In this photo you can see that I have painted the coupling after getting it back from the machine shop, and am in the process of fitting everything back into place.


A few words on solid couplings:


1) If you are removing a coupling that is more than say two years old, depending on bilge moisture, you probably should not re-use it. Solid couplings are very often a one time use after they have become "made on" Why is this?


When you remove an older coupling you will break a layer of rust free. This rust was your tolerance. A shaft and coupling are meant to fit together very, very snugly and are an "interference fit". In all my years of boating I have only had one or two couplings go back on and maintain the tolerance and those boat only had a few months of service on them. Re-installing a used coupling can ruin your shaft. Shafts are expensive couplings are cheap in comparison. The best rule of thumb is when removing a coupling to always take the shaft and coupling to a shafting/machine shop for a fit & face.


The shafting shop I use fits the coupling to the shaft with a light interference fit. This means it DOES NOT just slide onto the shaft. Western Branch Metals, the largest supplier of prop shafting in the US, suggests that the shaft OD be about 0.0003" - 0.0005" larger than the ID of the coupling. A proper fit will require some light tapping to get them together or some light heat, no more than about 160F, to get the coupling over the shaft.


A "clearance fit", where the coupling just "slides on" is 100% unacceptable for a straight or split coupling. It can cause excessive shaft, key and coupling wear and can eventually lead to a shafting fracture or failure. The coupling must be a light press fit or light interference fit.


2) The shaft is usually fine and can be cleaned and re-used. It is the steel coupling that gives up surface material, and this lost layer of rust means you can't re-use it. If the coupling just "slides" back on it is too loose.


3) When reinstalling a new coupling to an old shaft you should always have it fitted and faced by a COMPETENT machine shop or prop shop. Shops charge around $65.00+ for a fit and face. While you are at it have the shaft trued. Shaft truing is more costly than a just a fit and face but generally cheaper than a new shaft and it will get rid of any annoying vibrations. Another good practice to eliminate vibrations is to "lap fit" the prop to the shaft and you could actually do this yourself.


If you hit something and bent the tapered end of the shaft near the prop the shaft is often considered ruined and most any reputable shop will not straighten it but let them make that diagnosis.


4) When re-installing the shaft you should get it started with the machined in "lead" then lightly tap it home with either a rubber mallet, soft lead mallet or an oak or maple block protecting the shaft, and a hammer. Alternatively you can cool the shaft and warm the coupling. I am not talking blow torch here just warming one and cooling the other. For this job you'll usually need two people or many trips up and down the ladder. Tap it in while looking in the coupling holes until you see the dimples for the set screws. I color the spotting in the shaft with a red sharpie marker then use a flash light to see when they "dimples" or "spots" are lined up. Don't over do it cause backing it off is more of a pain than driving it in.


5) Shaft keys should fit tight in the shaft, mallet snug. The coupling fit should be snug but not binding. If the key has any play/slop in either the shaft or coupling have a machine shop make you a new one.


6) Anti-corrosives like Tef-Gel can sometimes aid in future removal but it is no guarantee and is generally advised against. You should also not use a never-seize product containing any aluminum, copper or graphite as it can add to galvanic corrosion issues.


I have been using Tef-Gel and had good to mixed results up to about two years with coupling removal. It does aid some in the removal but does not always mean it's re-usable. It is NOT recommended to use any lubricant between the coupling and shaft so do so at your own risk.


7) If you can, don't replace the coupling with a solid coupling and instead replace it with what's called a "split coupling". This will make future removal and re-install much easier. Even with a split coupling you should still have it fitted and faced after removal. The $65.00 +/- is well worth it. Split couplings require the utmost care in installation. The nuts should be tightened EVENLY and to proper torque. Uneven tightening of the clamping nuts can cause a split coupling to be thrown off in true. Tighten each one a little at a time.


8) Never use a large hammer, or a slide hammer, with any great force, to pound or pull directly on a coupling that is connected to the transmission. You can rather easily damage the bearings and brinel them. Brineling of the bearing surface creates an imprint of the roller bearing on the face of the bearings race. This brineling of the race will eventually lead to a bearing failure and gear box re-build. In extreme cases the gear box may shatter and there are many who have done just that. Slide hammers should be avoided at all costs and NEVER used to pull a shaft out of a coupling when it is connected to the gear box.