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View Full Version : Funky screws in Mercury 20hp carb.



Black_Moons
02-17-2015, 03:30 PM
Hi. Today I am working on a 20HP mercury boat motor. Good compression, spark, won't cough, got carb kit after seeing carb was filthy inside and a light cleaning didn't make the motor run.

Fun starts while trying to get new screws as most where VERY tight and a few had the heads half strip when removing them

1980's American engine (At least I assume), everything is imperial.. So I go down to lordco and ask for some #6 machine screws in course and fine.

Go to compare #6-32 screw with thread.. the 32 is wayyy too course. Find a metric M4.. and it fits.. but of course they don't have metric *screws* just bolts.

Ok so ID'ed one. Next was the 'fine' screws... Try and screw a M3 nut I have (At least I think its M3! bin was marked M3) I had at home and it does not go on. Measure threads... None of my gauges fit, but the 40TPI seemed to come closest, screw thread appears a little wider, maybe 42?

OD is 3.35mm or 0.132", Pitch of 0.604~ or 42

Have I found some kinda crazy 3.5mm screw?? Am I doomed to walk the earth forever more in search of 3.5mm screws?

lakeside53
02-17-2015, 03:38 PM
No idea.. but if it is a M3.5-0.6 screw - they are easy to find even here.

Maybe someone like me put them in years ago when the right screw wasn't in my box :)

Rosco-P
02-17-2015, 03:43 PM
6-40 extra fine thread

Black_Moons
02-17-2015, 03:45 PM
Looking at ebay I can get 10 SHCS M3.5x12 for $12 after shipping to canada.

Or 100 of em in cheese head, made outta cheese metal, guaranteed to cheese me off when they strip just inserting the things, for $6 from china.

Toolguy
02-17-2015, 03:46 PM
#6-40 is fine thread. #6-48 is extra fine.

A.K. Boomer
02-17-2015, 03:46 PM
can you re-touch up the heads of the old screws now that they are free'd up?

even if you have to put a huge flat blade slot through them or something?

Black_Moons
02-17-2015, 03:51 PM
6-40 extra fine thread

Hmm, my spec sheets say those should be 0.138", or 3.5052mm.. Very close indeed but rather larger then mine. and mine didn't really mate up with 40TPI, my cheapo thread gauge might be off but it was at least a half tooth off by 1/2" suggesting at least 41~ tpi.

M3.5 are 3.354mm to 3.479mm

I am suspecting metric a little more because the other screws in the carb are metric. Although I have no idea what an American motor is doing with a metric carb.

Just don't tell me I need british gas to use a metric carb :)

Black_Moons
02-17-2015, 03:55 PM
can you re-touch up the heads of the old screws now that they are free'd up?

even if you have to put a huge flat blade slot through them or something?

My bigger concern is some of them where so tight they felt like they could snap off at any moment, some even had a lot of torque after a turn or two before they loosened and iv had heads suddenly strip/snap in half after that much force for several turns. I don't trust screws after that much torque has been applied, especially such small screws. I could just barley remove them with a normal screwdriver (I hope to never use my manual impact on a carb!). they are also very rusted and this is a boat motor so I would like to go with new.

lakeside53
02-17-2015, 03:55 PM
Mikuni carb?

What's the carb manf and numbers?

Mike Burch
02-17-2015, 04:01 PM
I have just sold my Mercury 2.2, which I bought in 1989. It is actually a Tohatsu 3.5 with an aluminium disc with a small slot in it between the carb and the cylinder, to reduce the power.
So not all Mercuries from the 1980s were made in America, but rather in Japan, and it would be quite normal for them to have metric threads.

Black_Moons
02-17-2015, 04:14 PM
Mikuni carb?

What's the carb manf and numbers?
No idea. has a circle mark with.. X diagonal from a K, if you hold it upside down..
Another circle mark with a P and two dots. then a 1 next to that.

Numbers on the outside of the carb are: WMC9-1785. There are some numbers on the inside of parts (float bowl and butterfly flap) but I kinda doubt those are useful.

