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2004 Mercruiser 4.3 Fuel Pump Fittings - NPT, UNF, or Other?

J

_jp

Contributing Member
Hi all, I ordered a fuel pump pressure tester the other and all the fittings supplied with it are Metric (as i'm UK Based). I'm having trouble finding the correct fittings just to give a barbed end, I was under the impression they should be NPT from other posts I have read? However, a local marina didn't have anything in stock (all Metric) and said they think it's UNF not NPT?

Engine Specs
Mercruiser Inboard
MCM 4.3L V6 (190M C/Al)
Carb Type (Mercarb 2 Barrel)
Thunderbolt V Ignition
2004 (approx)
0W034153

What I am looking for is information on the correct version of this please (this is the Metric one in the kit), this size looks about right (by eye)...

IMG_2713-min.jpg


To fit the top of the fuel pump (either in the small pump hole, or the large one on the adapter that normally connects to the stainless fuel line ends)...

IMG_2673-min.JPG


I thought it was 1/4" NPT Male with Barb, but what turned up (no picture) was more like 1/2" in diameter (so it may be the wrong one sent to me, or I am asking for the wrong thing). Any guidance would be appreciated. Thanks
 
Bt Doctur said:
those pumps are about 4-6 psi .Are you having a issue with fuel delivery?
Click to expand...
Yes, the carb is flooding and I wanted to rule out the fuel pump; as advised by someone else. The boat is new to me, and it's possible the wrong fuel pump could have been fitted.
 
If the carb is flooding have you checked the needle & seat? That’s the most common cause although a defective pump could be responsible…that fitting on the top of the pump I’d guess 1/4” NPT (National pipe taper) but I could be wrong! It is confusing the way these fittings are measured; the 1/4” isn’t the diameter of the threads I think it’s the inside diameter of the tubing IIRC. On fuel fittings you usually have either inverted flare fittings with straight threads or NPT threads with pipe threads. A carb inlet on a marine engine (metal lines) usually has an inverted flare fitting but at the fuel pump (mechanical pumps) they use NPT threads. Not sure about electric pumps. That pump isn’t OEM its aftermarket.
 
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The one in your hand is most likely NPT, the one on the fuel pump is inverted flare

Go to your local auto parts house and take the pump with.
Look for something like the one on EBAY
 
louc said:
If the carb is flooding have you checked the needle & seat? That’s the most common cause although a defective pump could be responsible…that fitting on the top of the pump I’d guess 1/4” NPT (National pipe taper) but I could be wrong!
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Yes, i've striped the carb and bought a refurb kit, but I wanted to rule out the fuel pump too. I have a separate thread for the carb and the flooding issue and thought best to post this its own as just trying to confirm the correct fittings.

AllDodge said:
The one in your hand is most likely NPT, the one on the fuel pump is inverted flare

Go to your local auto parts house and take the pump with.
Look for something like the one on EBAY
Click to expand...
I've been to Eurocarparts and GFS and both couldn't help. That's why I tried a local marina too, thinking the problem is being based in the UK and the age of the boat. I've not heard of inverted flare before though - I can see if that sheds some light on it. I had thought about putting an regulator, but I think i'll be stuck with the same issue.
 
Do not connect at pump, connect at flared fitting at carb inlet fitting. I believe that is a 3/8 npt male flare fitting. I would think any automotive parts seller who sells american parts will have the correct fittings.
What you would need is a 3/8 npt flare female to Female NPT (what ever your barb is). Use teflon tape on you barb when threading into fitting. If the female to female is only the same size, get a barb fitting to match
 
kghost said:
Do not connect at pump, connect at flared fitting at carb inlet fitting. I believe that is a 3/8 npt male flare fitting. I would think any automotive parts seller who sells american parts will have the correct fittings.
What you would need is a 3/8 npt flare female to Female NPT (what ever your barb is). Use teflon tape on you barb when threading into fitting. If the female to female is only the same size, get a barb fitting to match
Click to expand...
Is there a specific reason not to connect at the pump? It does make better sense to at the carb inlet in, the only reason I was connecting at the pump was due to already removing it to look for model numbers (encase it stated "high-pressure" but there were nothing). So currently in the process of re-installing it again.

