Front Sprocket Install

[thussey42]

I just recently purchased a 1983 XS400 DOHC and it seemed to run and ride just fine. However, today as I was riding I had my front sprocket come off completely (I'm just as baffled as you all). Luckily I was able to keep the bike upright and pull over. I have no idea how this happened, I just know that the sprocket is nowhere to be found and that all I have is the sprocket holder (# 28 in this diagram: http://www.bikebandit.com/oem-parts/1983-yamaha-xs400-xs400k/o/m138137#sch207760 ). Now I just need a new one, but cannot see in any diagram how the sprocket is held laterally along the counter shaft. Does anyone know how the front sprocket mounts up besides the holder and the two bolts that hold it in place?

 

[Wolfe_11B]

The sprocket should slide on, then the piece #28 goes on, you spin it in the groove on the shaft to lock the sprocket lining up the bolt holes, then tourqe the bolts. At least that's how I've seen other bikes done, never messed with the DOHC xs.

 

[Lou Ranger]

There's no way that the sprocket should come off unless the bolts holding the retaining clip weren't tightened properly, or the sprocket was not installed properly.
It's hard to believe that could happen - unless your PO had installed the sprocket on backwards. This has happened at least once - it came up on my thread:

http://www.xs400.com/forum/showpost.php?p=82918&postcount=77
​

There's a raised centre section on one side of the sprocket and the other side is flat. If installed backwards (like Bentwrench's was) the retaining clip wouldn't be flat against the sprocket and it would be easy to imagine it flexing enough to allow the bolts to work loose. Different makers of the sprocket stamp the number of teeth on different sides. If someone used that as their guide when installing a replacement sprocket (i.e., having the stamp face out, then, depending on the maker, you could install it backwards.
The correct way (per the manual illustration) is side with raised section facing in, flat side out:

http://www.xs400.com/forum/showpost.php?p=83042&postcount=79
​

Just a shot in the dark - but I hope it helps. Good luck.

 

[thussey42]

Thanks for you and a wolves responses. Believe me I was in absolute disbelief. I thought I must have been seeing things when I looked up between the countershaft cover and the motor and did not see the sprocket, and just the bare counter shaft. I think the PO must have either not tightened things properly or put on the sprocket backwards as you suggested. I just ordered a sunstar sprocket and the #28 part from bikebandit and am waiting on them to come in. Do you know the correct orientation for the sunstars?

 

[Lou Ranger]

... Do you know the correct orientation for the sunstars?

Here is a picture of the 17-T sprocket I installed:

http://www.xs400.com/forum/showpost.php?p=83088&postcount=82
​

The one I replaced was a 16-T made by a different company. It was identical in overall shape, but the identifying stamp was on the opposite side of sprocket - the side which is flat. The one in the pic above has the stamping on the side with the raised centre section. According to the drawing in the manual, the side with the raised centre faces in and the flat side is what faces out to you. The lock plate bolts to the flat side.

You mentioned you ordered a new clip (part 28). Does it come with 2 new bolts? If not, you will have to order them too - unless they were recovered in the bottom of the side cover. Check out page 7-50 of the manual (top thread on "The Garage" page) for a pic showing the flat side out and the lock plate bolts and the torque spec.