RandyZ
02-17-2015, 06:02 PM
Why don't you just go down to a deal and order the screws you want? Probably cheaper than driving all over looking for odd balls.

I worked at a mercury dealership last summer, so as a word of advise, bring your serial number of the motor. The way Mercury made changes on the fly, the only way to get the proper part is by the serial number. Mercury's computer system needs the number to even open the search window.

Black_Moons
02-17-2015, 06:21 PM
Why don't you just go down to a deal and order the screws you want? Probably cheaper than driving all over looking for odd balls.

I worked at a mercury dealership last summer, so as a word of advise, bring your serial number of the motor. The way Mercury made changes on the fly, the only way to get the proper part is by the serial number. Mercury's computer system needs the number to even open the search window.

Not sure there are any mercury dealerships near me.. Nearest boating place is a 30 minutes drive away, although I do need go near there soon to drop some stuff off at recycling.

Willy
02-17-2015, 07:45 PM
Why don't you just go down to a deal and order the screws you want? Probably cheaper than driving all over looking for odd balls.

I worked at a mercury dealership last summer, so as a word of advise, bring your serial number of the motor. The way Mercury made changes on the fly, the only way to get the proper part is by the serial number. Mercury's computer system needs the number to even open the search window.

What Randy said.
Eight different carbs types where used for the 2-40 hp Mercs according to my Mercury shop manual. Manufacturers include Tillotson, Mikuni, Keihin and Walbro. So yes depending on what you have it could very well be metric.
Don't sound surprised to hear that metric threads are used on an old American product. Ever find a spark plug with imperial threads?:)

Black_Moons
02-17-2015, 08:48 PM
What Randy said.
Don't sound surprised to hear that metric threads are used on an old American product. Ever find a spark plug with imperial threads?:)

Next your going to tell me the OD is not even imperial and my spark plug sockets all lie.

lakeside53
02-17-2015, 08:53 PM
No idea. has a circle mark with.. X diagonal from a K, if you hold it upside down..
Another circle mark with a P and two dots. then a 1 next to that.

Numbers on the outside of the carb are: WMC9-1785. There are some numbers on the inside of parts (float bowl and butterfly flap) but I kinda doubt those are useful.


Well.. the numbers on the outside are very useful is you read them correctly :) It's a WMC9-178S. Type that into a search engine and you'll find a wealth of info.

http://www.bing.com/search?q=WMC9-178s%20carb&qs=n&form=QBRE&pq=wmc9-178s%20carb&sc=0-0&sp=-1&sk=&cvid=376697ae89494043aef06b64c3c05854

smalltime
02-17-2015, 09:01 PM
I have just sold my Mercury 2.2, which I bought in 1989. It is actually a Tohatsu 3.5 with an aluminium disc with a small slot in it between the carb and the cylinder, to reduce the power.
So not all Mercuries from the 1980s were made in America, but rather in Japan, and it would be quite normal for them to have metric threads.

Dammit!
I wanted to blame this on the Canadians.....Canadians do ya' hear me!

EVERYBODY knows that Carl Kiekhaefer is the founding father of Canada..........Don't they? Hell, the guy buried the first outboard motor on Oak Island in 10X.

Every home in Canada is required to have a Mercury outboard SOMEWHERE on the property, it's in the constitution.

Jon Heron
02-17-2015, 09:33 PM
LOL
Huh? Kiekhaefer was a yankee, a cheese head if I am not mistaken?
But yeh us canucks do like a good outboard, I have 4 classic merc's myself a 9.8 a 4 and two tower of power inline 6's, a 115 and a 140.
The nice thing is I can rebuild them with a set of standard wrenches and a screwdriver out on the water. :)
Cheers,
Jon

Jon Heron
02-17-2015, 09:37 PM
FWIW I have the shop manuals for all these classic pre 1988 mercs. When I get home from Charleston tomorrow (if I dont get stranded at Laguardia again :( ) I can send you any info you need.
Cheers,
Jon

