Exhaust manifiold plugs on 305 Chevy


Old 10-07-02, 07:49 AM
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Exhaust manifiold plugs on 305 Chevy

Can any one tell me what the thread designation is for the flared
tube ports for the air pump tubes. I want to fix this the right way,
with brass plugs. These fittings are not a std pipe thread. The air
injection tubes were already gone when I bought the motor.
I know this is illegal for street use, but this motor is going to be
swapped into an old Jeep that is used off-road only. This is an
'86 Chevy 305. I have the same engine in my truck, but I would
hate to have to take the tubes loose to check the fitting size.
I don't think they would come off without destroying them any

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Old 10-07-02, 09:00 AM
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Probably 1/2" or 3/8". Just ask the parts store to show you an air tube for that application. Then, match a pipe plug up to the appropriate thread size of the tube.
Old 10-07-02, 01:19 PM
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Thanks Joe. I'll try that. I have tried a handfull of different
size brass pipe plugs, but to no avail. The thread in the
manifold seems a lot finer than a std pipe thd.
Old 10-07-02, 07:52 PM
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Alternatively, you could screw in the new tubes, and then cut them off and crimp them/weld them closed. Kinda hokey, but would "work".

My vote is for finding out the thread by using a new tube as a guide and then finding the right plug. Dorman/Motormite/R&B is usually good at handling things like this. Most parts stores carry their AIR pipe line and emission controls. They are the red "HELP" packages you see in most parts stores.
Old 10-07-02, 07:58 PM
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Hi rgrogan!

I have encountered your situation myself before. The threads are odd...not any standard pipe thread. I have heard of plugs made for this purpose, but don't know where to get them. I have, however, seen the pipes cut, folded over a few times, and squeezed tight with a vise or hammer.
Old 10-08-02, 07:46 PM
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If it's truly off road buy headers or get older junkyard manifolds.The threads used to be standard untill the epa saw people defeating the air injection reaction system,which is what the tubes do.Same thing happenned to cat converter elimination pipes.The exhaust has bends at the inlet and outlet to discourage cat converter removal.
Old 10-09-02, 04:25 AM
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Thanks to all!
To move this project on to the next step, I bought a junk
yard air pump with brackets and tubes for $20. I don't
think I have room for the CAT on this vehicle. Is it OK to
run the air injection without tha CAT?
Old 10-09-02, 05:40 AM
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Sure. Air injection predates converters. 60's vehicles had air pumps and cats weren't standard issue until 1975.

Again, assuming it's used off road and won't see license plates, you should have no problem with any emission rules...

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