A brand new Cam post... Really

[Morris]

Okay, I lied. This is just a continuation of my old cam post, but I felt like I needed to "doll things up" a bit to get some attention. What can I say I am impatient.

So here's where I am with timing my cam. I have used a dial indicator to find TDC. My dial indicator is now following the intake lifter for #1 cylinder. I have a degree wheel on my crank. What I am having a hard time understanding is how to read my degree wheel to determine if my cam is aligned properly.

Here are the numbers I have for my stock 1500 cam.

Here are all the numbers I have:

Lobe centers 112°
Intake timeing 18°-58°
Seat duration 247°
.05" duration 205°


How do I translate my degree wheel readings so that they relate to these numbers. It seems that these values have many common names and I think that is what is confusing me.

Lemme throw this out: Starting from TDC before the intake stroke, I take a degree wheel reading when my tappet reaches .05" lift; I take another reading when it returns to .05" lift. I add these two readings, and if everything is aligned correctly, the sum should be 205°. Is that right.

If I use Hap's technique of finding the upper most part of the valve opening (see my previous thread), the result I should get will be 112°. Is that right?

I am so confused.

 

[GB1]

Morris

Sorry, I always have to look at the instruction when I do this, since it is usually once every couple of years.

I do remember finding TDC, attaching a piece of coat hanger to the dyno/alt bolt for a reference. Lining the wheel with this at TDC and checking it many times.

Then rotating it to where the first valve fully opens, seeing how many degrees it is, rotating back to where by eye, the piston is at TDC, adding the two numbers and dividing by two.

After I was comfortable with this, I would then do the exhaust. Checking each thing at least three times.

I do recall for the first measurement (intake) that it should be between 106-112 or something of the such. The cams usually give you the desired amount.

I hope this helps until Hap or someone more knowledgable gives an awnser.

Patrick

 

[Morris]

Thanks Pat. Maybe I should post this over on the Triumph forum.

I could totally do this on an A-series thanks to Hap's excellent advice. The only thing I am having difficulty with is interpretting the numbers I have for my cam.

 

[ChrisS]

Lobe centers 112°
Intake timing 18°-58°
Seat duration 247°
.05" duration 205°

Intake opens at 18° BTDC and closes 58° ABDC. This gives a 254° duration (18°+180°+58°) assuming it is a symmetrical lobe then the max lift is 128° into that duration or 110° ATDC (128°-18°).

It is impossible to see the start of the lift at 18° BTDC so they give you the duration at a given lobe lift (not valve lift due to rocker arm geometry) So 205° at 0.050” means that the max lift is at 102.5° after the lobe measures 0.050” lift. Since we know that the cam should be at the max lift at 110° ATDC then the cam should lift to 0.050” at 7.5° ATDC (110°-102.5°).

So if you found TDC accurately then you should be able to see that the cam will lift to 0.050” at 7.5° ATDC be at max lift at 110° ATDC, and return to 0.050” at 32.5° ABDC (7.5°+205°-180°)

The 112° Lobe centers is the offset of the intake and exhaust timing.

Take a look at this Isky Cam Timing

BTDC - Before Top Dead Center
ABDC - After Bottom Dead Center
ATDC - After Top Dead Center

 

[Morris]

YES! YES! YES!

Now I understand.


Thank you! Thank you! Thank you!

If I am able to correctly apply this knowledge I will compile this thread into an article for the knowledge base.

Thanks to everyone who helped me wrap my very unflexible head around this.

 

[JerryB]

Morris......

I will add.....

make sure you establish TRUE t.d.c.

Use a positive stop (a steel strap bolted to 2 head bolts across the bore with a drilled and tapped center hole with a bolt to stop the piston top as you rotate CW and CCW) on #1 cylinder. Split the difference on the crank dampener and use a good solid tab on the motor and scribe mark on the dampener.

If you want to have more fun.....after you find tdc on #1 then go to 2, 3, and four and see if they are the same @ tdc.....this will see if the net stack (crank throw, rod and piston) are the same in each hole. compare the tdc reading of those other 3 cylinders.

""""""My dial indicator is now following the intake lifter for #1 cylinder"""""

and make sure that the d.i. rod is parallel with the lifter.

Have fun.......

 

[GB1]

Great Morris

I am glad you got it straightened out.

Sorry I couldn't be of more help.

Patrick

 

[Morris]

Soooo... when I put my dial indicator on #1 piston and turn my crank, there is no dwell time at TDC. In other words, the dial hits its top reading then instantly begins to decline. I am unable to turn the crank in an increment small enough that will cause the dial to "hang" at the high reading.

Is it still neccessary for me to use the positive stop method to find TDC?

 

[Morris]

I posted a very similar inquiry on the Triumph forum over 8 hours ago. 19 views and no replys.

SPRIDGET FORUM ROCKS!

(best forum ever.)

 

[Trevor Jessie]

We certainly have many posts and threads considering we are focused on one model line (as opposed to an entire marque).

 

[Dave Russell]

You can turn the crank through several degrees of rotation at tdc without having measurable piston movement. The longer the rod length to stroke length, rod ratio, the more dwell at tdc. If you can't turn the crank in small enough degree increments to see it, get more leverage to turn the crank. Or possibly, there is too much friction or back lash in the dial indicator set up.

