Interstate Tank Variations

This started when yesterday I put 6.83 US gallons in my Interstate tank. I had always assumed from the literature that the capacity was 6.6 US gallons. So I did some research and it seems there were at least three different tanks and possibly six. And that doesn't take into account any aftermarket ones. I knew there was the fiberglass ones and assumed they were less capacity due to the thickness of the material. Even on this site there seems to be some variation all the way up to 7.3 US gal! Maybe we can settle this once and for all.

Maybe some measurements will help. Feel free to post any measurements you can provide along with the model year and if you know the tank is original. If you have a 72 be sure to indicate the material. For other years we'll assume steel unless you note otherwise. You don't need to post pictures unless you want to, I've provided them for reference. You can also include the maximum fuel you have filled, this won't be actual capacity unless you filled from dry.

I'll go first. 1973 and assume original.

Rear width = 10in
Front width (just behind steering stem) = 12in
Maximun width = 14in
Length from steering stem to back = 22in
Height not including seam at bottom = 9.5in
6.83 US gallons/25.8L

1974 steel:
Rear width = 9 1/2in
Front width (just behind steering stem) = 10 1/2in
Maximun width = 13in
Length from steering stem to back = 22in
Haven't measured capacity yet

One other measurement you might want to consider is the height. I understand that the early tanks were taller than the later tanks. I have a couple steel I/S tanks and a glass one that I'll try to get some measurements on tomorrow.

robs ss said:

1974 steel:
Rear width = 9 1/2in
Front width (just behind steering stem) = 10 1/2in
Maximun width = 13in
Length from steering stem to back = 22in
Haven't measured capacity yet

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I suspect those numbers are close enough to mine. I didn’t exactly put calipers to it.

Ron L said:

One other measurement you might want to consider is the height. I understand that the early tanks were taller than the later tanks. I have a couple steel I/S tanks and a glass one that I'll try to get some measurements on tomorrow.

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I’ll see if I can eyeball an overall height.

Mine is also original 73. Measures the same. Never filled it from empty with anything that would measure the amount.
the riders manual says 7.3 US gallons.

Bill C. said:

Mine is also original 73. Measures the same. Never filled it from empty with anything that would measure the amount.
the riders manual says 7.3 US gallons.

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When there are variations that aren’t noted in the documentation is the challenge. For example what year is your Riders Manual from? I saw one on the web that just said from engine 220000-on. Which should include the 72 fiberglass tanks? And it did say 7.3 US gal. Whereas some manuals cover through 75 and will say 6.6 US Gal.

This is why Matt at Colorado Norton Works stopped supplying Corbin seats for Interstates, for which he had the exclusive agency. 5 or 6 different Interstate tanks that he identified.

Dave, can you remove your seat and show a pic of the rear of the tank? I have two IS tank of different sizes, the rear ends are very different. I’ll get some measurements tomorrow.

swooshdave said:

When there are variations that aren’t noted in the documentation is the challenge. For example what year is your Riders Manual from? I saw one on the web that just said from engine 220000-on. Which should include the 72 fiberglass tanks? And it did say 7.3 US gal. Whereas some manuals cover through 75 and will say 6.6 US Gal.

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The riders manual I have is a reprint for a MK1. I have always been under the impression that capacity was about right. I could certainly be mistaken. I don't know if that would be absolute capacity or if it allows for air space. Be interesting to check sometime.

Bill C. said:

The riders manual I have is a reprint for a MK1. I have always been under the impression that capacity was about right. I could certainly be mistaken. I don't know if that would be absolute capacity or if it allows for air space. Be interesting to check sometime.

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Ok, that’s the right one. There are so many variations.

I’m guessing no air space.

Fullauto said:

This is why Matt at Colorado Norton Works stopped supplying Corbin seats for Interstates, for which he had the exclusive agency. 5 or 6 different Interstate tanks that he identified.

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I know @cNw is busy but maybe he can share some insights when he gets a chance.

Interbak said:

Dave, can you remove your seat and show a pic of the rear of the tank? I have two IS tank of different sizes, the rear ends are very different. I’ll get some measurements tomorrow.

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Mine looks exactly as Dave's. Tank cap flips rearward. Tank is a tight fit on the frame.
Bought it separately from the bike so don't have an exact way to date it.

Here‘s a comparison of two sizes. The blue tank measures 8” at the front, above the tunnel, 5.5” mid way back, 9” across at the back. Length from top of tunnel front to rear is 23.5” depth of 9”.
the black one is 9”, 7”, 10”, 23.5” and 9.5” for the same measurement.
My custom seat would work with either tank.
i don’t know which years either tank are from, got them both separate from any bikes.

Fullauto said:

This is why Matt at Colorado Norton Works stopped supplying Corbin seats for Interstates, for which he had the exclusive agency. 5 or 6 different Interstate tanks that he identified.

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At least Matt found Norton tanks were consistent (by being inconsistent)

Interbak said:

Here‘s a comparison of two sizes. The blue tank measures 8” at the front, above the tunnel, 5.5” mid way back, 9” across at the back. Length from top of tunnel front to rear is 23.5” depth of 9”.
the black one is 9”, 7”, 10”, 23.5” and 9.5” for the same measurement.
My custom seat would work with either tank.
i don’t know which years either tank are from, got them both separate from any bikes.

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You said they were very different across the back. I assume you mean the black one is wider by an inch?

After direct comparison they are very similar, except for width. Going from memory, I had thought the wider one had a deeper more pronounced tunnel, it’s actually almost identical. I use heavy foam on the frame rather than rubber pads in the tank tunnel, you can see it sticking out the back. The larger tank has a much tighter fit on the foam, I didn’t measure how much narrower it is than the blue tank, but I’d guess at least 1/4”.

Interbak said:

After direct comparison they are very similar, except for width. Going from memory, I had thought the wider one had a deeper more pronounced tunnel, it’s actually almost identical. I use heavy foam on the frame rather than rubber pads in the tank tunnel, you can see it sticking out the back. The larger tank has a much tighter fit on the foam, I didn’t measure how much narrower it is than the blue tank, but I’d guess at least 1/4”.

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Which one do you prefer? I like the shape of the blue one.

Blue one is more comfortable, being slimmer. I’ve been running it for over 30 years and the black one just looks too big in comparison.