Drive chain scraping inner primary

Mark F · Aug 13, 2011

Yes that's right folks. I was working on the bike today(still a non runner) and as I was spinning the rear wheel I noticed a scraping sound. At first I thought the chain was scraping on the chain guard, but on closer examination I noticed that the chain actually scrapes against the back of the inner primary. Not hard mind you but it does not look like there is any clearance between the chain and the back of the primary cover. The left hand edge of the swingarm also looks to be hard up against the cover also and even the left hand side of the gear box adjuster.
How much clearance should there be between the chain and the back of the inner primary? I thought maybe I am missing a spacer or packing piece or else something is bent. :cry: I hope not.

stockie2 · Aug 13, 2011

Mark, they do run close depending on what chain you have on there. H-Duty 530 with most likely rub. But if the primary has been fitted without carefully spacing the central cover bolt, it will pull the cover in toward the chain. Try releasing the nut on the centre cradle hold the centre stud for the primary. If is missing a spacer, maybe you can make up a C washer to insert in there to avoid tearing the inner primary off?

Cheers Richard

pdl999 · Aug 13, 2011

Hi
I had exactly the same problem. After much investigation and the help of Norman White, the lessons learned are:
1. there isn't much clearance there to begin with
2. the shimming of the inner primary centre stud is important
3. gearbox alignment is also critical (mine was slightly out and the main contributor. With the primary stripped it showed itself as misalignment between the gearbox and rear wheel sprockets).
4. Don't leave it like this! I only discovered I had a problem when a friend noticed I had a broken rear chain link. In fact I had three broken links! I had previously heard the scraping noise but foolishly ignored it. I was at the Colombres rally in Spain and had been enjoying a good thrash around the sweeping bends in the mountains. I shudder to think of the outcome if the chain had broken.
5. Converting to a 520 o-ring chain gives more clearance and means very infrequent chain adjustment is needed. Involves a slimmer gearbox sprocket (availabel from Andover Norton) and machining the rear sprocket.

All the best
David

DogT · Aug 13, 2011

Make sure you have the thick spacer on the 1/2" bolt on top of the trans between the trans and the cradle too. If that gets left out, it can skew things around. Item 34 here http://www.oldbritts.com/1971_g5.html

Dave
69S

maylar · Aug 13, 2011

Kinda like this?

Chain chewing into the chaincase, chips of aluminum in the links, a donut worn into it from the swingarm? Yup, BTDT.

In my case it was loose engine mount bolts allowing the engine to twist to one side and push the chaincase inward. Further investigation revealed oblonged and cracked crankcase mounting holes...

.. requiring a rather drastic solution...

I hope you're luckier than I was.

Hobot · Aug 13, 2011

Also can't forget about the swing arm spindle/bush slop that drive chain tugs to the LH to add to chain chew into case list. One over tight primary chain event may bend the the main shaft a tad toward chain case chewing too. Ugh must think down the whole line of power transmission and just finding one obvious fault don't mean there ain't more lurking in that line that add up too. Like bushes and bearing bores.

It is not drastic to enlarge the crank case holes its a very wise upgrade and 7/16" is what the hot rodders do with AN-aircraft bolts in essentially interference fit all the way through cradle. DynoDave who needs coke bottle glasses to see showed me a whole 'nother level of human observation resolution on examining for and finding case fractures, so thin a cob web strand is planner to see. Some of these develop below the surface. Wise to eliminate the sharp corner stress risers the factory left on the crank case seats. I ground them out, have melted horrifically deeper, both to find more cracks and then fill with Al lava, then grind back radius, ugh then spend a week or more daily pounding pecking to bring distorted seams back to sealing closeness.

Mark F · Aug 13, 2011

Lots of good advice there thanks. There does seem to be a lot of opportunity for things to get a little skewed.
Just out of curiosity just what sort of gap should there be between the end of the swingarm and the back of the primary?

DogT · Aug 13, 2011

I was just looking at that tonight, and it's impossible to see, even through a mirror from the bottom, the chain is in the way. I'm guessing it's 1/8" or less, real close.

