Topic: 73 to 77 750 swing arm swap

[Bailgang]

At the moment I'm assembling a 77K engine that will be put into my kids 73K and I already know that Honda changed the output shaft on the 77 engines which affected sprocket alignment. I have the cush drive/sprocket carrier from a 77K as well as a 77K swingarm to address the sprocket alignment issue so my question is will I have any issues installing the 77K swingarm into the 73K frame? Also there is axle spacing, will the axle spacers for a 77K provide the correct spacing when using the 73 rim with 77 cush drive along with the 77 swingarm? I'm trying to be as prepared as I can for this swap because it's a 3 hr drive to Bloomington, IN. where the bike is and I don't want to get there only to find out I forgot something.

[70CB750]

It should drop right in.  I put 78F arm in my K3 frame without any problems.

http://forums.sohc4.net/index.php?action=dlattach;topic=115305.0;attach=208443;image

[Tugboat]

Actually, you've got most of it. But do note that the 77/78 750's had a 10mm "shoulder" on the upper LH shock mount (on the frame) to keep that shock straight. Otherwise, the wider swing arm's lower mount will be further out than the upper mount, causing the shock to be off kilter. Dig?

The newer swing arm will drop right in - no spacers needed. But you will have to lose your chain guard, as it will be too close to your LH shock.

Best way to remedy all of this: use your stock swing arm, stock wheel & carrier, and use a 10mm offset rear sprocket from CycleX.

[70CB750]

Or replace the shock mount:

http://forums.sohc4.net/index.php?action=dlattach;topic=115305.0;attach=210578;image

[Tugboat]

That's great if you can weld. I can't. And for $50, with no other modification needed, why would you bother?!?

http://www.cyclexchange.net/Chain%20Sprocket%20Page.htm

[Bailgang]

Actually, you've got most of it. But do note that the 77/78 750's had a 10mm "shoulder" on the upper LH shock mount (on the frame) to keep that shock straight. Otherwise, the wider swing arm's lower mount will be further out than the upper mount, causing the shock to be off kilter. Dig?

The newer swing arm will drop right in - no spacers needed. But you will have to lose your chain guard, as it will be too close to your LH shock.

Best way to remedy all of this: use your stock swing arm, stock wheel & carrier, and use a 10mm offset rear sprocket from CycleX.

That's why I like this site so much because I was unaware of that shock offset you mentioned. I'm a certified welder so a new shock mount wouldn't be out of the question however I'm in no mood to haul all my welding equipment down to my kids place just to do it. I was aware of the offset sprockets that are available but as usual my kid's on a tight budget and costs are already adding up for him so I was kicking the idea around of using the 77 parts I had but that shock mount does pose a problem because the frame is 3 hrs away at his place so the offset sprocket does seem to be a more practical alternative right now. Thanks to all for the info.

[bjbuchanan]

The offset sprocket is a good option because it is a lot less work and there is always that high possibility that sprockets need to be swapped

[Bailgang]

I talked to my son about it and the offset rear sprocket seems to be the way to go however I have my suspicions about the swingarm that's on it right now. Too long of a story to explain why I have my suspicions but he's going to double check measurements on the swingarm that's on it now, we basically need to make sure the bike has what we think it has before we order anything.

[Bailgang]

My son searched around this site and found a thread that had all the info he needed and the swingarm that's on the bike checks out fine. As this thread progressed and learned a few things I didn't know I then had suspicions that a PO may have already put a later swingarm on the bike but turned out not to be the case, once we were sure the bike had what it was supposed to have my son then placed an order with CycleX for the offset rear sprocket. Thanks again for all the info.