biffco Posted July 2, 2017 Report Share Posted July 2, 2017 I have an OSSA that has come from Italy. It was sold as a 250 , which I beleve it to be, but today at a trial it was pointed out that as I have cutout on my fins, that it may be a 350?? The engine no. Matches the frame no. But the number doesn't show up on any of the lists I have seen. Any info in aging the bike etc would be appreciated. Or even recommended upgrades, mods etc... 1 Quote Link to comment Share on other sites More sharing options...
dadof2 Posted July 2, 2017 Report Share Posted July 2, 2017 That looks like a 1972 to 1974 model. The 350s (Actually 310cc) had a red stripe on the tank as well as the green. Easy to measure stroke through plughole and bore through exhaust port ten work CC out. Usual mods were to replace the IRZ? carb with a Mikuni, replace the front mudguard stay with Sammy Miller items, replace the sump plug with a countersunk / headless item and fit a kit with an extra rear wheel bearing to stop the spindle bending. Quote Link to comment Share on other sites More sharing options...
lotus54 Posted July 3, 2017 Report Share Posted July 3, 2017 What is the engine number? Looks like someone already changed the carb- but I had excellent results with an OKO conversion from Mid-Atlantic Trials. 2 Quote Link to comment Share on other sites More sharing options...
cleanorbust Posted July 3, 2017 Report Share Posted July 3, 2017 (edited) The fork sliders, exhaust pipe shape and rear shock position define it as a Mark 2 Mick Andrews Replica, ie 1974/5 model. Dad of 2 is correct about the 310 model having green and red stripes, and the tank on your bike looks to have its original stripes (many have aftermarket or homemade stripes which are obviously unoriginal) suggesting it's a 250, but I think you're right about only the 310 having the fin cutouts. 250 bore and stroke is 60 x 72 mm = 244cc 350 is 65 x 77 mm = 310cc Easy to whip off the cylinder head and measure the bore to confirm which motor you've got. The 350 was never that popular, the 250 certainly was, seemed to run better and smoother than the 350 and had enough power for the sections at the time Looks like someone has welded on alternative top mounts for the rear shocks to increase suspension movement. Edited July 3, 2017 by cleanorbust Quote Link to comment Share on other sites More sharing options...
pmk Posted July 3, 2017 Report Share Posted July 3, 2017 (edited) I did not see where you posted the frame and engine number. Possibly this was sold as a 250 and later had a 350 crank and cylinder installed. This link is one of the more complete list to ID the OSSA. You need to scroll down through the entire list to see the various 250 and 350 models. http://ossa.2y.net/ossa/bilder/modelos/index.html What is the frame / engine number if you don't mind sharing. Nice looking OSSA. Appears very complete. I think if I were to offer one tip, from my short OSSA experience, immediately and continually lube the spoke nipples with a quality penetrating oil. If the aluminum nipples are seized onto the stainless spokes, they need every bit of help to prevent breaking the spokes. Edited July 3, 2017 by pmk Quote Link to comment Share on other sites More sharing options...
pmk Posted July 3, 2017 Report Share Posted July 3, 2017 Looks as if the slot for the rear axle was modified as it seems very long. I did notice the wires for the ignition are possible very close to the exhaust by the tank / seat area. Again, neat bike. Quote Link to comment Share on other sites More sharing options...
cleanorbust Posted July 3, 2017 Report Share Posted July 3, 2017 Looks like the bike is running the spark plug in the angled hole to the side of the head, where I think the original had a blanking plug which was often removed to fit a decompressor. Probably advisable to swap the plug to the central hole. Quote Link to comment Share on other sites More sharing options...
woody Posted July 3, 2017 Report Share Posted July 3, 2017 Colour scheme or decals won't tell you anything as it's 40 odd years old and they can be changed - as per the tank which is a UK alloy tank, not the fibreglass original. A couple of possibilities. 1) It could be a 250 cylinder from a later green model which were also slotted but the head is MK2 MAR 2) It is a 350 MAR - If the engine number starts 430 then it is. Not all Ossa numbers appear on the charts. A 350 top end won't fit a 250 as the stud spacing is different - except a Gripper which had the same spacing for both The engine number will give you the definitive answer 1 Quote Link to comment Share on other sites More sharing options...
