Getting ducks in a row as we wait for the replacement swingarm to get chromed.
Will be ordering a new rear sprocket, as the current one has a gouge where the bolt bent/sheared.

I've heard/read that you should replace the sprocket when putting on a new chain, but what about the reverse? Should the chain be replaced when replacing the sprocket?

You'll get as many opinions as posters I suspect, but I've never subscribed to the "in sets only" theory. If the chain is not worn, put the new sprocket on and hit the road.

In case you aren't familiar, there are three major signs of a worn chain:

1. You've run out of adjustment at the rear axle.
2. The chain is kinking and not running smoothly.
3. It fails the "bow" test.

The bow test is done by taking the chain off the bike stretching it out on it's side on a bench and then picking it up in the middle. It should have a fairly small arc from normal play in the pins. If there is a noticeable droop to the ends the pins are worn and the chain should be replaced.

"Noticable" varies with chain length. As an example, a 110 link chain (pretty common size on UJM's, yours may be longer) the droop from center to either end shouldn't exceed 2 to 3 inches. Longer chains will naturally droop more. O-ring/X ring chains droop less.

As mentioned, if the chain isn't stretched you can run it again. When I was racing dirtbikes I replaced them as a set usually. Would find that the old chain would wear faster on the new sprocket given it was already worn to a different sprocket. On a street bike i would have no problem replacing just the sprocket.

Not required. Chain stretch being a fact of life, one should focus on chain safety now and measure its length before riding on that newly chromed swingarm and sprocket.

Nominally the answer is to measure the wear on the chain directly. It's pretty easy but it depends on you having some knowledge of what specific chain is involved. That's what I do racing motorcycles. On a street bike given the annoyance of changing it all and the low wear at least with normal mileage I simply would factor in both the history and specifics. If the current sprocket shows no abnormal wear and the chain shows no specific issues (wear marks, kinks, rust, and most importantly flotation (can you pull it off the rear sprocket)) then its probably fine. But given that the chain is cheap, the sprockets are cheap, but putting the whole silly mess together takes a bunch of time and energy, I generally subscribe to the just do it all at the same time.

NO!!!!

you have to replace the chain, both sprockets, the rear tire and the left front turn signal or you will die in a fiery crash!

but seriously;
Yeah, as stated before, eyeball everything, check the deflection in the chain, and cruise on.
PHPaul summed it up pretty good.

Seems I remember the old "standard" chains I ran on dirt bikes would wear kinda funny but even then we pretty much swapped sprockets when they got "rooster comb" pretty bad from our sandy soil. But I was a broke teenager back then also.

I don't have a clue about ads. But a common issue on some bikes is that the front sprocket is steel and to save weight the rear is aluminum. So those wear lots faster. If both are steel its not a real issue. But in any case what you are looking for is hooking at the teeth as the chain scallops the front face. Chains and sprockets can last a long time with care, but in today's world 10K would be a pretty good run IMO given the maintenance I see people do.

What king of chain OEM? How many miles on it?

I

Chain, LOL

Chains are pretty cheap. Why not replace it?

Bob

I would replace both together, and you can probably find a sprocket/chain set that will save you a few bucks compared to buying separate.

Replace both, and the front sprocket also while you are at it. If there was any wear on the teeth on the sprocket it was because the chain had "stretched". Chains don't stretch, they wear between the pins and links. When this occurs the rollers on the chain climb higher on the teeth as the sprocket rotates under the chain which takes metal from the tooth. If you check the front sprocket which is smaller, the wear will occur in a faster, more pronounced fashion. It will be a lot smarter to replace everything at this point and not have to do a tear down later and buy more parts that should have been replaced at the initial repair.

Not sure the type of chain, but on the current sprocket the teeth still look great.

Original equipment was belt drive, converted to chain long ago by a previous owner.

I believe the old adage is appropriate, but not a hard law for which lawbreakers will be cast onto the grill of an oncoming Buick.

That said, given the context of your in-process work, I personally would replace chain & sprockets.

Meanwhile, I’d like to applaud you all for not evolving into a “which chain lube?” thread.

I've always swapped out both sprockets when I replace the chain. Probably overkill?

Ive treated chains as a unit to include both sprockets, and as individual pieces.

I came to the conclusion I was happier spending the hour or so once doing it all vs. the hour or so changing 1 piece 3 different times. Each part on a different wear cycle just seems wrong.

If the sprocket you're replacing is at all hooked, replace the chain. How's the c/s sprocket?

Changing the front sprocket on an Indian/SS Harley clone isn't easy, you have to remove the primary drive system and backing plate just to get to the drive sprocket.

As with gears, or any intermeshing metal parts they set a wear pattern as they run, if you change them it's wise to change in pairs so the new part can mesh with the other new part and they create their own wear pattern.

At least that's the theory, since you have the wheel off, and you can get to the sprocket now would be the time to do it from a labor standpoint.

Not saying you should, I would if you have significant wear, rust, damage on the sprocket and/or the chain was in bad shape, anything that could be seen as a problem,

Other than that its a good preventative task,
and now would be the time..

You could also change the sprocket and chain length either direction for lower RPM in 5th and better cruise, or for a higher numerically (lower) gear ratio so it accelerates harder.

I always replaced chain with sprockets on my Shovel