Hi. I’ve been wrestling with the primary chain of my Dommie 99 today trying to get the tension correct. The chain, engine sprocket and the complete clutch are all new. However, when I’ve set the tension to 1/2 inch up and down movement at the tightest point of the chain, the rest of the chain tension is 3/4 inch up and down. At first I suspected a dodgy new chain but I put the old chain back on and it was exactly the same. I then checked the runout of the mainshaft and crankshaft and they were around 4 thou out at worst, so no big problem there. I just can’t work out why the tension varies like this on both a new and old chain. The clutch basket sprocket and alternator rotor both run true (within the 4 thou runout). I’ve tried moving the clutch centre to several different positions on the main shaft but the tension ‘issue doesn’t change at all. Is this deviation in chain tension ‘normal’? Should I be worried about that 1/4 inch variation? The chain doesn’t hit anything in the chaincase when the clutch is rotated.
Tension can only vary because of eccentric sprockets because its length is constant at any one time. The up and down movement is very sensitive to the gap when it is nearly tight. A slack chain is better than tight, especially with all that aluminium down there expanding more quickly than the steel chain. The difference between 1/2" and 3/4" is to be expected. It's fine.
As per the instruction book, 1/2" to 3/4" up down movement is ok. As long as it exceeds 1/2" on the tightest spot. Slightly more is acceptable. So Norton knew that some manufacturing differences exists, both on sprockets and definitely on chains. As David said, a warm engine, engine plates and gearbox tightens the primary chain. Caused a number of broken chains on the TT.
I suggest that you look closely at the chains you have. There are several makes that are basically a waste of money and this includes a few famous names from the past, previously manufactured in Japan, Italy and the UK.
The attachment shows my UK manufactured primary chain after less than 300 miles. The rear chain had to be junked after only 1000 miles despite a wax/oil dunk every 3 months and regular spraying.
Go for a good German make.
When you tension (or rather adjust the slackness) of the primary chain, the rear chain should be under load when you do it. I use the subtle method of putting a boot on the lower run. This removes any movement in the gearbox adjustment. If you don't do this, the greater tension of the rear chain when you are running will tend to pull the gearbox back and make your primary chain too tight.
I hope this makes sense...
... to overtighten the primary first then use the adjuster(s) to push the gearbox forwards till I get the right slack. This ensures that all the play in the adjusters is taken up so the gearbox won't be pulled back.
There is nothing a German officer can't do if he has the instruction book.
Surprisingly, it is all covered in the Norton instruction book. I usually check with it, before answering to a thread.
If length of the side plates differs one thou in manufacturing, up/down movement of the chain can vary more than half an inch between tightest and loosest points. Quality chains recommended.
Hi Mikael. That’s amazing - no wonder it’s so easy to get uneven chain tension. The only trouble is, what chain brands are now considered ’quality’ items? I bought a Renolds primary chain because when I was younger, it was the best. Now I understand it’s no longer any good. I’ve also got a Regina rear chain - no doubt someone will inform me that it’s another inferior brand. Oh well, that’s progress for you!
By the way, which Norton book were you referring to? - I couldn’t find the information about chain plate differences in any of my books.
Renolds allegedly lost interest and pulled out of making motorcycle chains because they primarily make their profit from industrial chains. Modern motorcycles operate at power levels far outside the Standard design parameters (BS, DIN, ASTM and the like) for chains - even more so when they are not even run in a full oil bath.
I still run Renolds chains. Primary chains as long as there is oil in the chaincase just go on and on. rear chains last up to 6,000 miles (as long as you avoid winter salt), much the same as ever.