Shaft coupling flange

[igillerman]

I’m in the process of installing a new prop shaft on a 1984 C&C 29-2 with a Yanmar 2GMF. The new off-the-shelf coupling flange was like 0.020” bigger than the old one in the red circled dimension so it didn’t fit onto the transmission flange. I had machined down to match the dimension of the old flange, but it’s still 0.004” too big! And the old flange doesn’t slip fit either, also 0.004 too big! It’s unclear how the old one ever fit on. I tried torqueing the new one down (no shaft installed) and unsurprisingly it had like a 0.020” gap on one side.
Both the prop shop and a diesel mechanic I talked to both were like “well those are standard parts, never heard of them not fitting”, so I feel like I’m doing something wrong. Anyone want to wager what’s going on?

 

[ggrizzard]

Not accusing, but...
Did you notice that the four pre-drilled holes in the new flange are not symmetrical?
If your tranny flange is symmetrical the new flange won't fit!

Of course, it could be my 80 year old eyes are mis-seeing the pic you posted.

 

[igillerman]

The four holes line up fine on the transmission. I tried installing it and torquing it down - didn’t self align or press in or anything. Any oddities are just the camera angle!

 

[dlochner]

The flange has to match the transmission, not the engine. Is this the original transmission or has it been replaced? I know this because I had to replace a brand new flange when the transmission was replaced.

 

[Barnacle Bill]

My guess is you don't have the right coupler.

 

[igillerman]

Closing the loop here. Unclear if I have the wrong coupler, the shop who sold me the shaft and sold the coupler after looking up my engine and transmission. Unlikely they made a mistake - the 0.020” could just one of those weird mysteries…

as to the last 0.004” that didn’t fit. Well, that was becuase the flange on the transmission side was bent during the process of getting the shaft off. During my first attempt jacking out the shaft, I accidentally let the socket fall off the shaft and so just torqued the flange forever with 2 bolts, not 4 - bent it like a banana and only realized that yesterday, a month after I did it. Doh.

I opted to just go forward and install the whole thing. New flange did fix with enough torque on the bolts, and the new flange being thicker than the transmission flange sort of bent it back straight. The downside/risk is that was impossible to check my alignment. But I figure it’s worth the gamble, because if it’s out of alignment, I’ll know real quick. And then I’m just back where I was in the first place. Fingers crossed.