3/8 pipe thread - tapping for fuel bowl

[LodiCal]

I searched the WWW for an answer, but I am still not certain about how far I should run the tap through this 5/16" bar stock. I am replacing the fuel bowl that had 1/8" pipe thread to this new fuel bowl that has 3/8" pipe thread. Do I essentially bottom out the tap in the bar stock or should I stop short a certain amount? Thank you in advance for your assistance.

 

[true temper]

About half way or maybe a little more. Tap bit and check it, don’t go too far or it will bottom out and wont seal.

 

[Seven Gables]

I’d probably put a slide caliper on the widest threads of the bowl and stop the tap couple threads shy of that width

 

[strantor]

If you don't tap far enough, you can always run the tap back in and take it deeper. But if you go too far, there's no un-tapping. I would go no further than half way, check, and go deeper if you need to. One or 2 turns at a time.

 

[Menagerie-Manor]

Rule of thumb for pipe threads is 2-1/2 to 3 turns by hand for a correctly cut thread.

 

[oldtimer 66]

To add a little information, the original female threads may have been dry seal threads. That it what we used on all of are diesel engine components. It eliminated the need for any type of sealant on the first assembly. The tapped hole is tapper reamed before tapping using the L1 and L2 gauges. This way you have full thread engagement / crest and root/ for the length of the thread.

 

[rScotty]

I'm curious why you would not simply use a brass thread adapter?

 

[LodiCal]

Thank you for the replies! It seems pretty straight forward/"common sense" like, but I just wanted to check with people who know much more than I.

It seems that I spend 25% of my time thinking about how I should do something, 25% of my time working on stuff, another 25% of my time re-doing stuff that I worked on, 25% of my time in the garage, pump house or shed wondering what I walked in there for and 25% of my time trying to figure out why my math/measurements did come out right... ;-)

rScotty - I looked at a couple of options from welding on a 3/8" fuel bung to utilizing a thread adapter. It's an old rusted fuel tank that I am cleaning with vinegar & bolts and if I used the adapter, the screen on the fuel bowl would have to be removed. I am thinking that I need all of the screen/filter I can get. I looked for a fuel bowl with 1/8" NPT to replace the one I took off, but I was not successful in finding one that had a decent review. About 40 years ago, I could go to the small engine repair place and get a gasket kit for a fuel bowl but it seems that those days are long gone at least the two local places I checked with.

Thank you again for the replies!

 

[rScotty]

Thank you for the replies! It seems pretty straight forward/"common sense" like, but I just wanted to check with people who know much more than I.

It seems that I spend 25% of my time thinking about how I should do something, 25% of my time working on stuff, another 25% of my time re-doing stuff that I worked on, 25% of my time in the garage, pump house or shed wondering what I walked in there for and 25% of my time trying to figure out why my math/measurements did come out right... ;-)

rScotty - I looked at a couple of options from welding on a 3/8" fuel bung to utilizing a thread adapter. It's an old rusted fuel tank that I am cleaning with vinegar & bolts and if I used the adapter, the screen on the fuel bowl would have to be removed. I am thinking that I need all of the screen/filter I can get. I looked for a fuel bowl with 1/8" NPT to replace the one I took off, but I was not successful in finding one that had a decent review. About 40 years ago, I could go to the small engine repair place and get a gasket kit for a fuel bowl but it seems that those days are long gone at least the two local places I checked with.

Thank you again for the replies!

Now I know why that sounds familiar. I recall going through a similar problem a couple of years ago on our1958 John Deere 2 cylinder. BTW, that task led me to find out which gasket & thread sealers that are impervious to gasoline. Many are not.
rScotty

 

[LodiCal]

rScotty, since you brought it up, I purchased Permatex High Performance Thread Sealant for both the hydraulics and the fuel system (recommended by the Hydraulics place where I purchase my seals, fittings and hydraulic hoses).

The Permatex website states the following: "Stainless steel fittings, head bolts into through holes, oil PSI sending units/sensors, oil and coolant lines, intake manifold switches, rear axle fill plug, and fuel fittings."

What were your findings in regard to thread sealers impervious to gasoline?

Thank you,

 

[rScotty]

rScotty, since you brought it up, I purchased Permatex High Performance Thread Sealant for both the hydraulics and the fuel system (recommended by the Hydraulics place where I purchase my seals, fittings and hydraulic hoses).

