TDC the measuring of.

[DigDug]

You dont find TDC, you just find when the piston stops moving, then set it to 0, easy

Isn't that effectively TDC?

[firekdp]

Isn't that effectively TDC?

It's not exactly TDC because of the dwell, but it is the at the top of piston travel.
I've seen several "tech sites" and even stickies book telling you to faff about finding exact TDC in the centre of the dwell. The gauge is measuring piston travel, if you think about it, the piston isn't moving here, so zeroing it anywhere at this time will give the exact same result. And makes looking for exact TDC a bit of a ridiculous thing to do .
If the stroke and rod length is known then the piston travel distance can be calculated for a given degree setting. EG for a 58mm stroke/107 rod at 19 degrees BTDC the piston will be 1.9973mm down the bore. Simple trigonometry!

[dirtyhandslopez]

That's all well and good Firedkp, provided that the con rod is exactly what it is suppose to be. With manufacturing tolerances being what they are, that isn't always the case, ie a 107mm con rod may not be exactly 107mm, 58mm stroke may not be exactly 58mm, so that would throw the whole equation out the window.

If using a dialguage and holding bracket isn't the best, easiest way, why did Innocenti proved two different size holding brackets(one 150's, one for 200's) to dealers? You'd think the manufactures would know what they were doing.

If you watch the dial closely when going to and past tdc, the dwell really isn't all that much to worry about in real world applications.

[Muppet]

That's all well and good Firedkp, provided that the con rod is exactly what it is suppose to be. With manufacturing tolerances being what they are, that isn't always the case, ie a 107mm con rod may not be exactly 107mm, 58mm stroke may not be exactly 58mm, so that would throw the whole equation out the window.

If using a dialguage and holding bracket isn't the best, easiest way, why did Innocenti proved two different size holding brackets(one 150's, one for 200's) to dealers? You'd think the manufactures would know what they were doing.

If you watch the dial closely when going to and past tdc, the dwell really isn't all that much to worry about in real world applications.

[firekdp]

That's all well and good Firedkp, provided that the con rod is exactly what it is suppose to be. With manufacturing tolerances being what they are, that isn't always the case, ie a 107mm con rod may not be exactly 107mm, 58mm stroke may not be exactly 58mm, so that would throw the whole equation out the window.

I don't know what tolerances they are made to, but I've always thought they were precision engineered. If the rod/stroke are miles out, then what's the rest of it like? However, using the piston travel vs crank rotation in Sticky's, look what difference a 110mm vs 107mm rod makes to piston travel at 19 degrees, 0.01mm. So a rod 3mm longer and massively out of tolerance would only give a discrepancy of a fraction of a degree. With parallex error etc, you're probably going to get more discrepancy than that making the mark on mag housing.
When using a pos stop and degree disc most on here seem to be marking 4 points (again totally unnecessary), piston stop marks, measuring and marking TDC, then measuring to mark firing point compounding this discrepancy.

If using a dialguage and holding bracket isn't the best, easiest way, why did Innocenti proved two different size holding brackets(one 150's, one for 200's) to dealers? You'd think the manufactures would know what they were doing.

I don't quite get what you're saying here. I think you're misreading my posts as I do think using a dial gauge/holding bracket is the best way. Either that or I'm misreading yours.

If you watch the dial closely when going to and past tdc, the dwell really isn't all that much to worry about in real world applications.

Again As I've said, if you're using a dial gauge, the dwell is absolutely nothing to worry about? You simply zero anywhere in this period. I simply wanted to know why some on here thought a degree disc more accurate than a dial gauge. I presumed that it was this dwell period that confused them. Alas, none of those who think that a degree disc is better will explain why/how they come to that conclusion.

[dirtyhandslopez]

We both might be reading each others post wrong.
From what I can gather, you are saying there is no such thing as tdc becuause of the dwell factor and that using the trigonometry equation is the best way to go about it.
I say the best way IS find what is close as dammit to tdc(when the pistion is at the top of the bore=tdc in the real world)
then measure piston travel in mm backwards from tdc and make your marks.
The easiest way to know what measurement relates to what con/stroke etc is to look it up in the Sip Lambretta parts book or Sticky's (if you have a copy, which I don't) .
Using this method, the discrepancy with be the width of the tool used to make the mark and how old the eyeballs are that are looking at said mark

[firekdp]

From what I can gather, you are saying there is no such thing as tdc becuause of the dwell factor and that using the trigonometry equation is the best way to go about it.

I think you need to re-read my posts Of course there is an actual TDC (mid dwell position needed for a degree disc), but what I'm saying is that whilst using a dial gauge true TDC is irrelevant. If you read some dealers tech sites, they say to find the exact mid point of the dwell to zero the gauge, which is a complete waste of time. As long as the piston is at the top and not moving, zero it.

The easiest way to know what measurement relates to what con/stroke etc is to look it up in the Sip Lambretta parts book or Sticky's (if you have a copy, which I don't) .

And all calculated for you by clever b*st*rds using trigonometry , albeit rounded up to the nearest hundredth of a mil.

[dirtyhandslopez]

So we are on the same page. Roughly.
Bet those smart b'tards used a calulator...It'd be well impressive if they did the math in their heads, without a pen or paper Still, however they did it, glad they did, saves us all a lot of work.

[10 inch Terror]

So, will it work ?

[firekdp]

So we are on the same page. Roughly.

What confused me was, in your first post you said, tolerances in rod/stroke length would make the dial gauge process inaccurate, but then went on to say that, Innocenti deemed it the best, and that you used the method yourself. Totally contradictory.
If you're worried about tolerances, then you should, wind the piston down past the required "mm" and then back up to it (in the correct direction of rotation), this will eliminate any discrepancies due to backlash, caused by said poor tolerances in bearings etc.