TDC

[Stevepshipley]

Hi, what can cause tdc to vary from old tdc markings on a mag housing and is the piston stop method of finding tdc reliable?.
Cheers
Steve

[dickie]

Damaged keyway or key?

How far are they from original ones?

Piston stop method is very good. I use 3 or 5 different stop positions to check my own work.

[MickYork]

Hi, what can cause tdc to vary from old tdc markings on a mag housing and is the piston stop method of finding tdc reliable?.
Cheers
Steve

How much has it moved by ?

New crank or flywheel would probably give different marks.

[Stevepshipley]

Its different by a few (3 or 4) degrees. I think the mag housing is original to the 64 li 150. The timing was set at 19 degrees from the old mark. The top end is a muggy small block 200, so new crank and flywheel have been fitted. When i measured TDC the timing was around 21 degrees from my measurement so ive trusted my measurement and set it at 17, which moves it about 4 degrees. Seems to run well, but im curious.
(I was replacing a duff stator)

[HxPaul]

I have a series 2 with Indian 12v system.The timing marks on the flywheel are about 4 degrees to advanced,I'd set my timing at 17 degrees before TDC using the marks on the flywheel,but the error took it to 21 degrees before TDC.Always check your timing with a strobe.

[Adam_Winstone]

As soon as you change a crank or flywheel then any existing markings on the mag housing mean nothing and can only serve to confuse and get things wrong! The original marks would only be relevant if the original crank and original flywheel were still in the motor. Machining differences between even the same components, off the same production line would have some variation, however, once you start changing manufacturer and or component, then new marks are required.

The marks are based on where the arrow on the original flywheel edge were at TDC and whatever the correct firing point was for that model (e.g. 23 degrees btdc for an LI), which is in turn governed by where the centre boss is riveted into the flywheel body, then where the woodruff key slot is machined into the flywheel boss, then where the keyway is machined into the crank taper, then where this is in relation to the crank pin position and how that in turn influences piston position in bore... and timing is all about where the piston is in the bore, in relation to TDC, when the ignition fires. All of these elements can be machined anywhere in their own 360 degree rotation available and manufacturers are not necessarily trying to work to what may have gone before (e.g. a MEC crank may have the woodruff key slot in a very different rotational position to a SIL crank, etc. etc.), and manufacturing errors would soon add up to mean that you couldn't trust any original marks anyway!

With new crank and new flywheel, you simply cannot trust the marks. You might get lucky but that would simply be a fluke... not something that you want to trust where ignition timing is concerned.

Adam

[Stevepshipley]

So, thanks for all that. Appreciate it.

How does "strobing" work to confirm the timing degree settings? I understand it may coinfirm timing marks.... But how do you know 17 degress is actually 17 degrees and you havent cocked up the measurement

[HxPaul]

Find TDC and mark the mag housing making sure that the arrow on the flywheel points to this mark,then find the firing point,in this case 17 degrees before TDC,mark the mag housing and connect strobe and run the engine,then turn the stator plate until the arrow on the flywheel points at the firing mark on the mag housing.Your timing of 17 degrees before TDC is at this point.

[Stevepshipley]

Yeah i think i understand that paul....
But
My worry is about my tdc measure and therefore the timing offset. How does strobing confirm that timing is correct? Or does it just confirm that youve set it to the marks?
I think im thinking about thus too much and should just be confident in my finding of tdc ( measured it twice on different stop settings) and my measurement of offset against that. I just worried that original mag house markings were so way off

[Scooterlam]

Yep, you're over thinking now.

[HxPaul]

If you use the positive stop and timing disc method to find your firing point it will be accurate,its the distance between the timing mark on the flywheel (especially Indian flywheels)and the trigger point that can be inacurate.If you strobe a motor this distance can be verified by moving the stator plate in one direction or the other until the arrow on the flywheel lines up with the firing point mark on the mag housing.

[Adam_Winstone]

Yep, if you're happy with your TDC and measured firing point then you should not even be concerning yourself with the original marks.

Strobing is simply an accurate way of seeing when the motor is actually firing. It is the best way to check the firing point on any ignition system (points or electronic) but doesn't influence the accuracy of your marks or the position of arrow on flywheel. The strobe will fire when the ignition fires, regardless of where anyone has chosen to put arrow or timing marks, which could be anywhere around the edge of a possible 360 degree rotation. Agreed, that this is talking and thinking about it too much!

I've had to work on old dog motors that have been built 100 times from whatever 10 different owners had kicking around their garage floors, with chunks of mag missing and/or multiple marks knocked into the alloy using various width drifts at different times. When faced with a mag like that I commonly ignore ALL marks on mag and also totally ignore the arrow on flywheel (remember that this is only an arbitrary mark given by the manufacturer for convenience) and start again with a new mark roughly opposite to the flywheel arrow, then put new TDC mark and firing point mark on the mag edge in relation to my new flywheel mark. This means that you're dealing with new 'clean' marks and are miles away from the confusion that others have left you. Strobing these new marks will confirm the accuracy of ignition. NB: All of these marks can be made with a pen, rather than hammer and chisel (or other), saving you from adding to any confusion of multiple marks knocked in.

Yep, too much chat... but hopefully this helps to explain why you should always ignore original marks, what the reason behind the difference to original marks might be, and give you a method (doing it with completely new marks of your own, incl. mark on flywheel) of double checking the timing if you have lots of marks that you're wanting to ignore.

Adam

[MickYork]

I use a bit of white paint (or Tippex) on the flywheel and mag-housing, then use a fine black pen to make my marks. The white background with black line makes it easier to see when strobing, and easier to rub off if it needs altering.

[Stevepshipley]

Thanks all, happy now.