Another TDC question.

[Tom]

I’m not sure what it is, but i feel like i’m overcomplicating this or just being plain stupid...perhaps both. Please help my sanity

I’m trying to locate TDC on my bike that has a banshee PMA conversion where the PO kindly didn’t leave any markings. From finding TDC I will mark the timing marks.

To try and locate TDC, i’m using a piston stop and degree wheel which i’ve read the process about on this site many times. From what i’ve seen the process is this:

1)Loosen off valves
2) Bring the piston up close to visual TDC
3) Insert piston stop and continue bringing the piston up until it hits the piston stop.
4) Mark this position on the flywheel/case
5) Bring the piston fully back down and up again to visual TDC
6) Insert piston stop and continue bringing the piston up until it hits the piston stop.
7) Mark this position on the flywheel/case
8) True TDC is halfway between these marks

Correct?

If not...help! If the steps are correct...help!

I follow steps but both my TDC markings come out at the same / very similar location on the flywheel...is that correct? For example, TDC mark 1 has the points locating pin facing 12 oclock and the other has it facing 6 oclock but my original marking is very close what i should be marking. I was expecting both measurements to be way off like the HHB example below....

Points locating pin at 12oclock

HHB example i’m looking at, the two differnt TDC range marks are far wider apart than mine.

 

[DaveO]

The piston stop is not supposed to be used at full Top Dead Centre but when the piston is some way down the bore.

Turning the crank one way will run up against the stop at, for example 23 degrees before TDC. (where doesn't matter).
Turning the crank in the opposite direction (leave the stop in position) will have the piston hit the stop at 23 degrees after DC.
If you have marked both of the positions then logically TDC must be half way between them.

I don't know why the pin on the cam is 180 degrees out but I do know that the cam and the crank don't match turn for turn. The camshaft will turn at half the cranks speed, I think (hope I haven't made an idiot of myself with that bit)

So if I'm right - if the crank is at TDC and the pin is up, if you turn the crank through 360 degrees the cam will have turned only half a revolution (180 degrees) and the pin will be pointing down.

Dave

 

[grizld1]

Good explanation, DaveO, you just don't want to set up the piston stop so deep that it fouls a valve. I prefer to take one degree reading BTDC reading piston depth with a dial indicator, then roll the motor over the top and take a reading at the same depth ATDC, but either way gets it done. You're right, the camshaft in a 4-stroke motor has to turn at 1/2 crank speed to achieve one crank rotation for compression and power strokes and one crank rotation for exhast and intake strokes.

 

[MaxPete]

.....the camshaft in a 4-stroke engine has to turn at 1/2 crank speed to achieve one crank rotation for compression and power strokes and one crank rotation for exhaust and intake strokes.

The camshaft must turn at EXACTLY 1/2 crankshaft speed - or much mirth will ensue.....NOT!!

As I understand it, that is one of the complications of mixing and matching early and late XS650 parts.

MamaYama changed the number of teeth on the timing drive and camshaft sprockets for the timing chain - NOT the 1:2 ratio - just the number of teeth. Soooo...you may NOT mix an early crank and a late camshaft or vice-versa.

 

[Jim]

I'll add that some of the PMA rotors don't have woodruff keys. If yours was set up that way, whatever mark you make now for TDC will most likely not be valid at a later date. The rotor can (and will) move without the key in place. If it was me, just for peace of mind.... I'd pull the rotor and check.

 

[MaxPete]

I'll add that some of the PMA rotors don't have woodruff keys. If yours was set up that way, whatever mark you make now for TDC will most likely not be valid at a later date. The rotor can (and will) move without the key in place. If it was me, just for peace of mind.... I'd pull the rotor and check.

Given that, I'd be tempted to put a tiny punch mark (or even a scratch mark across the crankshaft end and the rotor hub - to provide a future reference.

 

[5twins]

Yet another check mark in the PMA "minus" column, which is quite full already. The "plus" column, on the other hand, remains practically empty, lol.

 

[Tom]

I'll add that some of the PMA rotors don't have woodruff keys. If yours was set up that way, whatever mark you make now for TDC will most likely not be valid at a later date. The rotor can (and will) move without the key in place. If it was me, just for peace of mind.... I'd pull the rotor and check.

Mine has the woodruf key in place on the rotor.

 

[Tom]

Good explanation, DaveO, you just don't want to set up the piston stop so deep that it fouls a valve. I prefer to take one degree reading BTDC reading piston depth with a dial indicator, then roll the motor over the top and take a reading at the same depth ATDC, but either way gets it done. You're right, the camshaft in a 4-stroke motor has to turn at 1/2 crank speed to achieve one crank rotation for compression and power strokes and one crank rotation for exhast and intake strokes.

Thanks mate. I made sure to back the valves off fully and to make sure the piston stop didn’t hit them or go too deep into the cylinder to make a flase TDC reading. I spent some time doing this.

