peterb

Hi Ric, Piston/bore clearance in imperial units should be between 1 to 1.5 thou, I have the spec in mm somewhere and converted it to units that I understand better and can remember, but can't find them just now. Measure at the front or back of the piston skirt about a third of the way down from TDC. I have never seen specs for piston ring end gap for the Ossa, it will be the same as a 280 GG, safe to have 4 to 6 thou. I have had to replace a piston/cylinder that was nipping up from new, turned out it was tight from the supplier at half of a thou, no wonder. Bye, Peter B.

Hi ripgroove, You need to drain the oil asap, there is every chance that the water in the oil contaminates the clutch fibres making them swell, giving you a hard to pull in clutch action, generally needing a new clutch pack if this is the case. Drain the oil, fit the new water pump shaft and seal, re-fill with 350ml ATF and run this for 10 mins, then drain this when warm and re-fill with 500ml ATF rated to Dexron III. It's a bit of a muck about changing the seal and shaft, but persevere with it. Best to insert a self tapper into the back of the old seal to remove it by pulling on the self tapper with a pair of pliers. The impellor is screwed on, I can't remember if it was a left or a right hand thread. If this doesn't sort the problem, then the head O rings may be passing, they age harden and loose their seal over time. As advised earlier, you only need to remove the 3 water pump screws to do this. On the water pump, there is an outer bronze looking bush, this bush has a nylon sealing washer, then behind the end of the bush is a stainless steel washer that fits between the bush and the shaft, don't loose this washer. Ta ta, PeterB.

Check the head O ring grooves to see if these may have cracked, it would only be a hairline and difficult to see without a magnifying glass. Some of the early 280's had this problem, you can fit a later 2012, 2014 or 2015 head if this is the case. Bye, PeterB.

Hi Ric, Some of the early motors may have had the piston bore clearence too tight. I have worked on one that I measured at 0.0005 thou (half a thou) that had nipped up. The standard piston bore clearence is between 1 to 1.5 thou. Also , I would not run with a 140:1 fuel/oil ratio, 100:1 is the highest that I would go to and I am pretty hesitant about using that. All my previous trials bikes I have run at 50:1. It is possible that the motor nipped up early on in its life and has "bedded in" since then. Possible causes are the lean fuel/oil and possible tight piston bore clearence, and a lack of coolant, or non working water pump. When you get it going, from cold, have a look in the top of the radiator neck to see if there is any turbulence from water circulating. I have had a bike that from new had the water pump driving gear spinning on the shaft such that the pump never actually turned. If you are going to re-use the old cylinder, check the piston size if it is an A or B so you can fit a correct new one, I wouldn't use the old one due to the damage and it may have warped. Bye, PeterB.

The clutch centre bolt is an M5 cap screw with a centre hole. Generally, if oil is being forced out of the breather, then this bolt must be in place for this to happen. The problem for pressurising the case will likely be the centre gasket or the crank case bearing seals fitted the wrong way around, or the crank shaft O ring seals are passing. If you are doing the work yourself, take care when fitting the LHS main bearing as the oil holes must line up, make a mark with a felt pen on the case where the oil hole should align up to. With the oil seals, the spring side faces the pressure. Bye, PeterB.

These bikes were not so bad once the gearbox and clutch issues were addressed. As far as I am aware the transmission oil did and still does lubricate both main bearings. There is a 2002 parts book, the 2003 parts book would show an incorrect crankcase gasket for the 2002 model, the 02 is different. There were a number of improvements made over the years to the Pro motor after it's initial release, like other manufacturers would make. The 02 clutch case would flex when the clutch was used, accentuating the clutch drag and parts of the gearbox were altered over time. The 02 airbox was a bad design, always letting in water just crossing a stream, may have been replaced by now on your bike. I would have thought that a bike this old, may have had much or all of the needed changes made as a matter of course. In this case, the bike could be a good buy. We always used 450ml oil, any more usually finds it's way out the vent. Used ATF Dex III since 2002 (and on the earlier GG) and never had a problem. Some of the UK guys use synthetic, according to my UK GG guru friend, it tends to cling to the main bearings and better prevents them from rusting up when the motor takes in water into the clutch case, which could be often up north in the UK where there are plenty of proper stream sections. The Kokusan electronics are different to the later models, and are pretty rare at the best of times. I produced a tech sheet for basic things to do with a new GG bike that is somewhere in/on the web which may be of help to you, I think Jim at trials parts usa would still have this, was about a full A4 sheet size. Contact him for all things GG, he is really brilliant. I always ran my GG's at 50:1 with a mineral 2T oil, but I have been told that 70 to 80:1 synthetic has worked well too. The water pumps tend to wear the shaft and seal, which can be replaced. Biggest problem is water getting into the gearbox as this can swell the clutch fibres and this then increases the clutch pull on the lever, generally needing a new clutch pack. Bye, Peter B

