singer 112w139 timing issues: quick intro

[@piecetoyou8285]

The timing mark is to do with the starting place that make sure everything is in time , before removing the belt , think about the horizontal shaft at the top that the hand wheel connects to also on that top shaft it runs the set cam timed by the factory that the take up leaver is linked to ,,and needle bar and all its crank arm moving sections,
I am sure the thread take up lever you can not time it as its moves by the connections that are linked to the cam and set and timed at the factory with the cam at the back, to the hand wheel ,
The bottom horizontal shaft that has the timing mark you asked, I am sure you line the arrow up with the mark before moving the the shuttle hook drive, if you don`t keep it lined up I think you will send the top shaft out of sequence with the bottom shuttle drive shaft.
And the belt makes sure that the top and bottoms shafts move in time with each other,
So as you know the hand wheel is the drive to move all parts like the fly wheel on a car,
Now Ive seen videos on your model or type where they are removing the belt and them lining up the arrow and they explain what they are doing at the time.
Think of a car, much easier to understand the sewing machine then,
When you have a crank at the bottom part of the engine where the pistons are clamped on to the crank is a part that basically is shaped up an down not a straight shaft, hens crank and at the end of the crank you have a cog or pulley sticking out passed the engine casing that can be removed from its place,
Then you have a cam shaft at the top part of the engine, that moves the cams etc, with a cog or cogs at the end of that shaft or shafts sticking out passed the engine casing just like the Crank Shaft,but the Cam Shaft has pieces of oval shaped metal that was machined out from one thick shaft, like the crank shaft was, but the cam shaft move the cams .
Now to keep CRANK SHAFT and the CAM SHAFT turning in perfect time with each other as they are a distance apart ,you have to connect the ends to each shaft using a belt or a linked chain to there cogs or pulley wheels ,
But before you can just join the two up to work in sequence with each other leaving the belt or chain still on the cog or cogs , you have to turn the Crank Shaft cog or pulley , this will move the top Cam cogs to the respected markings to top dead center and then you lock the cam shaft cog or cogs in place or its moving the Crank shaft pistons to top dead center ,this stops them coming out of sequence with the each other, using a special jig so the cam shaft cogs don`t move out of there position so the timing is not messed up.
The cogs or cog will have markings on them ` so will the out side of the engine case i think,
Now I think moving back down to the lower part on to the Crank Shaft it should have a marking for top dead center T.D.C you can remove the timing belt or timing linked chain once you turn to the Crank Shaft to T.D.C .
Now if you have ever heard when some one says Owe!! you don`t want your timing belt to snap on you as that will be costly due to the damage,
This is because when the crank shaft is spinning so fast them pistons are going up and down at great speeds at all different heights depending on there compression strokes ,
and the Crank Shaft will be not in sequence to the Cam Shaft any more .
When you replace a new belt or chain you need to set the Crank shaft new belt pulley or linked cog to a set of markings inscribed on the engine block, as its a new belt or chain its not going to be stretched so must be set to the correct marking forward or back or in the center or else you timing will be off,and the car will not run correctly,
Now when the timing belt brakes the crank shaft spins at a different time to what it was set to, the Can Shaft ,
So now they are both out of sequence with each other, which leads to damage when thing hit each other,
Now as I don`t really now or understand the correct positioning on engines its not far off in my dialysis.some things are slightly off but close enough for you to get the jest, why timing is important,
And as you work with cogs etc possibly engineering you possibly know more about the sewing machine than I do, just need to stop second guessing your self ,
If you think about what can be adjusted at what can not then you know that certain things are not going to be out of sequence, certain things on a sewing machine you do not want to mess with,or it can be a night mare to get it back into sequence,
So now back to the sewing machine,
if you can build a sewing machine you can build a car engine , if you can build a car engine you can build a helicopter , if you can build a helicopter you can build a fighter plain if you can build a fighter plain from learning the sewing machine then we won`t have any more engineers to fix our sewing machine`s Sorry could not resist that🤣just that dam silly advert stuck in my head ,
So think about the car and now you will have a better understanding to your sewing machine, i
Its best to break the sewing machine into three sections front- top- bottom, think about how they are connected to each other,think about what you can mess with out causing a issue to the other section,
On your other video where you show the machines issues , I have put down the same issue I had it was the same you were getting on your machine ,stitching looping snagging and untwisting, you will find it should find it solving your machine issues and it possibly is not what you think or have been told by others,