View Full Version : bolt size for prop shaft coupling

09-22-2011, 07:17 PM
I may need to remove my drive shaft in order to get at some keel bolts which are leaking and need to be re-torqued. I will have to press out the prop shaft from the coupling and want to purchase some longer, hardened bolts for this job. What is the correct bolts size and thread pitch? And any idea how long a bolt I will need to press out the shaft. I will use the trick of inserting a socket just a bit smaller than the shaft and tighten the bolts to press out the shaft.

09-22-2011, 08:02 PM
The threads in the gearbox output coupling should be 3/8" x 24 (NF). Rather than turning into the threads, I would install long, say, 4" threaded cap screws with nuts and flat washers to apply the torque on the fully threaded (into output coupling) cap screws.

09-22-2011, 11:00 PM
Hanley, thanks for the information on the technique. I was afraid of using the threads in the output coupler to press out the prop shaft. I am assuming that the holes in the collar of the prop shaft are clearance holes and that the holes in the output coupler flange are threaded. The washers and nuts go on the outside of the prop shaft collar and you hold the bolt from turning with a wrench on the hex head of the bolt while turning the nuts to squeeze the collar against a socket (or similar piece). I have heard that you need grade 8 bolts for this job, is that your opinion?

09-22-2011, 11:19 PM
Yes, grade 8 can take the torque without stripping.

09-22-2011, 11:25 PM
I checked with McMaster, they only sell the fine thread in grade 8 at $4 each for 4 inches. Again, thanks.

09-23-2011, 08:56 AM
I've done this three times in the 20 years I've owned my boat, and while I did not need Grade 8 fasteners I did require a substantial repertoire of Grade 8 obscenities.

It's a horrible job, perhaps made worse in my circumstance with the V-drive which places the transmission directly over the coupler with very little access to swing a wrench, or even fit bolts beyond a short length between the transmission and the coupling.

The next time I have to do it I'm cutting off the coupling with a Sawzall and buying Don's split coupling.

09-23-2011, 01:31 PM
hanley & tartan...I am dense..I am afraid I don't understand what you two are talking about. Tartan, can you take a picture when you get this set up? I used your first plan with progressively longer bolts to press out the shaft with a couple of sockets. It was very tedious as tenders commented. Moyer didn't offer the two piece couple just yet last year when I was doing this project, although I may do the same thing as tenders..cut it off and buy the split coupler (if I ever need to take it apart again..) :D

09-23-2011, 01:37 PM
Shawn - The procedure is essentially the same as the way you did it. The only difference is that he will be using nuts on the threaded cap screw to apply the pressure rather than turning into the threads in the coupling.

09-24-2011, 07:01 PM
...and will be using smaller-diameter bolts to fit cleanly through the coupler holes (on the engine side and the shaft side) so as not to chew up the threads on the couplings.

09-24-2011, 07:09 PM
...and will be using smaller-diameter bolts to fit cleanly through the coupler holes (on the engine side and the shaft side) so as not to chew up the threads on the couplings.
I had not thought about doing it that way with, say, 5/16" - 24 NF, but I see your point. However, I do think the threads in the output coupler will stand the torque if the cap screw is stationary.

09-24-2011, 11:15 PM
I will take pictures tomorrow but I have already ordered three grade 8 bolts, 3/8x24x4"long fully threaded. In my setup, a direct drive A4, there is no room behind the flange on the transmission to put a nut even it I were to use a smaller size bolt, say 5/16. So the only way I can see pressing out the prop shaft is the way Hanley suggested, i.e. fully threading a long 3/8x24 bolt into the output flange, then using (grade 8) nuts between the hex head of the bolt and the prop shaft coupling, to squeeze the coupling against a socket or a stack of washers, to press out the shaft. That technique sounds much less risky than trying to screw the bolt into the threads with the same socket (or washer stack).

But I am confused as to why you all think this is such a tedious process. I have excellent access to the engine, the T30 has the engine just aft the mast with a cover box which is easily removed. The reason for my concern about time is that I will probably have the boat yard help me in this process and at their rates, from what you both say, this could cost me a lot.

I would think that with a 4 inch bolt, it would only take one or two passes with a socket or washers to drive out the shaft, what am I missing here?

09-25-2011, 07:57 PM
As promised, here is a photo of my prop shaft, output coupler and transmission flange. Clearly there is no room behind the flange to put a nut if I were to use smaller diameter bolts (5/16) as was suggested rather than thread new longer bolts into the flange to used with nuts ahead of the coupler to press out the shaft.

