might need the intermediate gear/sprockets

Joined
Oct 28, 2014
Messages
968
Country flag
who would sell a brand new idler gear with two sprockets? No such thing from AN. I am unable to take the slack out of the cam chain on my Atlas. At the slackest it measures 3/8". at the tightest it measures 1/16". Cannot do any better than that. New chain. I am guessing it is sprocket wear, everything else is appears good. AN sells the cam sprocket. I could remove the head to unload the valve springs. I wanted to install a new heavy duty Iwis cam chain, discovered the old one was smacking the tensioner bolt. The motor does run very well, though.
 

texasSlick

VIP MEMBER
Joined
Jan 2, 2013
Messages
2,975
Country flag
IIRC, proper tension should be 1/4 inch up/down movement at the center of the chain run. From your description, you are in that range.

If your old chain has interference with the tensioner bolt, a heavy duty chain will likely be worse, as HD chains have thicker plates.

Perhaps your tensioner is not set up properly. I have the set up in my Norton Manual, but I am not with it right now.

Slick
 
Joined
Oct 28, 2014
Messages
968
Country flag
3/8" is a lot more than 1/4" when it comes to the chain movement. The manual says 3/16" so I am running double that amount and no way to reduce it. I tried another used cam sprocket with MAYBE a small improvement. I can try a new sprocket and see what happens. If that produces a noticeable change then the intermediate sprocket is also suspect.
I am sure the tensioner is set up correctly.
I mickey moused the solution by using a button head allen bolt in the tensioner. I put it in the lathe and reduced the O.D. of the head. Now the chain doesn't hit it.
 

texasSlick

VIP MEMBER
Joined
Jan 2, 2013
Messages
2,975
Country flag
There is an old saying that "No two Nortons are alike"

It is possible the center to center distance of the intermediate gear to camshaft on your engine is slightly undersized. I have that measurement for my Atlas somewhere in my notes, and will post it if/when I find it.

I am confused by your original post where you say "..... At the tightest it measures 1/16".
That should be sufficient to get you to 3/16" up/down movement (which is the spec in my book as well, my memory notwithstanding).

Slick

Edit: 04.11.20

My Atlas measures 3.746" CC Intermediate shaft to camshaft. I remember measuring this very carefully several times as I wondered if it should be a nominal 3.750"

Fitting of tensioner: Manual states "The thinner of the two clamping plates goes on first with long end down, followed by the tensioner and the thicker plate with long end up."

I happen to have my timing cover off. Examining my tensioner, I find the nuts to be positioned well out of the way of the chain. Perhaps a picture of yours would help.
 
Last edited:
Joined
Oct 28, 2014
Messages
968
Country flag
It seems I don't have the plates positioned correctly but I don't see how that would make a difference. I will follow the manual however. I am using two button head allens in place of the stud and nut combination.
I turned it so there was the minimum amount of slack and gave it 1/16" which I think is not enough. Turning the motor over to the slackest point I come up with 3/8" which is quite a bit. Discussing this with JC and Mr. Radford the conclusion is that the sprockets were not cut perfectly and they have probably worn which exaggerates the "tight - slack" syndrome. So, I can either A) bolt it back up with a new chain and run it knowing the chain is whipping up and down like crazy or buy two new sprockets hoping to reduce the whipping problem. That will cost about $250 for a maybe. Phil was able to locate both sprockets. I would be much more inclined to buy the sprockets if I knew they were machined on the latest and greatest machinery giving dead on accuracy but I'm afraid it's wishful thinking. These come out of China or India and are a pig in a poke.
 
Joined
May 28, 2003
Messages
2,235
Country flag
Considering how much stress is on the chain from the camshaft resistance in both the top run while pulling against the valve springs then transferring to the bottom run to restrain the valve closure pushing the cam. I don't see where a loose chain is desirable at all.
To me it seems a slapping chain would encourage a quick wear out of the chain side plates against the adjuster and generally the sprockets too.
I made my own copy of the RMA auto adjuster since he does not make the twin chain version. It's on my 68 Dunstall atlas with rear points.

 
Last edited:
Joined
Oct 28, 2014
Messages
968
Country flag
I am using the support plate blessed by AN. From adjusting umpteen rear drive chains I know that a little added distance translates into a lot of chain movement. I suspect that it is a cumulative thing here. I removed the intermediate spindle to check the play in both the case and the cover. Usually the spindle does not come out of the case without heating so I might be losing a TINY bit there. Yes the spindle could be worn..slightly...it always runs in an oil bath. I don't see how it could be bent short of total disaster. The prime culprits should be the cam bearings, the quality of the sprockets whether machined poorly or worn, and a worn chain.
Everything in the timing side sits in 1/2 cup of oil and gets PLENTY of oil at all times. Very little wear occurs in this dept. Have you ever replaced any timing gears due to wear? I haven't.
ANOTHER CLUE...the magneto chain has very little flop to it. I am not concerned with how it is running. Does that put less suspicion on the intermediate gear?
I think the best route is to buy a new cam sprocket and intermediate spindle and see what happens. I just wish I could be 100% sure that the new sprocket was dead on accurate.
 
Joined
May 28, 2003
Messages
2,235
Country flag
Don't you have a dial indicator?
Put the gear (old or new) on a shaft held in a vice? Flop a chain over the sprocket. read the top of the chain rollers running as you turn the sprocket by hand. You should be able to read the non concentric as you read the tops as they pass by? not measurement rocket science.
I did the math to see that the chain cyclically does a complete revolve every ? 10 /15/? ( I forgot the #) revolution so wear is never due to the chain since it does not repeat. The sprocket are very repetitive and synchronized with the cam loading pulses. That is what you detect in the above test...
 
Joined
Oct 28, 2014
Messages
968
Country flag
update...I made a tool for holding the sprocket, while it was still mounted in the lathe I set up the dial indicator and measured the tops of the rollers. I did it twice to make sure I could repeat the results. generally it bounced between 0 and .005 but two rollers jumped up to .013. Not enough to account for all the slack I have but the valve springs are going to exploit any change in dimension. I will measure the chain on the intermediate sprocket tomorrow. It requires a different fixture.
 
Joined
Oct 28, 2014
Messages
968
Country flag
further update...I removed the head (loose valve guide) and that took all the pressure off the cam. At this point it is much easier to set the tensioner while using the AN holding tool. Doing it with the head on is very misleading.
 
Top