You cannot find tdc ACCURATELY with a dial indicator. The positive stop method is the only accurate way.

D

 

[RomanH]

The following is an excerpt from Practical Classics & Car restorer publication.
"Top dead centre was found by using a timing disc on the front of the crankshaft in conjunction with a mark on an adjacent bolt head as a pointerand a dial gauge which was set up to measure the movement of the piston crown. First, the crankshaft was turned until number 1 piston was (as near as could be judged by eye) at top dead centre. Then the timing disc reading was set to zero, as was the dial gauge. Then the crankshaft was rotated slightly until a reading of 10thou (.010) was indicated on the gauge and at this point a note was made of the timing disc reading. The exact dial gauge reading doesn't matter as long as the same figure is used troughout the procedure. Then the crankshaft was rotated the opposite way until the dial gauge reading was 10thou againand, once more, a note was made of the timing disc reading. Then the crankshaft was turned again until the timing disc gave a reading halfway between the previous two readings and number one piston was now at true top dead centre."

 

[Dave Russell]

Don't assume that authors are always accurate in their statements. David Vizzard usually is. I don't know about "Practical Classics & Car".

At plus & minus 0.010" piston travel at tdc, there would be about 5 degrees of crankshaft rotation each way. This is not nearly enough rotation or travel to get the needed resolution. A rod bearing clearance of 0.002" could easily introduce a 0.002 +- error or 20%, in piston location. This error could be reduced by approaching tdc from opposite directions, which the "article" makes no mention of. In fact it apparently says to just keep rotating the crank in the same direction. which will assure that the rod bearing clearance has changed as the piston goes over the top. The positive stop method requires reversal & effectively eliminates this error.

If the piston pins are offset from the piston center lines, which is often done to reduce piston rock & noise, this will introduce reading errors near tdc due to rod angularity.

When you combine the posible & likely small errors in the dial indicator & errors caused by not having the indicator exactly parallel to the piston travel, the result is not very accurate.

Taking readings at something like 30 degrees each side of tdc with .0200" or more piston travel would reduce the errors quite a bit.

The positive stop method completely eliminates any indicator errors or errors caused by the dial indicator not being exactly parallel to the piston travel, or bearing clearances & is actually easier & quicker.

Maybe the mentioned method is sufficiently accurate for the average low tune street engine, the top engine builders always use the positive stop method. Lots of folks just line up the marks, hope for the best, & never know the difference. However, this discussion was about precision cam timing so why go half way by using an inferior method.
D

 

[Hap Waldrop]

Okay, I lied. This is just a continuation of my old cam post, but I felt like I needed to "doll things up" a bit to get some attention. What can I say I am impatient.

So here's where I am with timing my cam. I have used a dial indicator to find TDC. My dial indicator is now following the intake lifter for #1 cylinder. I have a degree wheel on my crank. What I am having a hard time understanding is how to read my degree wheel to determine if my cam is aligned properly.

Here are the numbers I have for my stock 1500 cam.

Here are all the numbers I have:

Lobe centers 112°
Intake timeing 18°-58°
Seat duration 247°
.05" duration 205°


How do I translate my degree wheel readings so that they relate to these numbers. It seems that these values have many common names and I think that is what is confusing me.

Lemme throw this out: Starting from TDC before the intake stroke, I take a degree wheel reading when my tappet reaches .05" lift; I take another reading when it returns to .05" lift. I add these two readings, and if everything is aligned correctly, the sum should be 205°. Is that right.

If I use Hap's technique of finding the upper most part of the valve opening (see my previous thread), the result I should get will be 112°. Is that right?

I am so confused.

My numbers in my cam timing post were just examples, different cams different numbers.

 

[RomanH]

The positive stop method completely eliminates any indicator errors or errors caused by the dial indicator not being exactly parallel to the piston travel, or bearing clearances & is actually easier & quicker.

Thanks for the insight Dave.
I wish this discussion would have come up a week earlier as I timed my cam and found TDC with the dial indicator method, though I did increase the distance to .030 and rotated the crank both CW and CCW.

 

[Dave Russell]

Hi Roman,
Chances are pretty good that your timing came out just fine. The only folks who worry about one or two degree timing accuracy are hard core perfectionists & racers. Unfortunately, I fall in this category.
D

 

[RomanH]

Hi Roman,
Chances are pretty good that your timing came out just fine. The only folks who worry about one or two degree timing accuracy are hard core perfectionists & racers. Unfortunately, I fall in this category.
D

Hi Dave,
Thanks for the reassurance. I did need to hear that.
There is another group of people who would be concerned about this and those are the Worry Warts... I'm in this group! /bcforum/images/%%GRAEMLIN_URL%%/blush.gif

 

[Bruce_B]

I'll be assembling my 1275 engine within the next two months and one of the items I've been reading about is timing the came.

Just another example of the great "questions and answers that are avalable on this forum.

 

[GB1]

Hi Roman,
Chances are pretty good that your timing came out just fine. The only folks who worry about one or two degree timing accuracy are hard core perfectionists & racers. Unfortunately, I fall in this category.
D

Dave,

here here, I do as well.

Patrick /bcforum/images/%%GRAEMLIN_URL%%/driving.gif

edit sp