There is another alternative, that being to make a new dummy flange out of say 3/8" steel with three 3/8 clearance holes. Then I could unbolt the output coupler, slide the prop shaft/coupler back a few inches, install the new longer 3/8 bolts and use a nut on the back side of the dummy flange with the socket trick to push out the prop shaft. That technique would not stress the threads in the original transmission flange but its more work to make the dummy flange.

Do you all feel confident that if I screw the longer bolts into the original transmission flange and use nuts ahead of the output coupler to press out the prop shaft, that I will not screw up the threads on the flange??

BTW, the reason why I think I need to separate the prop shaft from the coupler is that I also have a pillow block between the transmission and the stuffing box, and the output coupler will not slide back beyond that pillow block. And finally, the reason for this whole exercise is to get a clean shot at my leaking keel bolts some of which are under the prop shaft.

09-25-2011, 09:50 PM
If you go to the on line catalogue on this site and check "specialty tools" you will see TOOL_06_135 which carrys the theme of your plans nicely.:)

09-25-2011, 10:28 PM
Hanley, Now you tell me! thanks, I will call Ken and see if the bolts they supply are grade 8, looks a lot less risky than my original scheme using long bolts into the output coupler.

09-26-2011, 11:40 AM
If you have room to swing a wrench around that coupler, and have the ability to push the shaft aft several inches without banging into the shaft seal with the coupling or the rudder with your prop, then this will simply fall into the "slow work" category rather than "abhorrent PITA" category. Count your blessings!

09-26-2011, 12:56 PM
Tenders, thanks for the explanation. I have excellent access to the shaft coupler, in my T30, the engine is located just aft the mast and not in the companionway as in most boats. I also have lots of room between the shaft coupler and the pillow block and lots of room between the prop and the rudder so I think I am OK in that regard.

However, Hanley pointed me to a special tool from Moyer, Tool_06_135, which is essentially is a dummy flange with three holes that match the hole pattern on the prop shaft coupler and a central bolt to press out the prop shaft. Or you keep the central bolt fixed and pull the prop shaft coupler into the dummy shaft, either way, you press out the prop shaft without using (and possible damaging) the transmission shaft coupler. But that tool is about $60 and I have already purchased three grade 8 bolts from McMaster Carr. So my question is do you think I am taking a chance of damaging the transmission shaft coupler by pressing out the prop shaft with the old way? I would thread the new long bolts fully into the transmission coupler, leave them fixed and use nuts between the head of the bolt and the prop shaft coupler to press the prop shaft out.

(I apologize to all for this lengthy rambling, but Gee Whiz this forum and Don are really great!)

09-26-2011, 01:09 PM
You should be aware that the output flange is not indestructible. There's a limit as to how much force it will tolerate.

Here's a picture of the aftermath on Shawn's marathon Indigo thread. Note that the deflection is right where and in the direction of the force applied when pressing out a shaft from the coupler using the socket technique.

Yikes and Gadzooks!!


09-26-2011, 01:35 PM
Aye, and the output flange can be had for $62 from the on-line catalogue. Sooo....you have to ask yourself one question---"do you feel lucky? well, do you...."?:)

09-26-2011, 01:37 PM
But I am confused as to why you all think this is such a tedious process.

Have you ever removed a coupler from a prop shaft?

It is an interference fit. After it's been on there for a few years and a little rust as developed in between the shaft and the coupler, it's practically welded on there.

After I removed my engine from my boat, I took the prop off the outside and slid the entire shaft inside the boat so I could take it home and work on it.

After what it took me to get that coupler off the prop shaft, while working at my workbench in a well-lit workshop, with the prop shaft held in the vise, (and after breaking at least one tool), I can't imagine trying to do it lying on my belly with my head down behind the engine. I'd say it would have been physically impossible for me to get that coupler off inside the boat. Your layout, though, does sound like it provides far better access to the coupler than I have in my boat.

But unless you've been very good and eaten all your vegetables (unlike me), it's not going to be a matter of simply tightening the bolts and merrily sliding the coupler off the shaft. You'll be tightening the bolts and nothing will be happening. You'll tighten them more, grunting and sweating, and nothing will happen. You'll tighten them as much as you physically can - assuming you've got some way to prevent the shaft from turning - and grit your teeth so hard you think you're about to crack that crown your dentist put on last year. And still nothing happens. Then you notice you've bent one of the bolts. Yes, it is entirely possible. Ask me how I know this.

