastic parts upward.

The rear brake light switch is very simple having a spring loaded metal bridge between the two spade terminals. Not much to go wrong.
I guess I shouldn't say much more, I have made a living for the last 15 years repairing and building Boat hulls and re-repairing cracks that people thought they could fix by throwing some resin in the crack. I thank you for my livleyhood. :D
Boat hulls

I only know cracks are best stopped by giving them a radias at each end. If fiber needs to be used I stand corrected. It seems like keeping this a one eighth under cut slit would work. Could fiber be used in a repair this small or would one need to open this up into a quarter inch V groove? norbsa
removing old liner

I bought the Hirsch kit so I can line my tank. But - the tank has been lined before. Problem is much of the old liner has apparently dissolved. Do I have to get the rest of that stuff out before applying the new liner? If so, what will remove it? The instructions don't talk about that at all.

Maybe I should just patch the tank and start looking for a good steel one :(


Debby, A trip to the hardware. Get one gallan of MEK and a box of 100 5/16 nuts. Pour in the MEK and nuts into the tank and shake and roll and shake and roll. be careful this stuff will bite the Glass on the inside of the glass so don't leave it sitting too long. Mike at Walridge Cycle is getting brand new Roadster tanks made of steel Depending on the price this should blow the market on old used Roadster tanks. Don't let this keep you off the road. norbsa
fiberglass tanks

Just a bit of related info on this subject; Norvil no longer will sell glass fiber tanks to the USA due to the nasty stuff in our petrol. Seems the new gasoline formula has something in it that dissolves the resin. A good long term plan might be to replace your fiberglass tank with a steel tank rather than spending a lot of time and money on it. nothing more depressing than to have your new paint job bubble up from a pinhole leak. :(
Yes the repop fiberglass tanks from Norvil is what started my whole learning curve on tank sealents. The old tanks had a much smother inter surface like an extra coat or a liner to start with. But I still use the tank with the Hirsh linner and it works fine. Hey if you have a fiberglass boat with integral tanks you have to deal with this and it is why this linner was developed. norbsa
Am *not* looking forward to the MEK ordeal. What would happen if I applied the Hirsch liner to the tank as is, without removing the old liner?? It hasn't come loose, it's just dissolving.

I know, dumb question. Just hoping to avoid a nasty job...

fiberglass tank

Hey Scooter,

It's on my inevitable list to replace the fiberglass tank on my '72, but I like the fiberlass tank. It can't dent, the color is in the gel coat (signal red) and it doesn't have any weld seams to spoil it's shape. Oh yea and the gas cap is hinged at the front which make much more sense than at the back like all steel tanks.

What I'm doing to delay having to get a steel tank now that California has been mandated by Mr Bush to using alcohol in our gas...I had the tank cleaned and lined with some very toxic substance that is supposed to be better than anything anyone else has...we'll see.
Debby, The first coat of the Hirsh coat knocked all kinds of crud out of my tank and that was after a hard cleaning. It's the melting thing that should concearn you. That stuff locks carb slides and does a job on valves and guides as well. MEK is the medicine sorry. norbsa
I'm not sure now if my tank has been lined. The half-melted stuff I thought was old liner is orange in color and is thin and hard, like resin. Was looking at a glass tank on ebay that the owner claimed was "like new inside". It had the same orange liner (but not melted). I thought the liners were usually white with a rubbery texture? Maybe I shouldn't try to remove this stuff after all???

Deb, The color you discribe would be the color of the resin used in origainal tanks. You would still need to use MEK to clean it but you don't need to strip it out. Good luck norbsa
Not open for further replies.