Black_Moons
02-18-2015, 12:16 AM
Well.. the numbers on the outside are very useful is you read them correctly :) It's a WMC9-178S. Type that into a search engine and you'll find a wealth of info.

http://www.bing.com/search?q=WMC9-178s%20carb&qs=n&form=QBRE&pq=wmc9-178s%20carb&sc=0-0&sp=-1&sk=&cvid=376697ae89494043aef06b64c3c05854

Ahh.. *finds a place selling the required screws.. gives no info on them other then wanting $2.25 each and some obscure part number*
Haha.. Maybe I should be buying those $10 pack of 10 off ebay after all.
Oh this is nice. 'To comply with Mercury policy, *****.net will not ship OEM Mercury parts outside of the United States.'
Everyone else only sells the gasket kit I already have.

LKeithR
02-18-2015, 12:16 AM
There are several marine dealers in Langley--M & B Marine specializes in Merc and Mercruiser engines and both the owners have been at this a long time so they would be familiar with the older engines. Might be worth a call....604-533-5255...ask for Mike...

Black_Moons
02-18-2015, 06:03 AM
Well.. the numbers on the outside are very useful is you read them correctly :) It's a WMC9-178S. Type that into a search engine and you'll find a wealth of info.

http://www.bing.com/search?q=WMC9-178s%20carb&qs=n&form=QBRE&pq=wmc9-178s%20carb&sc=0-0&sp=-1&sk=&cvid=376697ae89494043aef06b64c3c05854

Double checked, it really is a 5. Found a website that sells screws for that carb, only $3.20 each for the M3.5 and $0.90 for the M4. What a ripoff. I can get a 10 pack of M4's for that price in any style I want. 10 packs of M3.5 for $12 too. Hoping my metric screw guy can do a little better on the M3.5's though..

And people wonder why I didn't check a boat dealership, My wallet is not heavy enough to use as a boat anchor so I stay out of marine places.

Willy
02-18-2015, 06:20 AM
My bigger concern is some of them where so tight they felt like they could snap off at any moment, some even had a lot of torque after a turn or two before they loosened and iv had heads suddenly strip/snap in half after that much force for several turns. I don't trust screws after that much torque has been applied, especially such small screws. I could just barley remove them with a normal screwdriver (I hope to never use my manual impact on a carb!). they are also very rusted and this is a boat motor so I would like to go with new.

Just thinking out loud here BM.
If the screws were that hard to remove and came out that hard chances are the new screws will go in hard as well.
I'm thinking you probably have 20-30 years of galvanic corrosion built up between the dissimilar metals of the screws and the carb body materials. Have a good look at the threads in the carb body to see if they are clean, you may have to rum a tap into them to clean them out.
Marine equipment is bad for this stuff especially if it's spent most of it's life out in the salt chuck.

LKeithR
02-18-2015, 12:31 PM
...Hoping my metric screw guy can do a little better on the M3.5's though...

Did you check with Fuller Metric? They're also in Langley...

http://www.fullermetric.com/contacts.aspx


And people wonder why I didn't check a boat dealership, My wallet is not heavy enough to use as a boat anchor so I stay out of marine places.

Generally true--sometimes you have no choice...

Willy
02-18-2015, 12:57 PM
How's the old saying go?
If it flies, floats, or fornicates its usually cheaper to rent.:)

Black_Moons
02-18-2015, 02:13 PM
Did you check with Fuller Metric? They're also in Langley...

http://www.fullermetric.com/contacts.aspx
Generally true--sometimes you have no choice...

'Please be advised we have a $100 minimum for non account sales.'
Lovely, I guess they don't want my business either!

Black_Moons
02-18-2015, 02:15 PM
Just thinking out loud here BM.
If the screws were that hard to remove and came out that hard chances are the new screws will go in hard as well.
I'm thinking you probably have 20-30 years of galvanic corrosion built up between the dissimilar metals of the screws and the carb body materials. Have a good look at the threads in the carb body to see if they are clean, you may have to rum a tap into them to clean them out.
Marine equipment is bad for this stuff especially if it's spent most of it's life out in the salt chuck.