Then you get out the torch...

Unless you're a lucky duck (unlike me), that coupler is not going to come off that shaft easy. You have to completely convince it that it has no other choice but to let go. And it is not very open-minded about that possibility. As such, it needs substantial persuading to change its orientation on the matter.

09-26-2011, 03:42 PM
Yikes and Gadzooks squared. Sounds like unless I am very lucky, I had better plan on not using the output coupler (on the transmission) to press out the prop shaft coupler. The specialty tool in Don's catalogue, when turned around and used backwards to press out the prop shaft coupler is a much safer way to go. I spoke with Ken and while he had not thought of that possibility before, he could not see anything wrong with that approach. He specifically warned about not damaging the nut or the threads on the output coupler if I were going to go that route. He also suggested a good three arm gear puller might to the trick.

Also since there is no easy way to cool the prop shaft, heating the prop shaft coupler might expand it enough to get it off with a puller. also sounds like if this is originally an interference fit, you should heat the coupler before trying to re-install it too, has anyone done that?

Sounds like if I do get the prop shaft coupler off, that I should invest in the new split coupler, uggghh, more money. I got to think about this, get back to you all when I decide what to do, again thanks. Gee it sounded so simple.

09-26-2011, 04:02 PM
He also suggested a good three arm gear puller might to the trick.

Yes. Emphasis on a "good" one. The tool that I broke trying to get mine off was a Horror Freigh three-armed puller. One of the arms cracked right off.

Another thing to understand: being that the coupler is an interference fit with the shaft, it is not simply a matter of sliding the old one off, cleaning it up and sliding it back on. In fact, my understanding is that you likely should consider a new one in any event - particularly if you find rustiness in there once you get it off.

What happens is the rust ruins the interference fit. Once you get the old one off and clean it all up, you'll find that it slides easily on and off the prop shaft that you just had to practically kill yourself to get it off of. If it does slide on easily, that means it's no good.

The new one must be fitted and faced. "Fitted" means the bore of the coupler is reamed precisely to the correct interference fit so that it is a snug, driven fit onto the shaft. Once it is driven onto the shaft, the coupler must then be "faced", which means the whole shaft with the coupler on the end is chucked in a lathe and a light pass taken off the face of the coupler to assure it is truly perpendicular to the shaft.

I would recommend you give this thread a gander (http://plasticclassicforum.com/forum/viewtopic.php?f=4&t=5405&p=45367) and check out some of the links in the thread.

But ya know - I'm not trying to make you paranoid or anything. For all I know, you'll have no problem neatly pressing it off and then neatly pressing it back on again. I'm relating my experience with this, and I always research the heck out of anything new before plunging ahead. I always try to learn what is the "right" or at least generally-accepted-within-the-industry way of doing things. I figure with the time, effort - and money - I'm investing in my boat, I don't want to take risks with something like this. Your mileage may vary.

09-26-2011, 05:00 PM
Forget about three arm pullers - garbage. The tool from Moyer Marine is better by orders of magnitude.

09-26-2011, 05:43 PM
I suggested this to Tartan off list and thought I would repeat it here.

If conventional means (without massive force applied) prove unsuccessful, I'd be inclined to cut the side of the coupler lengthwise with a Dremel tool and replace it with one of the new split couplers.

09-26-2011, 06:25 PM
Hanley and Neil, thanks for the advice. I agree about Moyer's puller especially since I have already ordered three grade 8 bolts from McMaster's. I will definitely order Don's puller so as not to damage the output coupler, that is cheap insurance against a much bigger mistake. Now the only issue is whether I should invest in a new split coupler, aggrrhh, this is getting so complicated. I am not sure I have to pull the shaft (The yard may be able to get at the keel bolts with the swivel on the socket) and not sure that even if I do need to pull the prop shaft coupler that I will need a new coupler. I will let you know what I finally decide.

Again, thanks.

09-26-2011, 10:43 PM
For what it's worth if you can get the old coupling off I don't think you need to be too worried about it if you clean it up and use it again. If the shaft was rotating ten thousand times a minute and running finely-geared machinery on both ends, that would be one thing...but it isn't.

