Shaft coupling shoulder sizing

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   Hey CCfans! I have been working on a 1984 BFN that was taken apart by the PO before I purchased it. I am expecting that I will need a new prop and shaft but I want to see how the original gear all lines up before I place the order. One thing I noticed is that the shoulder on the shaft side coupler is very loose inside the transmission coupler. I can move the coupler roughly a 1/4” up and down and side to side between the shoulder and recess in the transmission coupler. The shoulder doesn’t look worn down so I am wondering if this coupler is even right. I thought a 4” flange should be standard but based on this mismatch I want to make sure I have the right information. From the outside, the transmission output looks standard and the shaft coupler looks like a typical 4” shrink fit coupler. 

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BF_skier Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: June-22-2021 Location: Hamilton, ON Status: Offline Points: 5	Post Options Post Reply Quote BF_skier Report Post Thanks(0) Quote Reply Topic: Shaft coupling shoulder sizing Posted: July-20-2023 at 11:45pm
	Hey CCfans! I have been working on a 1984 BFN that was taken apart by the PO before I purchased it. I am expecting that I will need a new prop and shaft but I want to see how the original gear all lines up before I place the order. One thing I noticed is that the shoulder on the shaft side coupler is very loose inside the transmission coupler. I can move the coupler roughly a 1/4” up and down and side to side between the shoulder and recess in the transmission coupler. The shoulder doesn’t look worn down so I am wondering if this coupler is even right. I thought a 4” flange should be standard but based on this mismatch I want to make sure I have the right information. From the outside, the transmission output looks standard and the shaft coupler looks like a typical 4” shrink fit coupler.

KENO Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 11125	Post Options Post Reply Quote KENO Report Post Thanks(0) Quote Reply Posted: July-21-2023 at 7:09am
	What's this thing got for an engine and transmission? 454's came hooked to a Borg Warner 72C or a 10-18 and they had 5 inch output flanges Small blocks usually had a 71c or 10-17 and had 4 inch output flanges Silly question...............Is it a good assumption that both of your couplers are the same diameter?

BF_skier Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: June-22-2021 Location: Hamilton, ON Status: Offline Points: 5	Post Options Post Reply Quote BF_skier Report Post Thanks(0) Quote Reply Posted: July-21-2023 at 8:48am
	Hi Keno! Thanks for your response. My assumption that they are the same diameter is based on the fact that the bolt patterns and outer diameter of the couplers match. I assume that means they are both the same size but I could be wrong. The engine is a 351 and the model tag on the trans reads 10-18-002. I uploaded a video showing how much play there is when the coupling faces are held together. https://youtu.be/lKB8usT1_Gw

GottaSki Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: April-21-2005 Location: NE CT Status: Offline Points: 3363	Post Options Post Reply Quote GottaSki Report Post Thanks(0) Quote Reply Posted: July-21-2023 at 9:00am
	Yeah thats jacked up, but i think if you just center it by feel about the OD as you snug it up, should be inconsequential
	"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing." River Rat to Mole

KENO Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 11125	Post Options Post Reply Quote KENO Report Post Thanks(0) Quote Reply Posted: July-21-2023 at 12:31pm
	I knew it was a silly question but had to ask it. I measured the shoulder on a prop shaft flange for a 71c (4 inch) and it was 2.49 inches The recess on the transmission flange was just a teeny bit bigger.........it slid in, but no free play like you have. Same dimensions for the shoulder and recess for a 5 inch setup So since it seems obvious that one of yours is the wrong size to mate properly, with these measurements you can figure which is the bad one. I wouldn't put it together with what you have now and expect things to stay concentric. If you're planning on a new shaft and coupler, cross your fingers and hope the transmission flange is the good one, so you're not buying one of those also. And hopefully you're thinking about a double taper shaft and coupler, it's worth every penny in saved aggravation

BF_skier Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: June-22-2021 Location: Hamilton, ON Status: Offline Points: 5	Post Options Post Reply Quote BF_skier Report Post Thanks(0) Quote Reply Posted: July-21-2023 at 3:09pm
	Thanks again Keno. Based on your measurement it looks like it is the coupler that is out of spec. I measured 2.25” on the coupler shoulder and 2.5” for the recess on the trans output flange. The OD of the flange is 5”. Looks like I will be putting in a order for a double taper shaft and prop while I am at it. Is there a go to prop for an 84 351 BFN?

KENO Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 11125	Post Options Post Reply Quote KENO Report Post Thanks(0) Quote Reply Posted: July-21-2023 at 3:50pm
	Wherever you're getting the shaft and coupler , make sure you give them all the info they're looking for and measure according to their instructions. A phone call would be a real good idea. As far as the prop, an ACME 540 is probably a good choice assuming you're wanting to maintain a good top speed and all around performance

Shaft coupling shoulder sizing

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