They smoothed up once I got them out 2 or 3 turns, inserting them back in seems ok, its mainly just the heads on a couple that are buggered up from the high removal torque. And yea, I'll likely add some anti-seize when I put new screws back in, and wash the holes out with lots of carb cleaner/compressed air and maybe a tap on the M4 ones (Not buying a M3.5 tap.. lol)

Mainly I know I can A: Spend an hour, replacing screws now or B: Spend 5 hours setting up, drilling out and helicoiling the screw threads later when it needs to be taken apart again.

RandyZ
02-18-2015, 07:04 PM
Dammit!
I wanted to blame this on the Canadians.....Canadians do ya' hear me!

EVERYBODY knows that Carl Kiekhaefer is the founding father of Canada..........Don't they? Hell, the guy buried the first outboard motor on Oak Island in 10X.

Every home in Canada is required to have a Mercury outboard SOMEWHERE on the property, it's in the constitution.

I own a 1947 Kiekhaaefer " Mercury Rocket twin" 8 HP outboard. This was just a few years before Brunswick corp. bought him out and name the whole company, Mercury Marine

RandyZ
02-18-2015, 07:09 PM
Next your going to tell me the OD is not even imperial and my spark plug sockets all lie.


You should try working on a Volvo penta stern drive. The engine block ( 350 chevy ) is imperial and all the other stuff (fuel injectiion, alternator, Ignition ETC) is metric.

Cuttings
02-18-2015, 07:44 PM
You mentioned putting anti-seize on the threads.
I would not advise that if it is going anywhere near salt water. You are introducing another metal into the galvanic equation.
I have had very good luck using Teflon pipe sealer on stainless in aluminum. The Teflon gives it some lube and also seal it so no moisture can get into the treads.
Twenty-five years and they still come out easily.

RPM22
02-18-2015, 11:29 PM
Those hard to get out bolts may have been put in with Loctite, Mercury is very fond of doing that in awkward places, and also using the red permanent Loctite instead of the blue removeable type - I recently did an overhaul on my 2007 60hp, and I thought I was close to breaking many of the bolts as they came out - but it was just loctite - good luck with yours

Richard in los Angeles

Black_Moons
02-19-2015, 02:03 AM
You mentioned putting anti-seize on the threads.
I would not advise that if it is going anywhere near salt water. You are introducing another metal into the galvanic equation.
I have had very good luck using Teflon pipe sealer on stainless in aluminum. The Teflon gives it some lube and also seal it so no moisture can get into the treads.
Twenty-five years and they still come out easily.

Salt water? what person in their right mind would let anything metal get near salt water? I don't even want to put my boat into clean lake water!

Rosco-P
02-19-2015, 12:06 PM
Try http://www.asmc.net/ next time, their stock does change, they do ship to Canada. They have 3mm socket caps, but no 3.5mm at this time.

Black_Moons
02-19-2015, 03:34 PM
Try http://www.asmc.net/ next time, their stock does change, they do ship to Canada. They have 3mm socket caps, but no 3.5mm at this time.

I doubt they are going to get in M3.5 since its a rare screw.

Wow most of there stock is expensive. Also did a shipping estimate on a 25pack of M6x1x30 screws. $5.50usd for the screws + $18usd shipping to canada.

I can get that for $2usd + $2.50usd shipping off ebay from a local canada seller, and he sells everything in 10 packs, with a 10 pack being more like $0.80usd + $2usd shipping.

Rosco-P
02-19-2015, 05:43 PM
That's okay. Skip it then.

I've found him to be cheaper than conventional suppliers, his quality is high and the shipping calculator is an just estimate. He doesn't profit off of the shipping. He also doesn't want to sell 25 or 100 pieces, he really wants to move volume, 1k, 5k, etc. "Real" fastener suppliers are funny like that. Repacking fasteners eats up their labor time.

He does like to use USPS flat rate. One issue for you, Canada Post.