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thanks for your comments
according to the records that came with the bike(81 sport) the belt is thee years old and was a nlm A belt, as i just want to take it easy should i just order an rs belt before i look inside?

As far as I know, the only current source of fresh graded belts is NLM. Any NOS belts you might find will be extremely suspect due to age.

If you look up gaussian distribution, B grade pulleys and belts fall in the middle of the curve, with the A and C grades to either side. There are far more engines with B pulleys than the others, since more of the belts produced fell into that grading. Production techniques have generally got better over the last 40 years and the 'width' of the distribution curve has generally narrowed, so fewer belts are likely to fall into the A and C grades.

Given the cost of graded belts, it might make sense to track down a B pulley.

Heat and light (particularly UV) are the enemy of rubber products and I'd rather use a bearing and transmission specialist who store correctly and have a high stock turnover.

I have never seen A or C pulleys. They must be quite rare.

Simply Bearings also sell belts.

gods teeth, i thought getting the right belt was hard without thinking about pulleys!
so i should ring nlm i presume?

scbond2@yahoo.co.uk wrote:gods teeth, i thought getting the right belt was hard without thinking about pulleys!
so i should ring nlm i presume?

Why not establish for certain which grade belt you need. What letter is marked on your cam pulley?

My K1 has a C pulley-so if this is rare does it add value? I tried a belt from RS and I could make out a slight noise in use,so reverted to a C belt.so it looks like tracking down a B pulley would be something to aspire too. Well that task will beat having to watch Sickly Come Dancing or whatever it's called.
John

so i looked at my pinion cant see letter where should it be?
anyhow the isoran seems tight and in good condition..has anybody broken one and under what circumstances?

cheers steve

A letter 'A' can be seen (upside down) at the 9 O'clock position on this pulley

If there is no letter, it´s suposed to be a "B" (the norm). The first pinions had no letters stamped.

i thought that the belts were just sorted to the pulleys in which case why did they have different pulleys and can you change them or are they related to the timing in some way? ( 81 sport)

No, it´s like Paul said:

The results of the production of he belts (today it´s a bit better, but the problem in general is still there) could and can even in the same "Kalotte" of 1 meter not garantee always the desired exact length of the belt ("B"). The production of the diferent pulleys was a result of this phenomen, not the other way around.

norbert

New belts from a specialist stockist will have a date code for example 01062017 either on the belt or on the box or packaging. I have one marked 10/96 on the box. I will not be using that! Only once a broken belt it was OEM. The manufacturer was Pirelli I think. It was most likely new old stock passed off as new. No damage just one slightly compressed push rod and bike has done 40k miles since and hasn't been taken apart.
Mark

thanks for the comments but answers breed questions like is it possible to fit non oem pulleys or are they special sizes?

scbond2@yahoo.co.uk wrote:thanks for the comments but answers breed questions like is it possible to fit non oem pulleys or are they special sizes?

Special sizes? No.

Specially machined for the application? Yes.

A cam pulley might easily be made from a stock pilot bore pulley, but the crankshaft puley would have to be machined from scratch.

The standard suggestion has always been to sweet talk your local bearing supplier, take home several belts, try them and use the one that fits best and then return the others. It's a lot less hassle than changing pulleys. Whether the belt production process still has the same variability as 40 years ago who knows?