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Am I right in thinking that the Mono exhaust systems were all 38mm?

I ask since my ST special is sporting one and I was thinking of getting a shinier example!

I can't give you measurements but I can confirm that all main stream R80 and R100 models after '85 (R80, R80RT, R100, R100RS and R100RT) use the same exhaust headers. The R65 models do not. Don't know what all the differences are but one of them is inevitably that the shape of the bends in the headers is going to be different due to the fact that the cylinders on the 650cc engine are closer together than on the 800cc and 1000cc engines.

The silencers (both front and rear) are common to all engine sizes.

Rob

Thanks Rob,
I wonder if the long serial number stamped on the side of the silencer would identify its type/size...

The BMW part numbers for the silencers are:

Right - 18 12 1 458 362
Left - 18 12 1 458 361
Front - 18 12 1 452 097

These three components are common to all Monoshock 'main stream' airheads and, by that, I mean excluding 'GS, 'ST and 'r/mystic models.

I'm not sure I'm understanding your query correctly. The 'ST, as it came from the factory, has a two into one system The silencer is either common with some years of the R65GS. Part numbers are:

Front - 18 12 1 242 966
Rear - 18 12 1 242 965

None of the major components of the 'main stream' bikes exhaust are compatible with the 'ST (and vice versa)

If your bike has been fitted with a 'main stream' exhaust, then I would anticipate that there has been some modifications to make that possible. It would be a good idea to identify these before buying new parts. Apart from anything else, if the bikes has a low level silencer, you need to know which side silencer you need to replace it.

Rob

Yes, Rob, the bike has a low level exhaust. I'm not even convinced the headers are standard given the way they both meet near the lh header end. Very nicely shaped though and look standard. The collector is a one off, again nicely made in stainless and then onto what looks like an inverted lh Mono silencer with an after market silencer 'hanger' to connect it to a specially welded strut on the rh pannier rail. It certainly has standard BMW printing on it. I'll compare the serial numbers you gave me to those on the pipe.

The single silencer was a bit rorty - being fed by two headers - but a set of 4 stainless pad scourers (Lidl 99p) up the entrance end has made it sound standard BMW twin and no obvious effects on performance.
Many thanks for your help. Simon