 

[Tom]

Yet another check mark in the PMA "minus" column, which is quite full already. The "plus" column, on the other hand, remains practically empty, lol.

Thanks...apart from not having any TDC or timing marka (that i assumed would be easy...) the PMA has been great.

 

[Tom]

The piston stop is not supposed to be used at full Top Dead Centre but when the piston is some way down the bore.

Turning the crank one way will run up against the stop at, for example 23 degrees before TDC. (where doesn't matter).
Turning the crank in the opposite direction (leave the stop in position) will have the piston hit the stop at 23 degrees after DC.
If you have marked both of the positions then logically TDC must be half way between them.

I don't know why the pin on the cam is 180 degrees out but I do know that the cam and the crank don't match turn for turn. The camshaft will turn at half the cranks speed, I think (hope I haven't made an idiot of myself with that bit)

So if I'm right - if the crank is at TDC and the pin is up, if you turn the crank through 360 degrees the cam will have turned only half a revolution (180 degrees) and the pin will be pointing down.

Dave

Thanks Dave.

Should this be done on the compression stroke or doesn’t it matter? How much difference should one expect between the two TDC markings?

I’m going to check the pins tonight and insure they’re both in the right place

 

[DaveO]

How much difference should one expect between the two TDC markings?

Just for the sake of clarity lets define some terms.

TDC = Top Dead Centre - This is the point at which the piston has reached the highest point in it's up stroke and will then start to descend down the bore again.
BDC = Bottom Dead Centre - is correspondingly the bottom (lowest point) in the piston's travel up and down the bore.

So, you can see there is only one position in the pistons movement when it is at TDC.

For various reason we might want to know and refer to other positions in the pistons travel. For example the spark has to occur just before the piston reaches TDC so that the fuel mix is ignited before going over the top. (It takes a while for it to get going burning and everything is moving pretty fast). This is usually referred to as "5 degrees before TDC".

So we are interested in TDC and we may also need to have a handy measure of where the piston is at other points. Because it is awkward to measure the physical distance of the piston in inches or centimetres we use the rotation of the crank as a proxy for the position of the piston. So we do this in degrees. Sometimes we use a degree wheel if we are interested in more than just ignition timing. But most of us are not designing camshafts etc.. and it is sufficient to physically mark on the rotor three marks only: TDC, 5 degrees before and "full advance" position (25 degrees or so??). Once we have determined TDC we can use a degree wheel to find the other points and mark them on the rotor too. (Of course, these are marked on the original rotor so I'm guessing you've changed it.)

With the head on its difficult to tell where TDC is. So we need a way of finding it out.

If we physically impede the movement of the piston when the crank is rotated (with a piston stop) so that it stops at some point before TDC we can mark this position on the rotor. If we now rotate the crank back the other way the piston will descend down to Bottom Dead Centre, past this point and start back up again. When it reaches the bump stop it will be exactly the same depth below TDC. So TDC is in the middle of these two marks.

In your original description you set the stop at where you estimated TDC to be. You then ended up with two marks which were very close together. True TDC is in the centre of these two original marks. This shows you must be a good guesser.

But for reasons connected to the mathematics concerning travel of the piston and its relation to physical measure of depth of piston travel don't use these marks. This is the average of two guesses (which is itself a guess).

Sorry this is so long but its actually only when you try to write it down that you realise how difficult it is to be clear.

Here's a youTube video. I think the introduction of the degree wheel confuses things a bit (you don't at this stage need to know anything about the value in degrees, just make a mark) but it does show clearly how the piston stop is used a point away from TDC but near it.

Hope this helps.

Dave

 

[MaxPete]

That’s a great video Dave - thanks for sharing.

 

[Tom]

Just for the sake of clarity lets define some terms.

TDC = Top Dead Centre - This is the point at which the piston has reached the highest point in it's up stroke and will then start to descend down the bore again.
BDC = Bottom Dead Centre - is correspondingly the bottom (lowest point) in the piston's travel up and down the bore.

So, you can see there is only one position in the pistons movement when it is at TDC.

For various reason we might want to know and refer to other positions in the pistons travel. For example the spark has to occur just before the piston reaches TDC so that the fuel mix is ignited before going over the top. (It takes a while for it to get going burning and everything is moving pretty fast). This is usually referred to as "5 degrees before TDC".

So we are interested in TDC and we may also need to have a handy measure of where the piston is at other points. Because it is awkward to measure the physical distance of the piston in inches or centimetres we use the rotation of the crank as a proxy for the position of the piston. So we do this in degrees. Sometimes we use a degree wheel if we are interested in more than just ignition timing. But most of us are not designing camshafts etc.. and it is sufficient to physically mark on the rotor three marks only: TDC, 5 degrees before and "full advance" position (35 degrees or so??). Once we have determined TDC we can use a degree wheel to find the other points and mark them on the rotor too. (Of course, these are marked on the original rotor so I'm guessing you've changed it.)

