feetupfun

Welcome AJ. There is an OSSA forum on here where you might get more info. Go to Forums, then Bike Specific Forums, then OSSA

I don't have a motor apart to measure but there are two pushrods to minimise clutch drag (the ball between the two rods means that the actuator end push rod doesn't spin while the clutch is disengaged)

If you can get the studs out you can use countersunk head screws instead and sandwich the spacer

Even heavily corroded crankshaft halves can usually be returned to serviceable condition by an engine re-conditioner and rod kits are readily available

Yes that's true. Many shocks do have a floating piston to separate the oil and gas. Standard Falcon shocks don't have a floating piston or a bladder or a piece of closed-cell foam. The oil and gas mix.

They look like early Falcons. The valves are there to allow them to be pressurised with gas. They need to have gas pressure inside to work properly.

https://www.frankiemountain.com/en/compra/bultaco-alpina-exhaust-new-bultaco-alpina-muffler-model-138-bultaco-1591

Having a stepped spacer like that makes it super easy to push the old bearings out compared with a spacer that has the same ID as the bearings

If you want to be certain of having enough oil, set the oil quantity by level rather than volume. A base-line starting point is 125mm dip to the oil surface with the forks fully compressed metal-to-metal. At that point, extend the forks fully and look down and see if the top end of the damper rod is still covered by oil. If it is exposed, add enough oil to just cover the top end of the damper rod. Then compress the forks again and see what the dip to oil is. More than 110mm dip is enough air space. I suspect that 48mm preload may be too much preload. I have the same forks, damper rods, oil weight and same springs and weigh 220lb dressed to ride and run about 32mm preload. My preload spacer is not the adjustable one from Magical Springs.

Warm the aluminium with a hot air gun and the bearings will emerge with much less persuasion

I reckon I could work out pretty much all that you wrote except what does the F stand for in FNG?

Yes a 320 Majesty would have come with TY250D or E forks. They are pretty much the same design inside as the forks on just about every dirt bike from the 1970s. Yes the TY250 top caps have internal hex drive, as do many Yamahas. A parts manual for TY250B,C,D or E and a service manual for twinshock TY250 will give you lots of info. As for doing the job, the fact that you are asking how to take the caps off tells me that you haven't done much work on motorbikes before. Here are a few things that might not be obvious but doing them will save you trouble. Loosen the top clamp bolt for the fork tube before trying to undo the top cap. Loosen the bottom fork bolt before you take off the top caps. It's the steel internal hex drive (allen head) bolt in the bottom end of the forks (not the screw in the side at the bottom). You may have trouble undoing the bottom bolt, because the part inside that it screws into can turn unless it is held still. There's lots of ways to hold them still. One way is to loosen that bolt with the fork spring compressed inside. When you do get to the point of unscrewing the top caps, beware that the cap might fly up and hit you in the eye. They usually have spring force pushing up on them. People have been blinded by doing this part of the job without precautions. Oil and sometimes a bit of compressed air will come out as you undo things. The oil can squirt out pretty fast if you undo things before letting any gas pressure out. The little screws near the bottom are for draining the oil. New aftermarket fork tubes are available. Re-chroming is also available but can be more expensive.

You may well use Dexron ATF in it now, but if the wrong sort of oil has been used at some time in the life of the clutch plates (before you owned the bike), the surface properties of the friction plates may have been (permanently) affected. The worst "wrong" oils in my experience for wet motorbike clutches are "friction-modified" 4 stroke engine oils and "extreme-pressure" or 'EP" transmission oils. There is no way to tell if your friction plates have been contaminated with those types of oil at some point, but you can be certain that if you buy new plates, they will not have been contaminated. One thing that can be seen in your photo is that that friction plate has a shiny friction surface. That's not a good thing because it will not be as good at letting go of the steel plate surfaces when you pull the lever in, compared with a friction plate that is not shiny.

The sticking pushrod may be making the action jerky. The pitted cam face will be making the action jerky and heavier than it needs to be. The photo labelled Hub is the Basket and the drive fingers on it look OK. The steel clutch plates look OK but they would look better if you roughened them some more. You didn't show the friction plates. They can cause what you've described when they get old and/or smooth. I never seen a set of friction plates that wore down to the service limit but I've seen dozens of sets of friction plates that were useless because of old age or because someone had used the wrong sort of oil at some stage or because they were shiny smooth. Are all the springs the same length? Does the pressure plate stay parallel with the plates as it moves in and out? Does the hub have grooves where it contacts the steel plates?

The only way an ignition system can cause carbon build-up on the plug tip is if it doesn't spark each time. If the bike runs reasonably normally, then the cause of the carbon build up is something else. It may be the carby causing it to run too rich or there may be too much oil in the fuel or it might have a plug with the wrong heat range or it might have the wrong kind of oil in the fuel.

Yes <1.0mm squish is a good thing. I've recently modified a trials motor to achieve this, and at the same time, copied the combustion chamber shape from a modern KTM. It goes well, however I didn't increase the compression ratio because it was perfect already. A good person to ask with experience in what you are seeking is Paul Arnott from The Hell Team in Sydney.

That indicates that it probably has one of these things happening: Pilot jet or pilot jet passage restricted. Too small a pilot jet. Air leaking into the motor somewhere other than through the carby venturi. If it didn't have the problem before you cleaned the carby then it's not option 2.

http://ossaengineering.com/wp-content/uploads/2012/02/IgnitionTiming.pdf

Sounds great. I'm all for your improvements. I was confused because I had thought that collar/bush was mounted on a taper rather than being an interference fit

Excellent! Can you explain how this process works?

Dry blast then paint works very well

Yes, I've got one that I'd be happy to sell. It's too wide to fit the bike I bought it for. Location Gladstone, Queensland

A standard TY175 motor is 66 mm bore and 50 mm stroke which is 171cc swept volume. Your cylinder at 67 mm bore and 50 mm stroke has 177cc swept volume. After seeing your photo (showing 8 mm studs a lot longer than standard studs), I suspect that your crankcase cylinder mouth was earlier enlarged to match a bigger cylinder sleeve to allow the cylinder to have a bore somewhere in the range 70 mm to 72.5 mm and that "200cc" cylinder was later replaced with the standard cylinder that you are showing. If this is the case then it would be interesting to know if there has been something done to match the diameter of the standard cylinder sleeve with the enlarged crankcase cylinder mouth.

For a period of about two years a few years ago I had very severe reactive arthritis which is a normal and lingering after-effect of Ross River Virus. It mainly affected my fingers, wrists and elbows. It meant I couldn't hold on well. Through that period, I avoided any sort of heavy loading on the front end. To help with comfort in smaller stuff, I backed off the fork spring preload slightly and ran lighter fork oil and experimented with handlebar position all of which made the bikes easier to hold onto. All my trials bikes are twinshock era which I find are generally easier on the arms and hands than modern bikes. I also found that my heavier twinshocks were easier to hold onto and rode smoother than the lighter ones.

A DT175 barrel has different finning to a TY175 barrel. A DT175 head has different finning to a TY175 head and has two plug holes. A DT175 head and barrel is held on the same as a standard TY175 head and barrel is held on. Your next question is probably going to be where to get an oversize piston from for your big-bore motor. For that answer, you'll need to take some measurements. Bore diameter (accurately) or read the numbers on the piston crown. Piston pin centre to crown dimension. Was there a spacer under the barrel?