Ossa Mar Exhaust

[old trials fanatic]

Currently modifying the OSSA for next year and i was wondering if anybody has had any experience with making an exhaust for a MAR?

One thing i have never got my head around is why the exhaust is such a large bore? A 320 Majesty seems to run fine on a much smaller pipe as does my James so why does the OSSA and incidentally the Bultaco have such a large bore pipe?

Has anybody tried a smaller bore one on their bike?

The reason for asking is i can easily get hold of some 38mm s/s dairy bends which is what my James exhaust is made of. Also the TY twinshock exhaust is about the same diameter and a 320 seems to have no problem breathing well on that size.

If i could get hold of a 310 OSSA pipe then i coud just tuck it in a bit better, wouldnt be hard lets face it, as the 310 gets bashed about on the rocks as it sticks out like a spare stabiliser. Probably explains why i cant find one anywhere.

So chaps anybody tried a smaller bore pipe on an MAR ?

Edited August 10, 2009 by Old trials fanatic

[brian h]

OTF,

I own two 1973 Mk I Ossa MAR 250,s and one 1974 Mk II Ossa MAR 250 , I am currently working out an exhaust system for the Mk II. I have an original front pipe and looking to see if I can design an expansion box , tail pipe and silencer. I see no reason why a smaller diameter tube can be used, the Mk I pipe has to go into a rectangular section behind the expansion chamber to provide tyre clearance, then open out to a full diameter to carry the silencer, the Mk I end pipe is approx 1.375 ins O/D to accept the silencer.

The engine needs some form of expansion chamber - space will allow up to 3.0 ins O/D fitted with a perforated pipe inside and the cavity packed along its length with exhaust fibreglass packing to dampen the somewhat noisy exhaust note. I think the orginal silencer had a 0.875 I/D bore at the outlet. I made two silencers of 2.0ins O/D MS tube X 9.0 ins long with a 1.0 ins O/D MS centre tube X 10 ins long which I perforated drilled myself, fitted a detachable alloy end cap to allow fibreglass packing to be installed ongoing and works fine and good sound suppression.

I think it is possible to fabricate the entire system as you suggest, I believe there must be an expansion chamber of sorts, you will need to make rectangular section aft of the expansion chamber to provide exhaust pipe attachment to the rear frame and to provide adequate tyre clearance, this rectangular section is 1.625 ins X 0.625 ins with approx 0.062ins wall tube. There is likely mathmatical calcs for two stroke exhaust design,etc, etc, however, the Ossa is a robust engine and pretty grunty and I think you will soon know if you have strangled the exhaust.

Please excuse the use of fractions ! Hope this helps Brian H