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30 x 1.5 mm is correct.

Steering head bearings are 25 x 45 x 12. Available at www.pyramid-parts.com.

An old practice used on the bell housing of farm tractors, and applicable to the magneto housing, is to install a small cotter pin in the drain hole. The jiggling effect of the cotter pin will prevent the hole from becoming plugged.

Rear wheel bearings are 17 x 40 x 12, or series 6203

The rest of it.

Perhaps the attachment will help.

Go to www.mid-atlantictrials.com and click on OKO Carburetors and Accessories.

If you search some of the TC forums I think you will find that Caswell's tank sealer is most often mentioned and recommended. I've personally used Caswell's on at least 6 tanks, with excellent results.

Look at the rear of the inner engine case on the right hand side and you will see the head of a bolt. That is not it, but it holds a detent spring and ball in place. Now look in the approximate same location on the rear left hand side. You will see a small drilled hole, perhaps 1.5 mm (1/16 inch) in diameter. That is the gearbox breather.

All Beta Rev 3 series use 38x50x8 fork seals.

Review the Beta forum. There are numerous articles on the Beta Rev 3 series. Pay particular attention to the "Beta Clutch Fix" and articles on water pump corrosion, which involves replacement of the inner clutch housing.

Recently purchased two from www.bosioffroad.it

450 cc of Maxima MTL 75 wt transmission oil.

The glass (plastic) eye is not threaded. It it secured inplace with an o-ring and sealant. You'll have to remove the glue from the inside of the case and the eye will push out. As far as the clutch sticking, go to the TC Beta forum and follow the Beta Clutch fix.

If you never plan to fall or hit anything they will work fine. But like most of us, after you replaced them a few times, you'll wish you had spent the money on a good pair of forged levers.

There is no workshop manual for the 348. The best you can do is the old Clymer Service manual for the 123 - 360 cc singles - 1965 through 1975, and the Trials Central Montesa forum.

From Wiki ... LHM (Liquide Hydraulique Minéral). LHM is a mineral oil, quite close to automatic transmission fluid. Mineral oil is hydrophobic, unlike standard brake fluid; therefore, gas bubbles do not form in the system, as would be the case with standard brake fluid, creating a 'spongy' brake feel. Use of mineral oil has thus spread beyond Citroën, Rolls-Royce, Peugeot, and Mercedes-Benz, to include Jaguar, Audi, and BMW. LHM, being a mineral oil, absorbs only an infinitesimal proportion of moisture, plus it contains corrosion inhibitors.

The assembly you have shown is the gear shift detent. It is secured by a 17 mm bolt, located on the upper rear, sprocket side of the case. Detent pressure can be "adjusted" by adding or removing washers. Increasing the spring pressure will make it shift slightly harder, but with less chance of jumping out of gear.

The serial number is pretty strong evidence that you have a 1983 Cota 350. I have a 1983 Cota 350, serial number 51M26546. That's close enough to your Cota to confirm what you have, but doesn't really address the front end differences on your Cota.

I'd consider the tether an unnecessary and attendance killing requirement for vintage meets.

It appears to be an 82 Cota 248. See image from Southwest Montesa Model Gallery.

Just did it on a 2014 300 Evo. Required removal of the rear fender, air box, carb, carb intake manifold, and exhaust manifold. With those parts out of the way the fuel tap can be pulled out

It is quite a distance, but Lewisport in the US carries them. Try http://lewisportusa-com.3dcartstores.com/ Home > MOTO TRIAL > PARTS > EXHAUST > OTHER PRODUCTS

Standard torque for M8 hardware is: Class 8.8 - 22 Nm Class 10.9 - 32 Nm Class 12.9 - 37 Nm

Check www.pure-gas.org for ethanol free gas stations.