Trouble tapping shift cable threads on MC-1 (pre-Alpha) outdrive

[tylermac]

I have a 1979 Mercrusier 140 with a MC-1 outdrive. Shift cable has gone bad and I'm in the process of replacing it. Discovered it's the old style cable. Did my research, read through the forums, learned how to do the conversion to the new style cable that threads into the bellhousing from the rear.

What size tap do I need?

1/4"-18 NPS
1/4"-18 NPS
1/4"-18 NPS

One thread said it's 1/4"-18 NPSF, and a little research shows there are minor differences between the two. Small enough I bought the cheaper NPS tap.

Tap arrived yesterday. Won't thread into the hole, feels like the tap is too big. I'm chasing from front to aft, trying to thread by hand. This is the one I bought:

https://www.amazon.com/gp/product/B07VDSWY2B/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

I'll admit I've only run taps to clean up threads once or twice, and they were NPT threads. But from what I understand, it should thread in easily by hand. What is going on?? I'm worried about damaging my bellhousing if the tap I bought is junk. It could be junk. It's a cheap tool from China.

Help?

 

[kenny nunez]

Try some thread tapping oil. Since the housing is aluminum the tap will go through. If you have a stove bolt square socket to grip the tap will help or sometime a 12 point socket will grip the tap enough to drive it. Just go a 1/4 turn at a time and back the tap out enough to clear the cut threads until the tap goes through.
I still prefer the earlier style Cable because they are easier to tighten up in the housing with a fuel injection line socket but a deep socket and vice grips will do the job.

 

[tylermac]

I shouldn't have to cut new threads, right? I believe the threads go all the way through the hole, so the goal is just to clean them up? I was assuming the tap would thread right in from the forward side where the old cable unscrewed, and I could clear out all the crud that's built up on the aft threads over the years.

Or is this actually a new thread vs. the old style, and I'm cutting fresh ones into the bellhousing?

BTW, I just moved from Algiers. How's the fishing down there these days?

 

[kenny nunez]

I always started the tap in the front and just went all the way through it.
I am the guy who goes fishing the day before and the day after they were biting!
I went fishing with my cousin at the La. marsh in his 27’ Parker, every one was getting 2-3lb specs and I kept catching the same black tip shark or 1/2 a spec.
Where did you move to?

 

[tylermac]

Front meaning the side the old cable threaded in? That's what I'm trying. Neither the tap nor the old cable will thread in from the other side. Did yours thread easily, or did it feel like you had to force it through?

I moved to Utah. Definitely didn't take advantage of Louisiana fishing as much as I wanted. I went a few times with my boss, and he seemed to have the redfish figured out. Was a lot of fun. Fish are smaller in Utah, but still fun

 

[achris]

Front meaning the side the old cable threaded in?..

Front = toward the bow of the boat. Yes, a term many people get wrong when referring to parts of boats. If I had a dollar for everytime someone called the diffisuer ring on a propeller, the 'front', I'd be retired now.

Only use the tap Merc specify (1/4"-18 NPSF). It's a parallel pipe thread (counter-intuitive, I know). Don't try to use any other tap, it will necessitate the replacement of the bell housing.

Chris.......

 

[tylermac]

Only use the tap Merc specify (1/4"-18 NPSF). It's a parallel pipe thread (counter-intuitive, I know). Don't try to use any other tap, it will necessitate the replacement of the bell housing.

Chris.......

Ok, this is where things are getting confusing on the site.

These posts all refer to 1/4"-18 NPS. Some use NPS and NPSF interchangeably:

https://forums.iboats.com/forum/eng...alpha-one-gen-1-shift-cable-brass-bolt-thread
https://forums.iboats.com/forum/eng...an-210-mercruiser-260-shift-cable-replacement
https://forums.iboats.com/forum/eng...rives/547049-snapped-the-shift-cable-head-off
Etc etc etc

Going to a machinist forum, this thread states that NPS female threads = NPSF female threads, and the difference is the male fitting. NPSF male fitting is tapered to create the seal, where NPS is sealed by tape/sealant/gasket/etc.
https://www.practicalmachinist.com/vb/general/nps-tap-questions-344721/

Supported by these ANSI drawings linked in the thread:
https://www.gewinde-normen.de/en/npsm-pipe-thread.html
https://www.gewinde-normen.de/en/npsf-pipe-thread.html

Then, in talking with my brother, he found one of his old engineering standards books that lists the specs for each thread. There are differences, but in the thousandths of an inch. It appears to be so close that one should thread the other without difficulty.

THEN, there are drawings linked in these forums stating to apply sealant to the cable threads, implying it is an NPS thread instead of the self-sealing NPSF standard.

Long story short, I'm thoroughly confused. Which is which, where is it listed, and does it matter if I use NPS instead of NPSF or vice versa?

 

[achris]

I go by the Merc genuine service manual... Forget other sources, use the source that built the machine....

 

[tylermac]

Alright, so I have the threads tapped and the new bellows installed. Now I can't get the outer cable through the bellows. The opening on the narrow end is so tight, I've spent a couple hours trying to work it in. No amount of grease or swearing is making more than about a half inch of headway. I have a feeling this is supposed to be easier. FWIW, I'm using the GLM kit sold here:

https://www.amazon.com/gp/product/B004AR1C06/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1

 

[achris]

... I'm using the GLM kit ...

That's the problem. Use genuine. Yes it costs a bit more, but it's the right one, and you wouldn't be going through the trouble you are.

I have installed literally dozens of these. I used ONE non-genuine kit, and swore it would never happen again!

Chris........

 

[Bt Doctur]

lube the brass cable end, lube the bellow end, like a virgin it needs a bit of force to enter but without a core guide be careful you dont insert it into the flywheel housing and get it stuck

 

[tylermac]

Thanks for all the feedback. Dish soap ended up doing it. Somehow the marine grease I have wasn't doing the trick. I took the bellows' virginity with a 1/4" extension bar and then the cable slipped right in with the soap.

I'm slowly putting her back together. Getting the outdrive back on was a bear. Guessing I need to pick up the alignment tool and check it the next time it's off. Should be smooth sailing from here. Hoping to be out on Friday!

 

[tylermac]

Follow up: Everything went smoothly after getting the cable through the bellows. I made the adjustments and it goes forward and reverse with ease. I really appreciate everybody's feedback on the thread. I got the boat out for the July 4th weekend and have had it out every weekend since.

I did end up buying a NPSF tap because I chickened out with the NPS. I'm pretty sure the NPS would have worked just fine but decided to not bet on it.