Forum bmwbike.com

Hello Mark,

Did all r25/3 come with the chain tensioner as standard?. I've taken off the chain cover and discovered that there is not one fitted. What 's more the chain has about 2.5 cm of play in it. It has already done some damage to inside of the cover. Could the chain have stretched so much?. Should I fit a chain tensioner with new chain?
Also there is a spring loaded clutch mechanism on the timing sprocket. The end with the two holes fits into the chain cover. What should the clearance be between the recess in the cover and this mechanism.Is it a bearing surface?
And lastly if you still have that sketch, re: electronic regulator could you send it please Fax 0030 0286 32440.

Many thanks
Ben

My R27 has the tensioner. I think the R26 did not come with a tensioner, but can be retrofitted. I don't know about the R25. I have a Stemler parts book that says:
"Kettenspanner orginal R27, bei R26 ohne Anderung nachrustbar!"
I can't read German, but it sounds like it is talking about retrofitting.

Initially, three different chain lengths were available for the R25/3 and the R26 in order to provide for proper tension.
The tensioner was introduced 1960 as a standard outfit for the R27. At the same time BMW advised their workshops to retrofit earlier models with the same tensioner set once maintenance/repair of the chain would be due.
The extra sizes for the chain came out of use.
When fitting the tensioner the old chain should be replaced by a new standard size chain.

Thanks Allen and Thorstan

I'll fit a new chain and retro fit the tensioner. I've opened up a real can of worms here. Not only was there enough chain to go twice around the sprocket, but when I dropped the sump I found the remains of an oil ring and a compression ring! Somebody had obviously changed the rings without clearing the damage. Restorations, great fun!!

Ben