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LM318 exhaust manifold drain plug thread specs?

D

DaleWindle

New member
Hi all. I have a 1984 Silverton 31C with twin LM318s.

One of the drain plugs on the exhaust manifold (underneath, front bottom end) has a cross threaded drain plug. So I need to tap it with a tap to create new clean threads.

I got a 27 NPT thread drain plug from my boat yard to go in, but am not convinced it's the right thread. I want to match the original threading and make sure I get the right threaded plug as well.

Can anyone help with that thread spec?

Many thanks!

Dale
 
This applies to osco log type only, if its the smaller one that threads directly into the exhaust port, its 1/8 pipe thread. if its the larger one for water I believe its a 1/2" pipe thread.

will
 
buick430 said:
This applies to osco log type only, if its the smaller one that threads directly into the exhaust port, its 1/8 pipe thread. if its the larger one for water I believe its a 1/2" pipe thread.

will
Click to expand...

Correct. It is the 1/8. But I need to know if the original is a 27 NPT thread (fine thread) or and 18 NPT thread (course thread).

Thanks!
 
DaleWindle said:
Correct. It is the 1/8. But I need to know if the original is a 27 NPT thread (fine thread) or and 18 NPT thread (course thread).

Thanks!
Click to expand...

Ayuh,.... Pipe thread is pipe thread, there's No fine, 'n course,....

I believe 1/8" is 27tpi,....
 
Actually, there is a 27NPT thread which is a fine thread and there is a 18NPT which much courser that the 27. There actually IS a course and a fine. In this case I eventually did find from another source this drain plug is a 27 thread and the threading is tapered, hence the NPT as opposed to NPS, which straight, like a bolt. But thanks anyways.
 
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