To hold the 19mm nut, grind quite a large flat on an old 19mm socket, such that the socket can slip over the nut when in situ, the ground flat then catches up on the frame when tightening and loosening the torx through bolt. You need to grind off quite a lot of socket, just try not to overdo it as then the flats won't catch on the frame and you will need to weld a strip to the back of the socket to give you something to hold on to. Bye, PeterB.

I haven't read the MCN article but there is no merger taking place. Gas Gas have a very large facility in Girona that is under utilised as far as space goes, Ossa require larger operating premises, both companies are looking to save money and have agreed to run the Ossa business in the vacant space in the GG facility. This is planned to take place in January. Both companies will maintain their corporate identity and individual operating regime. This is from a source very close to the Ossa factory. Bye, Peter.

Hi jsp, You can use a wheel from a 2001 GG, if you can keep air in it. Use a smaller diameter disc to suit the Ossa set up, fit it with washers between the disc and the hub as you need this offset to fit, countersink the mounting holes and use allen screws, make up a spacer for the sprocket side, then make 2 sleeves to take up the difference in the Ossa axle diameter and the GG, which is smaller. Do this if you want to use the original GG axle. If you ever want to go down this track, I can measure up the parts I made and give you the details. The sprocket needs to sit out a further 6mm from the hub, easy, use an old sprocket as a spacer and cut off the outer ring so you end up with a neat aluminium ring. Probably no more than 5 or 10 mins work............Have you got a lathe? Bye, Peter.

Hi Tim, email me. You were good at coasting over drop offs! 13 stone - dream on. Bye, PeterB.

Hi Mike, These linkage kits are available ex stock in a local area at a lower cost than the AB kits. Bye, PeterB.

Hi Tony, Have a look at the electrics, if the ignition system is a Kokusan, then there is likely to be nothing wrong with this except for there being a bad ground/earth on the voltage regulator unit, located under the headstock, if it is a Ducati, then there are 2 problem areas to look into. There is a finned aluminium control unit under the tank, this may have a fault, try exchanging with a known good unit. Some of the Ducati ignition systems have a very weak pick up sensor, try adjusting the air gap between the pick up sensor, located on the outside of the magneto in the 2 o'clock position, and the flywheel to 4 to 8 thou and see if this helps. From reading about your problems, it may well be this. Bye, PeterB.

If the DellOrto is from a 250/280/300 GG, then you only need to change the main jet to a 95, the std 33/35 pilot is fine. A D34 needle works best with the bigger Bultaco motor. From memory, the std set up with a GG would have: 270K needle jet (emulsion tube) 60 slide 35 pilot 120 main D36 needle (Fine with the 250, D34 better with the big bore motor) Fitted these carbs successfully to about 4 or 5 Bultaco's, 250's and 325's. Bye, PeterB.

Hi tankygsy, As mentioned earlier, the clutch pack height determines the leverage from the "fingers" and the take up distance. When this is at the optimum height, there are only two things one can do to alter the clutch take up "dwell" either alter the master or the slave cylinder diameters. Easiest one is to change the master cylinder, instead of using the std 9.5mm bore master, replace this with a master cylinder from a TXT BOY which has a 12mm diameter piston, this will provide more clutch travel, with less lever travel, but with more force required. I used one of these larger cylinders for the 02 PRO to overcome the clutch case flex problem, it worked. Bye, PeterB.

It is not usual for these to snap off, must have taken quite a knock. Only solution is to buy a new seat base. Bye, PeterB.