Dave
69S

CanukNortonNut · Aug 13, 2011

DogT said:
Make sure you have the thick spacer on the 1/2" bolt on top of the trans between the trans and the cradle too. If that gets left out, it can skew things around. Item 34 here http://www.oldbritts.com/1971_g5.html

Dave
69S

I am with Dave on this one as well. Check for this spacer. It is .125" thick and it's not a 1/2 inch bolt. More like 9/16" or .5625"
Regards,
CNN

Mark F · Aug 13, 2011

Dave I just had a look then and I can look down on the end of the swingarm through the battery area while shining a torch from behind.
Is the primary cover center stud the only fixing for the rear of the primary?
Can it be shimmed out more off the cradle?
The shim(item 34) on the gearbox looks right. About 3 mm thick.

rx7171 · Aug 14, 2011

Do you have both items 9 and 11 in place

http://www.oldbritts.com/1972_g8.html

Josh Cox · Aug 14, 2011

Mark,

The way I would suggest:

Tighten the three front bolts into the engine, check the chain clearances, insert enough washers to suit your needs onto the centre bolt that mounts on the cradle, it does not matter which end of the long bolt that has the washers, i.e. you can easily put the washers between the engine / gearbox cradle and the long through bolt.

Mark F · Aug 14, 2011

Items 9 & 11 in place. They do not look very thick to be honest. One of them is referred to as a shim in the parts list. Does that mean the thickness can vary?

DogT · Aug 14, 2011

You really need to get all the stuff out of the primary, pull the inner cover off, get a new gasket if you're using one, and install the primary inner with just the 3 bolts into the engine and no washers on the center fixing bolt. See what kind of spacer/washer you need from the center fixing bolt to keep the inner cover from stressing either way when finally installed. Install inner cover with the correct spacer, they all vary. Then check the clearance at the back of the cover to the swing arm. If it doesn't clear or your drive sprocket still doesn't work with the chain, you need to find out what is out of tolerance, bent or installed incorrectly. It could be too the engine is skewed in the cradle but something is not right. It could be the shims under the drive sprocket too. I would fix this before proceeding very far. Don't forget to line up the primary sprockets so the chain is pretty straight too. That has to do with the shims under the drive sprocket.

Dave
69S

Mark F · Aug 14, 2011

Yes Dave I really think that is the only thing for it. I was going to pull the clutch for a clean so I've got go in there anyway. The bike is off the road at the moment waiting on head work to be done etc so it isn't about to be ridden. It is something that needs sorting out although it has probably been like this for some time.
I expect the inside of my cover probably looks like Maylar's.

pete.v · Aug 14, 2011

Mark F said:
Items 9 & 11 in place. They do not look very thick to be honest. One of them is referred to as a shim in the parts list. Does that mean the thickness can vary?

Yes, this thickness can and does vary. Different gasket thicknesses at the crank end is a common. A thick crankcase to inner primary gasket will cause the cover to deflect in to the chain when tightened to the center stud. I needed, you have to hold the primary inner tight to the crankcase to determine the shims needed to get it all squared up. This process will also make the outer cover seal up nicely, keeping the inner seal surface flat.

Pjr1440 · Aug 15, 2011

Hi guys, interesting discussion here, especially given my chain has made a scraping sound when being wheeled in and out of the car park which I have assumed was the chain against the chain guard! I better get in and have a good look at it just in case!
:roll:

DogT · Aug 15, 2011

I got a look at the clearance from my primary to swing arm through a mirror last night and it looks to be about 1/16". You have to put the mirror back between the drive chain next to the rear sprocket. The chain (std) sits pretty much centered as far as clearance and has maybe 1/8" or more each side. I'll see if I can get a picture of it.

Dave
69S

Mark F · Aug 15, 2011

1/16 inch isn't much clearance. Doesn't leave a lot of room for things to go wrong does it.
I'll probably get to have a look at it this weekend.

maylar · Aug 15, 2011

It's so close on my bike that I have to grind a bit off the posts of the master link.

Drive chain scraping inner primary

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