With the head on its difficult to tell where TDC is. So we need a way of finding it out.

If we physically impede the movement of the piston when the crank is rotated (with a piston stop) so that it stops at some point before TDC we can mark this position on the rotor. If we now rotate the crank back the other way the piston will descend down to Bottom Dead Centre, past this point and start back up again. When it reaches the bump stop it will be exactly the same depth below TDC. So TDC is in the middle of these two marks.

In your original description you set the stop at where you estimated TDC to be. You then ended up with two marks which were very close together. True TDC is in the centre of these two original marks. This shows you must be a good guesser.

But for reasons connected to the mathematics concerning travel of the piston and its relation to physical measure of depth of piston travel don't use these marks. This is the average of two guesses (which is itself a guess).

Sorry this is so long but its actually only when you try to write it down that you realise how difficult it is to be clear.

Here's a youTube video. I think the introduction of the degree wheel confuses things a bit (you don't at this stage need to know anything about the value in degrees, just make a mark) but it does show clearly how the piston stop is used a point away from TDC but near it.

Hope this helps.

Dave

Thanks Dave. I understand what TDC is and perhaps my terminology wasn’t quite correct when typing this out. When refering to visual TDC, i was meaning that it was a small amount of travel to the very top of the piston movement. I understand that i need to impede the piston before TDC and mark it and then cycle the piston again and mark it, then true TDC is in between those points.

My main question is, how far should those two markings typically be? From reference images i’ve seen, they’re quite far apart from each other but mine appear very close. I’ll try again and report back.

On another note, my advance rod locating pins are pointing in the same direction. Do the location of the pins refernce anything? I.e if the pins are at 12oclock?

Thank you.

 

[DaveO]

Thanks Dave. I understand what TDC is and perhaps my terminology wasn’t quite correct when typing this out. When referring to visual TDC, i was meaning that it was a small amount of travel to the very top of the piston movement.

Hi Tom, yeah I guessed as much. But defining terms if there is some doubt, is a good idea so we are "on the same page". No offence - honest.
Also, there are always people who will be reading now who maybe aren't sure and a lot of what is posted may be useful in the future for others to refer back to

From reference images i’ve seen, they’re quite far apart from each other but mine appear very close. I’ll try again and report back.

Yours are close because you are fixing the piston stop at TDC. You need to drop the piston down a little from TDC once you've guesstimated it. Then raise the piston so that its firmly against the stop; mark; then do the same the other side.

How far should the piston be dropped? I don't really know, but try it out so that it gives a difference similar to the image you've posted.

OK - confession time. I know and understand how to do this. But I've never needed to do it as my stuff is standard. So you're operating at much advanced stage to me. Perhaps someone with more experience (or better maths) can jump in? E.g. I'm pretty sure that using the stop too close to TDC will not give accurate results but my maths isn't up to it. (Something to do with a larger number of degrees at close to TDC equating to a very small up/down movement of the piston)



Dave

 

[MaxPete]

It’s an iterative process - as shown in the video. You basically use smaller and smaller steps and “close in” on the true position of TDC.

 

[grizld1]

As long as you're setting the depth below the "dwell " level where piston movement is small you'll be alright, more isn't better where piston depth is concerned. I prefer to use a dial indicator directly on the piston crown or a dial test indicator with horizontal action on a piston locater with a moving plunger. Take one reading BTDC, lift plunger while the piston is on the rise, take second reading ATDC. Whatever method you use, make sure your measurements repeat. For our purposes there's no need to try to dial things in to within a fraction of a degree with a monster degree wheel and repeated iterations. You're doing mechanic work, not engineering.

 

[XSLeo]

On your advance rod both pins should be on the same side of the rod.
Now on the advancer side there is a pin that locates the advancer on the cam. Might be hard to see with the advancer installed.
When the piston is at TDC this advancer pins will point to 12 or 6 o-clock. If it points to 11 or 1 / 5 or 7, then your cam is not timed right. If the bike is running fine then I doubt that's an issues.
Can you rely on the advancer pin to locate TDC, not really. Cam chain wear and tension can effect that, so use you piston stop to find TDC then compare that to the pin.
Leo

 

[TwoManyXS1Bs]

The piston doesn't move much near TDC. Recommend setting the stop where the piston is at least 0.028" from TDC, +/- 10° from TDC. The amount of piston travel +/- 1° from that position (about 0.006") is significant enuff to ensure that you'll be within 1 degree of correct...

 

[grizld1]

Thanks for the chart, 2M--a handy reference for anybody wanting to confirm their marks! I usually use .040" drop just because it's easy to read on the dial and gets below the dwell range.