09-27-2011, 12:40 AM
If the prop shaft coupling is truly an interference fit then I estimate from the coefficient of thermal expansion, that the diameter would grow about 0.3 mil per 50 deg F so you might have to heat the coupler to about 300 deg F to get a 2 mil increase in diameter. Not too bad for a torch plus a little persuasion from a mallet and a block of wood. At $175 for a new split coupler, probably worth a try.

09-27-2011, 01:15 AM
I vote for swivel on the socket.

09-27-2011, 09:17 AM
As he's applying 350 ft-lbs to the keel bolts with a 4 foot breaker bar, I think a swivel will pack it in early.

Now that I think of it, 350 ft-lbs seems like a lot. The Catalina 30 has 8 - 3/4" keel bolts on a 4200 lb keel. Their torque spec is a shade over 100 ft-lbs.

09-27-2011, 11:52 AM
I agree that the 350 ft-lbs sounds like a lot of torque especially for bolts that are 39 years old, even if they are stainless. I will make sure to use anti-seize and go carefully, that is why I want the yard mechanic to take charge, I am hoping he has some experience in this matter.

09-27-2011, 11:54 AM
Neil, here is the torque schedule for Tartan, how does this jive with your experience?

Keel bolts should be torqued to the following specifications:
1" bolts should be maintained at 350 ft/lbs
3/4" bolts at 250 ft/lbs and
1/2" bolts at 180 ft lbs.

09-27-2011, 12:32 PM
I will make sure to use anti-seize and go carefully, that is why I want the yard mechanic to take charge, I am hoping he has some experience in this matter.
And insurance!

Regarding your post and torque, I'm certainly in no position to second guess Tartan's specifications but they are quite different from Catalina's.

I will add this though, the more coarse the threads, the higher the required torque to achieve the same holding power. This discussion arose in a Perkins diesel class I took decades ago. The head bolt torque for the 4-107 was half that of the 4-108 which are essentially the same engine. Being an inquisitive sort I raised my hand and asked.

Not that it applies here. The Catalina 30 keel bolts are 3/4 - 10 or 10 threads per inch which equates to coarse threads. In our case of comparison it may be the adhesive Catalina uses that makes the difference.

09-27-2011, 12:47 PM
My C&C 30 has 1" keel bolts and the torque spec is 350 ft lb.
I retorqued mine a few years ago which helped seal up a hull to keel crack, common on C&Cs of that vintage.
I found a used torque multiplier on ebay which allowed me to use my standard torque wrench which only goes up to about 150 ft lbs but still torque the bolts to an accurate setting. This was an expensive purchase but didn't cost me any more than letting the yard do it.
I agree that you better be comfortable with the integrity of the bolts and the glass around the area to be torquing that high.

09-27-2011, 03:17 PM
ms, when you re-torqued the keel bolts, did you also drop the keel to re-bed the keel to hull joint or just re-bed the nut to hull opening? and if so what did you use for sealing, butyl strip, 3M5200, etc?? I will see if the yard has a 3/4 inch torque multiplier attachment. Right now what I have is a home made 3/4 inch setup consisting of a 1 1/2 socket, swivel, 4' extension (a short extension cut in half and welded to a 4' pipe to get clear of the engine) and a 2' breaker bar with a 4' pipe. The fellow who made this to do his T30 kindly loaned it to and said he estimated he could pull about 80 lbs, so 80 lbs x 4 ft = 320 ft-lbs, close enough. I will see what the yard has in terms of 3/4 drive tools.

Neil, when I get the check nuts off the keel bolts, I will check the thread pitch.

09-27-2011, 05:20 PM
I did not drop the keel. I cleaned what I could out of the crack between keel and hull (less than 1/8" at the forward end) the caulked with polysulfide. I caulked around the keelbolts (under the nuts and washers) with polysulfide also. I avoid 5200 in most cases.

I still have the torque multiplier and a 1-1/2 deep socket, 3/4" drive. I'll loan it to you if you're willing to pay shipping both ways (could ship priority mail for about $8 each way). My yard, which is well-equipped, didn't have one.
I used a 1/2" drive torque wrench with a 1/2 to 3/4 adapter to drive the torque multiplier. It didn't break.

09-27-2011, 10:08 PM
Aye, and the output flange can be had for $62 from the on-line catalogue. Sooo....you have to ask yourself one question---"do you feel lucky? well, do you...."?:)

Yes, you certainly can have it...and I wasn't so lucky, so I did, obviously.... :cool: