Mercruiser Manifold Riser Drain size

[sam.thompson]

Does anyone know the size of the drain plug for the Riser on the old Mercruiser inline 6 manifolds?
Was going to use my 3/8 tap to clean it out, but the hole seems too large for the tap.

I was lead to believe it's 3/8s just like the drain on the bottom of the manifold itself, But I'm not so sure about that now.

Anyone?
Thanks...

 

[81 Checkmate]

Re: Mercruiser Manifold Riser Drain size

Its probley NPT thread... National Pipe Thread...which is tapered threads for pipe fittings or pipe plugs!

 

[Don S]

Re: Mercruiser Manifold Riser Drain size

1/4" NPT thread.

Being tapered, do not go too deep when cleaning the threads. Go just enough to clean the threads.
This is why after draining you should put the plugs back in. Then the threads won't get all rusty. A bit of gasket sealer also prevents rust and help seal so you don't have to over tighten them.

 

[sam.thompson]

Re: Mercruiser Manifold Riser Drain size

Can't be 1/4 NPT because a 3/8 slides through the hole with no grab. So it's not even 3/8.

Possibly 7/16?

 

[sam.thompson]

Re: Mercruiser Manifold Riser Drain size

It's an older motor. Mid 70s

 

[Don S]

Re: Mercruiser Manifold Riser Drain size

Can't be 1/4 NPT because a 3/8 slides through the hole with no grab. So it's not even 3/8.

Possibly 7/16?

Yes it can be 1/4" NPT. To tap a hole for 1/4"NPT thread, you first drill a 7/16" hole.
NPT- National Pipe Thread Taper- ANSI B1.20.1

The 1/4" indicates the inside diameter of a pipe, not the thread diameter.

It's a tapered thread, DO NOT try to use a regular tap for bolts, they are not the same.

 

[dan t.]

Re: Mercruiser Manifold Riser Drain size

Good link Don, will put that one in favorites for sure.