# Gas tank swap



## Rick Loncosky (Dec 14, 2002)

I was under a C30 in the junk yard nearby getting the gas tank switching valve for my 77K20. I'm planning on running dual tanks. My other tank is coming off an 81 C10. The tanks on the C30 look longer, Are they? Wow what a rear axle this thing has under it. I popped off the hubcap and it's a full floater 10 1/2". Would this be a direct bolt in my 77 ? What kind of brakes does this thing have, I saw no vacumn booster but some kind of hydraulic pump or ? behind the master cylinder. Sorry for so many questions but the size of that center section says HEAVY DUTY all over it and I think it would look better under my truck than in the yard. Thanks, Rick.


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## Chuck Smith (Dec 30, 1999)

Your K20 and the C30 should have the same 20 gallon tank. THe short beds used a 16 gallon tank. I suppose it is possible you have a 16, but unlikely. Gas tanks are the same from 73 - 81, so make sure it is an 81 you get the donor from.

Something to keep in mind is there are numerous sending units, and a few different valves. Not all of them will work properly with the gauge in your dash.

There are sending units with 1 outlet, 2 outlets, and 3 outlets.

My 77 K20 had a 3 outlet. One feeds the carb, one is a return, and one is a vent. The vent goes to the atmosphere on HD emissions models, and to the charcoal cannister under the hood on normal emissions models.

Just leafing through an LMC catalog, I don't see the switching valves, but I know there are a few different ones, to coincide with the various outlets of the sending units. IF I am not mistaken* on my 77 it was a 7 outlet switching valve.

As far as the dually 14 bolt, the swap has been discussed in detail here a few times, even recently.

~Chuck


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## wyldman (Jan 18, 2001)

The gas tanks will still fit even if they are longer,just gives you a bigger tank.

I have seen the smaller tank on long box models,mostly V6's,so make sure you get the big one.

If you can get the single outlet senders.Less to hook up.Only one line,straight to fuel pump (or switching valve if running dual tanks).Then you only need a three outlet switching valve,which is cheap.If your existing truck uses a return line,just block it off at the fuel pump.

The brake system you saw one the one ton is a hydroboost system.They use hydraulic pressure to assist braking instead of vacuum on the smaller trucks.Doesn't make a difference to the rear brakes at all,they work the same.That rear should swap into your truck just fine.The only change may be the driveshaft yoke.

Does your 77 K20 not already have a 14 bolt FF ? If not,what does it have ?


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## Rick Loncosky (Dec 14, 2002)

My 77 does have a 10 1/2 FF rear. The one in the C30 looks different, it has a finned center section and just looks bigger. I see that on this rear the "U" bolts are in the opposite direction, no big deal I guess. I was wondering if the internals of the rear changed because I have a posi to install. I'm going back today and drop the left tank to get the wire harness out for the switching valve. I did see that there are 3 lines to each tank and mine only have 2. The only thing that happened is that the wires from the switch in the dash were cut under the hood when the engine was removed. You know how those guys are in the yards, heat wrench & bolt cutters and it's out. I don't know where the few wires went under the hood . Chuck what's different on the 82 and later tanks, the saddle locations? Thanks Guys!


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## wyldman (Jan 18, 2001)

The only difference between the older tanks 73-81 and the newer ones 82-86 (and 87-91 R\V series) is the size of the filler pipe inlet.

Older tanks were 1 1/4" ID and the newer ones are 1 5/8".Everything else like saddles,straps,senders (except for # of pipes),are all the same.Sending unit resistances did not change either so any unit will work with your gauge.

If you can find a tank out of a newer one,then all you will need to do is get the matching filler neck and hose with it,or make some kind of adapter to make your filler neck fit onto the newer tank.

The wiring is pretty easy to do,you can even use a generic switch if you wish.When you go to wire it up,let me know,and I'll draw you a wiring diagram and explain how to do it.

If your going to run the 3 outlet senders,then one is fuel to pump (3/8"),one is return (5/16"),and the small one (1/4") is a vent.You should have the correct switching valve to make it work,but if you don't you can still hook it up to a switching valve will fewer ports.You will have to block off a few lines to do so.

You might want to check and see if your fuel pump has a return line on it,so you will know.Let me know what you have on your truck,and what all you pick up from the salvage yard,and I'll explain how to make it all work.


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## Rick Loncosky (Dec 14, 2002)

Thanks so much for your willingness to help Chris. I'm in the process of a body off resto- so, it will be about 2nd week of July. I almost have the rears out, then sand blast & paint, IF IT WILL EVER STOPS RAINING, I'll be able to continue. Later, Rick.................:salute:


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## wyldman (Jan 18, 2001)

Sounds like a fun project Rick.Hope the rain lets up for you.If you need any more info when you start to put it all back together let me know.

One other thing you may want to look into is a kit which allows you to fill both tanks at the same time.It runs a crossover pipe between the filler necks so you only have to fill from one side.Saves moving over or turning around at the gas pumps.It will also save you from adding a second fuel door if your box doesn't have one already,or will give you other option of filling from either one side or the other (nice when the pumps are busy).


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## Chuck Smith (Dec 30, 1999)

On 1977 models, there is no fuel door. Unless I am mistaken, the fuel door appeared in 1978. The filler neck comes right through the body, with a flush fitting gas cap. My 77 Chevy has no doors (though it does now with a newer bed) and my 80 GMC has the door.

~Chuck


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