biffco Posted July 3, 2017 Author Report Share Posted July 3, 2017 Thanks for the replies. In responce to some of the questions/ comments. The tank isn't the original. When I got the bike, it had been imported from Italy, and came with the fibreglass tank. I have since fitted an ally one. I too have fitted a Minuki carb as the original Amal 626 was very much worn out. I have found out the hard way about the sump plug, as I caught the original and have cracked the sump What is the thread size for this so I can get a replacement plug ? I'll get the engine number net time I'm near the bike. I had noticed the rear axel slot had been lengthen too. This will of shortened the wheel base, is this a good thing ? I'll take a look at the spark plug , the middle hole currently has a decompressor gizmo in it. Does anyone else think this will make a difference as it is or should I put the spark plug back in the centre hole? Quote Link to comment Share on other sites More sharing options...
cleanorbust Posted July 3, 2017 Report Share Posted July 3, 2017 Ossas had a shortish wheelbase to start with, reducing it further would probably be too much of a good thing. In fact, some folk lengthened the swinging arm back in the day, though perhaps more commonly on the Mark 1 MAR, which was notably shorter than its rivals. If it was me I'd move the plug to the central hole, simply because the motor was designed to run that way with respect to flame travel and thermal efficiency. I'd definitely retain the decompressor though, because: 1) with Ossa brakes, any supplementary retardation is useful 2) if the bike is dropped and starts revving its head off, a decompressor is a foolproof way of stopping it, unlike a kill button. Something these Ossas were known for was bending rear spindles, you could be in for some fun if you haven't tried removing it yet. Hard to tell from the photo but looks like your bike may still be running a dished rear sprocket, as originally fitted (aftermarket kit to convert to flat sprocket was a popular fitment). Perhaps a sign the bike hasn't seen a lot of use? Quote Link to comment Share on other sites More sharing options...
2stroke4stroke Posted July 3, 2017 Report Share Posted July 3, 2017 I seem to recall some people extending the slot fully to an open end in order to be able to remove the wheel in the event of a puncture once the spindle bent, though clearly that's not what has happened to yours. The third bearing conversion was not foolproof as the too small diameter spindle would still bend but just be more difficult to remove with the extra bearing. A top rider here got a spindle made up in stainless; it sheared. I found it intriguing that the 21st Century Ossa also had a badly designed rear wheel setup. Quote Link to comment Share on other sites More sharing options...
dadof2 Posted July 3, 2017 Report Share Posted July 3, 2017 Regarding the sump plug. Some riders used to shim the sump guard down a bit. The method I and quite a few others used was to slightly countersink the hole in the sump, then turn Lathe) nearly all the head off the sump plug leaving just a couple of mm to match the countersink in the sump. A screwdriver slot was then cut in the plug with a small diameter slitting disc in a grinder. Plug was then fitted with threadlock or PTFE tape. Some riders had special plugs turned and threaded from the head of a large socket head cap screw. Quote Link to comment Share on other sites More sharing options...
pmk Posted July 4, 2017 Report Share Posted July 4, 2017 23 hours ago, woody said: Colour scheme or decals won't tell you anything as it's 40 odd years old and they can be changed - as per the tank which is a UK alloy tank, not the fibreglass original. A couple of possibilities. 1) It could be a 250 cylinder from a later green model which were also slotted but the head is MK2 MAR 2) It is a 350 MAR - If the engine number starts 430 then it is. Not all Ossa numbers appear on the charts. A 350 top end won't fit a 250 as the stud spacing is different - except a Gripper which had the same spacing for both The engine number will give you the definitive answer Always solid advice. Thank you for correcting my mis statement about swapping cylinders and crank. As much as I learn about OSSAs, seems the learning curve continues. Thanks for being here, you are a true resource with great info. Quote Link to comment Share on other sites More sharing options...
biffco Posted July 5, 2017 Author Report Share Posted July 5, 2017 On 3 July 2017 at 7:12 PM, woody said: Colour scheme or decals won't tell you anything as it's 40 odd years old and they can be changed - as per the tank which is a UK alloy tank, not the fibreglass original. A couple of possibilities. 1) It could be a 250 cylinder from a later green model which were also slotted but the head is MK2 MAR 2) It is a 350 MAR - If the engine number starts 430 then it is. Not all Ossa numbers appear on the charts. A 350 top end won't fit a 250 as the stud spacing is different - except a Gripper which had the same spacing for both The engine number will give you the definitive answer The frame number starts B 2425## I have found the below chart that doesn't go this high, but on the marked column it is just below. If this is correct then it is a 350?? Has anyone else got a different list that has my frame no on it ? ( I don't see it on the link above) Quote Link to comment Share on other sites More sharing options...
biffco Posted July 5, 2017 Author Report Share Posted July 5, 2017 (edited) This list also seems to confirm it. I guess my bike is one of the last '75 350's ?? Edited July 5, 2017 by biffco Quote Link to comment Share on other sites More sharing options...
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