feetupfun

It's not the original type of flywheel for a TY250D but it will work fine. As expected, it's a TY250A flywheel. Do you still want dimensions or photos of a TY250D flywheel?

I got curious and just checked 1974 DT100 and 1974 DT175 and both have 4371543100 as the part number for the clutch cover

I'm saying that I believe ABCD models DT100, 125, 175 covers are all the same part. They changed the cover design with the E model. If you want to check that out for yourself you can look up their part numbers. I just did a search on these forums on this topic and from that, it seems you can use a TY175 kickstart shaft with a DT cover, but need to make a spacer for the shaft to stop it floating towards the right, so that sounds like both shafts are the same length overall but have the shoulder that bears against the casing in different locations. I have a DT175A motor that I can compare the kickstart shaft and cover from with TY175 parts if you want.

The YZ125C/X cover will also fit on the TY175, but has no up stop for the brake pedal and they are very scarce

Yes, also DT100, DT125 and DT175 ABCDE models clutch covers will fit on the TY175 and yes the kickstart shaft length is different. The DT type cover is also more robust than the TY175 cover

It's standard and is there to hold a wiring bundle away from the exhaust

They look a lot like HondaTL250 yokes with the lower yoke fitted upside down. However I think that TL250 tubes are 33mm diameter. Having the lower triple arch the wrong way up magnifies the forces on the yokes.

A single copper washer is standard. Yes not the easiest thing to remove if the screw has been done up very tight and spread the washer. It doesn't matter if you remove it or not so long as it seals by the time you put the forks together. You can buy the special copper washers from Yamaha so don't be concerned about damaging them while taking them out if you need to.

The TY250A flywheel has a smaller steel band on it than the TY250B, C, D and E models, but is otherwise the same as the BCDE flywheel. I can do photos if you need. Yours is a D model bike The part number for the TY250A flywheel is 434-81350-10

To start with, I would just tidy up the gouged section to make sure nothing was sitting proud. Once that was sorted, I would test the fit again, checking for wear and tight spots

Yes you can replicate the original cross hatch finish with fine paper. Old forks only have issues if there is something wrong. Checking the yokes: Fit one tube and do the clamps up. Slide the other tube through the bottom hole and up to the top hole. If it lines up it's likely that the bottom yoke is straight on that side. Do the same again but insert the tube through the top clamp towards the bottom clamp. If it lines up, it's likely that the top yoke is straight on that side. As for the fork brace, it's made of thin mild steel so should be pretty easy to shape it to fit perfectly.

If there are slight bends in your tubes, align the bends fore and aft with both bends the same way and you will minimise the friction. A good test for if you want to tell if you are making an improvement in alignment is to make an axle spacer to replace the wheel, suspend the bike, remove the oil, fork seals, springs, mudguard/fork brace and assemble the front end without them and move the axle up and down by hand. There are left field things to consider as well such as on one TY175 I bought had a fairly twisted lower yoke that was causing alignment issues. When assembled, the tubes looked fine but if you slid one tube down out of the upper clamp while the other tube was still in the other side, the bent yoke became very obvious. Another TY175 had a bent fork brace which was pushing the sliders away from each other (causing friction). Another way to look for the cause of friction is to look for areas on the tubes that are more shiny than others. Another cause of sticky forks is dents in the sliders. This is easy to test for by feeling for stickiness with the spring out and cap off and the fork slider disconnected from the fork brace and the axle. To answer your first question, TY175 forks working properly have very little friction and TY250 forks working properly are no different. I'm unsure if polishing the tubes is a good idea because the standard ground finish is intended to hold oil to lube the seals and to lube where the tubes rub against the sliders. Polishing the tubes may make the seals feel sticky and may also overheat the seals if you ride along a corrugated dirt road.

Larry, if you do a Google search for "Bultaco model 23 images" you will find lots of photos of model 23 Metrallas

France Trial Classic. They don't call them Magicals but they have the same dimensions. They are black whereas the Magicals are bright plated.

The standard Yamaha rebound springs will work fine in combination with the aluminium damper rods that In Motion sells and with aftermarket fork springs. I'm 95 kg and have three bikes with TY250 twinshock forks and all have aftermarket fork springs in them. Two of the bikes have the aluminium damper rods and the third has modified Yamaha TY250 damper rods. Two of the sets of fork springs are Magical brand and the third set is from France Trial Classic. All three sets of forks work great for my weight with the standard quantity of 10 weight fork oil and 31 mm spring preload. All three bikes have standard Yamaha rebound springs in them. I haven't tried them without rebound springs. The Magical TY250 springs (dual rate) have an initial rate 0.36 kg/mm and a final rate of 0.54 kg/mm. Spring travel to reach the higher rate is 134 mm. Standard TY250 springs (dual rate) have an initial rate of 0.26 kg/mm and a final rate of 0.42 kg/mm. Spring travel to reach the higher rate is 137 mm. I also have some B&J Racing TY250 springs (single rate) that I shortened by 8mm for use with Gold Valves. They are 0.39 kg/mm. The France Trial Classic TY250 springs (dual rate) share dimensions with the Magicals springs.

You'll need to say which Betor shocks you are referring to if you want fitment help on here

Yes 360 Montadero. It's a very rare trail bike with a Pedrables engine design which is quite different to the common 5 speed Bultaco motor and hard to source parts for. What do you need help with?

Probably because of the cost for photo hosting. I think if you become a Supporter of Trials Central you can post large images

Yes it works. Fit them in opposition. Two narrow seals was the original setup.for lots of Bultacos

Sorry, I didn't look for stampings when it was apart last, but will next time

Bultaco cylinders are usually stamped with the model ID of the first model that came with that design. I would have thought however that the 137 had a protruding sleeve cylinder while the Sherpa T 250s went to a soft metal head gasket/flush sleeve before the 198B. My 198 has a flush sleeve and soft metal head gasket. They did do lots of strange stuff to get bikes out the door towards the end and a 137 Alpina type cylinder and head would be fine to use on a 198B. As for the large diameter bore, yes it's fairly common to bore a 238cc motor much bigger than standard to get more grunt rather than because the bore is worn.

Width at pivot including end caps 178 mm Length from centre of pivot to centre of axle slot 380 mm

Be aware that some 2 stroke ignitions have a curve that advances the timing as the RPM rises and at some point then start retarding it as the RPM rises further. You will be able to see what your ignition does when you put a strobe on it. Yes make your marks somewhere near where you think you want the spark. If it's got a flywheel similar OD as an MAR I would think that 22-24 mm measured around the rim of the flywheel BTDC would be in the ballpark. With a trials engine, the spark timing at low RPM is probably the most important thing and trying out different timing settings for low RPM will yield the best result.

There is nothing new under the sun. Here is what a 1970s Maico MX clutch spring looks like. When stacked up top to top and bottom to bottom, these are used in Maico clutches in a similar way to the helical compression clutch springs of other brands. Maico were onto the unusual spring rate benefits of diaphragm springs for motorcycle clutches over 50 years ago https://maicoatchippys.com.au/clutch-spring-cupped-belleville-washer-maico-all-to-81-and-82-gs/

Is it possible that the eye of your conrod has had some mechanical damage that has deformed the little end of the rod, causing the hole to no longer be perfectly round? To answer your question, if the hole is still round, the hole can be honed larger. If the little end has been deformed then it is time for a new rod.