My preference with the Keihin is to use a GJH needle, offers much better low end control than the JJH, but then you need to use a 128 main jet. Try Allens in the UK for these bits. With the GG250, the standard DellOrto works very well and fitting a Keihin did not seem to give any vast improvement, unlike the 300 model, where the Keihin noticeably improved carburation. Bye, PeterB.

Hi Bob, it would appear that the magneto is coming up against the stator plate, therefore the crankshaft must be located too far to the right of the cases. This means the bearings may not be fully home, or the crankshaft may simply need to be centralised in between the cases. Bye, PeterB.

It does seem you will have to split the cases, don't lose the ball bearing inbetween the clutch pushrod and the "mushroom". I had a similar problem with a new 1993 bike years ago. This one was because there was too much end float on the shift drum and needed shimming out to reduce the endfloat to approx 10 thou. The selector forks and the rest of the gearbox are built quite strong, should have no problems there. I got the shims during a factory visit while chatting with the guy who was building up the motors, there was no part number for the shim. From memory, I think the gear shafts stay in the right side casing when splitting. Bye, Peter.

Hi John, If your bike has Kokusan ignition, then it is likely to be an earthing problem, perhaps the voltage regulator under the headstock, if it is a Ducati ign system, then it is likely to be the aluminium electronic box under the tank. Substitute this for another to confirm. If not this, then it will be the stator. Bye, PeterB.

Hi jsp, There is a mechanical stop adjustment at the throttle butterfly that you can use to set the idle/tickover, a small bolt with lock nut. Bye, PeterB.

Hi jsp, The GG axle is a different diameter. To remove the axle lock bolt, you need a really good quality Torx bit, like a Koken or Facom or similar, other lesser makes will twist and tend to stuff up the Torx head. Some of the lock bolts are done up a little too tight and tend to gall up on the snail cam, there should be a washer there under the Torx bolt to prevent this. I would imagine that if you kept on turning the Torx head, to undo it, the snail cam should come up against a stop allowing the bolt to be removed. When you do get this off, you can do the following mod to replace the Torx head (not a great fan of these), if you want. Turn down the shoulder on the outside of the snail cam, until the area is almost flush with the snail cam surface. Then source a high tensile 17 or 19mm hex head bolt, zinc plated to replace the Torx bolt and include a washer, use lots of grease. I did this on a bike and it is so much easier to use a 19mm ring spanner to loosen off the rear wheel. Bye, PeterB.

Hi blomster, The piston ported model is 1992, have a look for the engine number to correctly ID the motor. From 1993, the motor was case reed induction. As mentioned, a photo of the problem would help. It may be possible to use a helicoil and maybe carefully grind back the part of the helicoil that would protrude through the bearing housing, using a Dremmel or similar die grinder. Sounds a bit odd for a stud to be located so close to a bearing housing. Crank area or gearbox? Bye, PeterB.

Hi osssaguy, If you are unable to adjust this out using the eccentric adjustment, then there may be one of two problems, one, it is likely that one or more of your selector forks have bent. The 02 model was a problematic year, some motors required the selector shafts replacing and the 3 selectors as some shafts were found to be non-concentric and the forks were not properly heat treated. You can check 2 of the forks by eyesight, holding them back to back to see if they are square, eg; no daylight visible between them. Later forks are more solidly built. Two, it may just be something rather simple like the shift spring retainer, which is a top hat shape, at the top is a strip that engages into the end of the shift shaft, at the bottom is a separate pressed on piece that has a tab for the spring to engage on. Sometimes this top hat unit can separate or twist around slightly causing the problems you describe. You can braze this together carefully, or put a new one in. Bye, PeterB.

Around that time, 2003/4 the bearing was changed from a caged needle roller to a teflon lined sleeve. While the ID of both types of bearings will be the same, I am not sure if the OD is the same. Bye, PeterB.

Hello Kay, The Ht coil sits with the HT lead on it's side. The problem with the hot starting with Ducati ign on the GG is the ign sensor sir gap has to be set really close, 8 thou is fine. (Distance between the sensor and the flywheel. Ta ta, PeterB.