I seem to have the same issues looking for an 3/8 NPT Flare Female fitting too. Most autoparts stores over here don't stock anything US like that, I can sometimes find it online, but then there's a 3 day wait for delivery each time (annoying when the part doesn't fit and limited dry days to install and test). There's one small American Auto Parts specialist about a 3 hour round trip from me, who I have emailed asking for a 3/8 NPT Flare Female to 1/4" Barb. I'll see what they come back with. Thanks for the info, I feel that's going to be my best shot.

AllDodge said:
Inverted flare is also what they use for brake lines
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Good to know, cheers (y)
 
UNF threads are typically used with the inverted flare fittings...NPT threads are NOT. a set of calipers and a thread gauge should identify what you need.

If you are going to deadhead the pump for the pressure measurement is doesn't matter if you do it at the pump's outlet fitting or at the carb end of the fuel line...
 
makomark said:
UNF threads are typically used with the inverted flare fittings...NPT threads are NOT. a set of calipers and a thread gauge should identify what you need.

If you are going to deadhead the pump for the pressure measurement is doesn't matter if you do it at the pump's outlet fitting or at the carb end of the fuel line...
Click to expand...
What ever the thread size is at the steel fuel line end (male flare ) where it installs at the carb inlet fitting, that thread will be much easier to find then the fuel pump fitting.
In my opinion.
 
Ok I did some measurements.

The male threaded flare fitting that threads into carb inlet is,

5/8-18 thread. The thru hole is for 3/8 dia tubing.
 
makomark said:
UNF threads are typically used with the inverted flare fittings...NPT threads are NOT. a set of calipers and a thread gauge should identify what you need.

If you are going to deadhead the pump for the pressure measurement is doesn't matter if you do it at the pump's outlet fitting or at the carb end of the fuel line...
Click to expand...
I'm struggling a little to wrap my head around all the different types (it's a bit beyond my skillset). I really thought it was going to be a standard "off the shelf" product thing, something that would have simply been in the tester kit, and it has got far more complicated. I honestly have no idea what i'm looking at when it's not metric.

kghost said:
Ok I did some measurements.

The male threaded flare fitting that threads into carb inlet is,

5/8-18 thread. The thru hole is for 3/8 dia tubing.
Click to expand...
I really appreciate you taking the time to go and measure (that really is above and beyond). Would you mind clarifying what I need to buy @kghost? Just so that I don't get the wrong thing due to my ignorance on the topic?

Is it NPT or UNF, ? e.g: 5/8" NPT Female to 1/4" Barb?

I have presumed the 1/4" barb is the outer diameter, but do I need to use the internal hole size. Sorry I'm needing to be "spoon fed" on this.
 
The inverted flare (3/8) referes to the tube diameter.

The thread size is 5/8- 18
The outside diameter of the threads is 5/8" the thread count is 18 threads per inch
 
Dont blame us, we did not devise this standard for threads but its been around for well over a hundred years in the USA
 
kghost said:
Dont blame us, we did not devise this standard for threads but its been around for well over a hundred years in the USA
Click to expand...
No blame from me, I'm just used to something else and have little knowledge on the different threads, we've got others over here to apparently like BSF.

The only thing I could seem to find is https://www.ebay.co.uk/itm/235360740242. There's nothing coming up in the UK online. I'll contact the American Auto Parts store i mentioned early with the info you've supplied and see if they have anything.
 
If you have the pump removed, why not go to a marine repair shop and have them test the output pressure. More than likely they will have the proper fittings and anything else needed to test it
 
I did try that @kghost, when I visited the marina looking for the part. They said they weren't able to help, no mention of money. Tbh, I think i'm just going to bolt it all back together, try the refurbed carb and if it's still got the problem then i'll order a new fuel pump. If it's still got issues after that, i'll book the boat in somewhere. But, thanks for everyone's help on this and I hope the information is useful for someone else to use.
 
The pump "should" be low pressure. Take the line off the carb and slide the fitting away from the end to expose the steel line. Take a short piece of fuel line and push over the end of the line and hose clamp. Connect other end to gauge
 
I think @AllDodge's method is probably going to be the best approach for me at the moment, but thanks @Fun Times for the information regarding the official part designed to do the job of a pressure test. Good find!

I have searched ebay with the part number, which returned zero results and Google only references two websites (one being Hardin Marine), both in the US and delivery notices saying 10+ days, which doesn't include the additional UK times. I will keep an eye out and if I come across one, i'll get it for the boat toolbox.
 
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