It could be that your track is too lose and the drive cogs are skipping???? What’s your track tension like?

 

Track is tensioned to spec as the original manual says, chain case is vibrating bad and the chain seems super tight . Maybe something looses up on track and we have to recheck tension after driving it around a bit? To me it alsmot seems like something is going on with the bottom cog in chain case , long day putting it all back together, going to drain chain case oil out and have a look down there tomrorow

 

The 71 and 72 Olympique 300 & 335 used the same axle. The 399 and slider sleds used a different one as did the T'NT's. I'd check the track to see if it had shrank and also make sure the drive axle is straight. On my 72 T'NT that I just did the sprockets were shot/crumbled on the axles. No big deal as I can replace them from Goose. Got everything all back together and couldn't adjust the tension or alignment. Track had shrunk. Got a brand new one and all is good.

 

How are they differnt, the 399 engine swap happend 40 years ago, dad can't remember what went on with the drives shaft and chain case etc, the old shaft we took out of his looked identical to the one we rebuilt and put back in from the other sled. Or is there slight difference in spline? Diameter?

 

I don't think anything with the track can cause what you're describing in the chain case.
Does the sled have the original chain case? I believe it should be a pressed sheet metal body that is painted black? with two plastic chain tensioners, one on each side, and the chain is a double link type. Without the tensioners the chain should be quite loose, for it to be tight it must be pushing the tensioners back completely.. I wonder if the chain is riding up on the lower sprocket.
Looking at your photo of the front axle, it does not have the spacer that my 72 oly had. The spacer prevents the bearing from going too far towards the seal. If your bearing was driven on too far or moved as you were riding, your bottom sprocket might be shifting towards the bearing.

This is when I redid the '72 bearings and seals, you can see there is a ring between the bearing and the seal.

 

Also there is a spacer / shim / washer that goes between the bearing and the sprocket. You can see it on top of the bearing in that photo.

 

See step 7 on page 1-04-05 of the 72 workshop manual:

https://vintagesnow.com/Ski-Doo_files/1972%20Ski-Doo%20Shop%20Manual.pdf

 

The 399 has a spacer on the drive axle like John said Number 11 on the parts page. Also the chaincase is different on the 399 than the other sleds. Probably the reason being its a twin vs. a single.

 

yeah, my 72 was the 399 twin..

 

Step 8 from the link that John posted

8. Place a bearing with shield facing sprocket
on each end of axle. With an appropriate pusher,
push the bearings into position. The bearing on
the splined side of axle must be pushed until it is
seated on bearing stop. The end housing bearing
must be flush with end of drive axle

 

Sorry, I forgot he said it had a 399 put in it. That probably means someone swapped a 399 chain case in it.

 

Yes it allready has the chain case as stated above , but the 335 chassis we took the other shaft out of was identical. Everything looks good, only thing may be the spacer that we discarded off the original shaft we took out and brain farted thinking it was part of the old bearing. May have caaued the new shaft we put in to slide back and forth and rise on the splines not properly to make the chain go super tight?

 

yes I think something is not right with the geometry there. You'll probably see once you drain the oil/have a look down there.

 

FYI. John posted a pic of a axle with a spacer between the bearing and seal, that is correct for a twin axle. That spacer is about the width of the difference in spacing from a single to twin. It is not really need for the drive to function ( in a twin case) but with out it the seal has a very good chance of going too far into case, it is a backer for the seal. Ie: on a single the bearing is the backer for the seal but the twin has a extra 1" from seal to bearing.

 

Update , fixed. After looking at all options before tearing everything apart, I was looking down into the chain case threw the top opening and noticed a peice of metal stuck in the bottom cog , long story short, we where able to fish out enough fragments to say the thin washer that was suppose to go betwen cog and chain case was not placed in and got grinded up, makes sense after the test run was fine . Got it all fished out , was causing chain to ride up and tighten chain then snap.thank you for app your help

 

Glad you found it. I would drain and replace the oil, and make sure there are no small metal bits in the chain case. I had a similar problem where one of the chain tensioner springs broke and half of it got chewed up the same as your washer, the oil was full of small gritty bits that worked their way between the seal and the seal cup and ground deep scratches into the cup as the seal was rotated by the driveshaft. Was never able to stop it leaking oil after that.