The Permatex website states the following: "Stainless steel fittings, head bolts into through holes, oil PSI sending units/sensors, oil and coolant lines, intake manifold switches, rear axle fill plug, and fuel fittings."

What were your findings in regard to thread sealers impervious to gasoline?

Thank you,

I took the technical route to fuel resistant sealers. From my motorcycle & boating days I had done a lot of experimening with sealing gas tanks suspended over hot engines.

The cured material has follow threads, stay flexible, and resist petroleum based fuels plus various alcohols forever. Silica thickened polyurethanes are one group that works. None of the RTVs or silicons I tried worked long term. Epoxies? I don't know.

My favorite "go to" goo for gas tap threads is the time-honored Hylomar Universal Blue - it's an aviation product, but you can probably find it. Get the old style type. which is a proven product, but does outgase when curing so is being replaced by a newer formulation. I've used the old formulation many times. Developed in Britain, maybe by the same engineers that designed machines that leaked so many fluids in the first place.

Let me know how that Permatex works. It would be nice to have an alternative to Hylomar Blue. There are three Permatex products that are similar to Hylomar and might even be better. I just don't have experience & would want to run a few basic experiments just to convince myself they are good.

So I'm still using Hylomar Universal Blue (old style)

The Permatex products I was considering - just based on chemistry - are:
Permatex 80017 Aviation Form a Gasket
Permatex 85420 Orange Permashield
Permatex 80057 300 Form a Gasket

rScotty

 

[34Willys]

Hylomar may not be available in Lodi.
I used to overhaul large radial aircraft engines in Lakeport, and we would use Permatex brand Hylomar on the rocker cover and push rod adaptor flange gaskets. Those gaskets never leaked oil IF dressed with Hylomar.
Late 90's sometime, unable to get from parts house. Said the state banned it due to something EPA.
Went to using Permatex 80017 Aviation Form a Gasket. Messy but worked ok.
Used thread lube or fuel lube for pipe thread fittings into Magnesium cases. It was paste in a small can.
Had to be careful with fuel lube as if it got into fuel system, would not dissolve and could cause blockage.
Aircraft Spruce carries these products.
Patrick

 

[California]

... Developed in Britain, maybe by the same engineers that designed machines that leaked so many fluids in the first place.

rScotty

"That's not a leak, its the chain oiler".

'53 BSA here. AJS. Matchless. Austin small sedan that needed a NLA water pump. BTDT!

When the new Honda motorcycles didn't drip, that ended the British bike era.

 

[rScotty]

"That's not a leak, its the chain oiler".

'53 BSA here. AJS. Matchless. Austin small sedan that needed a NLA water pump. BTDT!

When the new Honda motorcycles didn't drip, that ended the British bike era.

My buddy still has his vintage race bike - an AJS 7R. It is now a living room sculpture. No, it doen't leak; no oil. The drip pan underneath is just for ambiance.
rScotty - still missing the old Gold Star twin. But it couldn't possibly have been as good as the memories it left.

 

[strantor]

NPT connections should not need thread sealant. The seal is metal-on-metal. That's why the threads are tapered, so they get tighter and tighter until seal is achieved. Thread lubricants like Teflon tape are used to help you be able to achieve the tightness required for sealing, before galling and damage occurs.

 

[rScotty]

NPT connections should not need thread sealant. The seal is metal-on-metal. That's why the threads are tapered, so they get tighter and tighter until seal is achieved. Thread lubricants like Teflon tape are used to help you be able to achieve the tightness required for sealing, before galling and damage occurs.

They shouldn't need sealants if the world was perfect and static, but it isn't and they do. Any connection that vibrates or has relative movement benefits from a sealant.

 

[LodiCal]

Thank you for the thoughtful replies. If the Permatex HP Thread Sealant does not work, I will try one of the aviation products. (grammar edit)

 

[oldtimer 66]

NPT threads do not seal on the crest and or root of the thread only the on flank of the thread by design. Therefore you can have a spiral leak. That is one the reasons sealant is required on NPT threads. The dry seal thread design eliminates this problem. ASME B1.20.3 Dryseal Pipe Threads, Inch - ASME