Top dead centre accuracy

[wherearewe]

This is not an old thread...................3 or 4 months is not very old, threads come back to life after years sometimes.

Its fine for HHB to say just install the PMA rotor and then find/mark TDC. What happens if you need to remove the rotor, say to replace a shaft seal? How will you be sure that its replaced correctly? I suppose you just take the time to find TDC with the crank/pistons again.

If you sell the bike, the next owner may pop off the rotor at some time and not even be aware that that he would need to find TDC each time.

Using a proper size woodruff key ensures that no rotary index can change, and that any present or future owner will not have the ignition timing go out of spec.

I agree. In hindsight ill admit its worth taking the time to make a key. Its one of those things you do once and thats it. However the PMA rotors are unmarked, so even with a key when you first install it you would need to be able to accurately find TDC using a stop etc. Once its done that first time though its good to go.

 

[Gcraay]

I made mine out of a spark plug, a long screw, and two what I think are called rivet nuts. Look at Fastenal online and you'll see what I'm talking about. I then shaved down the receiving end of the screw/bolt it was flat head receiver. Dipped it in some of that tool handle coating stuff and bam! no welder needed, adjustable, and worked fine.
'Sorry about the pic, I had others but my computer was made by engineers not people who actually use them and it made just getting this one a chore.

 

[glennpm]

Hi,

I have one of Hugh's PMAs and would also have liked it if provided with a proper key slot and new Woodruff key and thought it odd that it didn't for the money. I set mine with a degree wheel and piston stop but didn't like the angle that the spark plug is to the piston top, a shallow angle. When I've done this on car motors, the angle is much better and not likely to put bending into the piston stop bolt and threads. Yes, it is the same in both direction but ...

I also checked mine visually to confirm what I'd done. Everyone so far has talked about the small amount of movement at the top of the piston. It doesn't matter. Use the same technique of first locating TDC roughly as explained above as your starting point. You can then rotate the engine manually CW and CCW about that point looking for a good reference mark. I used a line between the head gasket and the block. It doesn't matter as long as you are consistent. I had a helper, my wife read the angles when I told her, so I kept my view line constant. You could and I did do it by myself as well and I did it this way first before confirming the angles with my wife helping. Make sure you have your head in the same spot, use a nice bright LED flashlight and find the angles before and after TDC where you can see your reference line. You then check if your before TDC and after TDC angles are the same and if not rotate your wheel accordingly so they are and verify. Once good, i.e., the same angles before and after, you have located TDC.

 

[peanut]

this thread really is a lot of silly fuss about nothing by folk that know nothing about how an engine functions

ffs what does 1/2 a degree matter one way or another when some owners put the damn camshaft in back to front ! or set the valve gaps incorrectly ! or have float levels all over the place !... or don't even line the cam shaft and crankshaft up when they put the chain on!..or don't understand how to ground their chassis and components properly

Nobody is half as exacting about setting their camchain tension accurately or adjusting their points properly with the right gap or setting up their carbs properly with the correct gaps, float levels and eliminating air leaks etc .

As I've said before you need to keep this ridiculous obsession in perspective.

The TDC setting is only one small (allbeit important) part of a whole host of settings of various components that need to be fitted and adjusted correctly and accurately if an engine is to start and run reliably and efficiently

Its pointless obssessing about a fraction of an inch of travel of the piston if you are not equally as meticulous about every other aspect of engine fuel delivery and electrics

rant over!

 

[fredintoon]

Gee Peanut,
you been putting vinegar on your cornflakes again?
What the OP wanted to know was how to make a piston stop when he didn't have a welder.
Got access to a length of wooden dowel or even a pencil to shove down the plug hole, don't he?

 

[peanut]

yeah you're right Fred
I'm guilty of getting involved in a debate about how best to establish TDC
I should have just left it with my first answer to the PO's question .
Lifes too short

 

[chizler62]

Actually, I would like to get an answer to my earlier question.

Why are XS650 owners looking for TDC?

I'm in the understanding that a company is selling Chinese YFZ350 mags that dont have the same key size. Was told it was for a "pamco" which is a pick up coil, right? That should have an indexing backing plate, right?

Posted via Mobile

 

[5twins]

You don't need any marks as long as you never intend to set your valves or ignition timing. And from what I see around this site, that's just fine with some people, lol.

 

[wherearewe]

Well yeh like 5twins says in his own way, you need to find TDC to check valve lash for example, so it's not some weird forum fetish... and it's a lot easier with a mark on the rotor.

Anyone doing the common PMA swap will need to mark their new rotor. The key is kinda irrelevant, since the banshee rotors (copy or OEM - i have one OEM and one from Hughs kits), have no ref mark on them and 5mm keyways.
Unless you can remove the old charge system, install the new rotor and tighten everything up without moving anything & keeping it bang at TDC the whole time, you should know how to find it using a stop method right? Double checking never hurts anyways. The OP asked just how accurate he needed to be.

 

[retiredgentleman]

Actually, I would like to get an answer to my earlier question.

Why are XS650 owners looking for TDC?

I'm in the understanding that a company is selling Chinese YFZ350 mags that dont have the same key size. Was told it was for a "pamco" which is a pick up coil, right? That should have an indexing backing plate, right?

Posted via Mobile

I always assumed that most people use a timing light on occasion to check that the ignition timing and advance is correct. How would they do that if the rotor is not accurately marked???? Maybe 5twins is right......................why waste time checking timing

 

[I am Carbon]

Actually, I would like to get an answer to my earlier question.

Why are XS650 owners looking for TDC?

I'm in the understanding that a company is selling Chinese YFZ350 mags that dont have the same key size. Was told it was for a "pamco" which is a pick up coil, right? That should have an indexing backing plate, right?

Posted via Mobile

I said it was for a pamco, But it could be for points or a boyer ignition too.
And as far as I know there is no marks on them for timing

 

[XSLeo]

On any engine you need to know when TDC occurs using reference marks. These reference marks are used to set cam position. With other marks referenced off the TDC mark you can set ignition timing at idle and check it at full advance.
How accurate your TDC mark is depends on how well you want your engine to run.
Be off on your TDC mark and the timing will be off.
Be as accurate as you can be, your engine will run better.
If I was to install a PMA, I would mark it at all. I would install a stock rotor and stator to accomplish ignition timing. Once the timing was done using the stock parts, then install the PMA. If I ever need to check timing, just swap the stock parts back on.
From all the trouble PMA's are having lately I doubt I will ever run a PMA.
The stock system is a good system.
Leo