How to find TDC?

[neoknight88]

I know I'm a newb, but aren't the cams and the crank marked for TDC? If I recall correctly, when the crank is at TDC by the marking on the crank pulley, there are marks on the cam gears that line up with metal fingers that indicate they're lined up correctly.

Yes, as Steiny notes above. But here's the problem. Set your crank to TDC with the cam gear marks also lined up. Now rotate the engine one full revolution on the crankshaft. The crank is again lined up to TDC, but the cam gear marks are 180 degrees from their marks. So which is right? The answer is that when the cam gear marks are lined up and the crank is at TDC, cylinder #4 has closed its valves and is at the end of its compression stroke, and the spark plug is set to fire. When you rotate the crankshaft a full revolution from that point, cylinder #1 is set to fire (and the cam gear marks are 180 degrees off).

-Bryan

So when the marks line up, cylinder 1 is just about to start the intake stroke, correct?

[18Fiatsandcounting]

So when the marks line up, cylinder 1 is just about to start the intake stroke, correct?

Man, you got it! Yes, when the marks line up (the stars are aligned), cylinder #1 has just opened its intake valve and the next 180 degrees of crankshaft (not camshaft) rotation will draw in the air fuel mixture. A short while after BDC (bottom dead center) the intake valve on cylinder #1 will close, and the piston will move upward to TDC to compress the air/fuel mixture. Shortly before TDC, spark plug #1 will fire, pushing the piston down. After BDC again, the exhaust valve will open on cylinder #1 and the upward-travelling piston will push out the exhaust gases, and the cycle repeats.

So, all chant in unison now: Piston goes down, air and fuel are sucked in. Piston goes up, air and fuel are compressed. Spark plug fires, and expanding gases push piston down again. Piston goes up, pushing burnt gases out through the exhaust. And the band plays on. 4 strokes: Intake, compression, combustion, exhaust.

-Bryan

[RRoller123]

deleted until I get my engine back together and can take a video.

[aj81spider]

Just to briefly return to the original question with a suggestion about accuracy.

Using a screwdriver or dial indicator and looking for the flat spot will get you close to TDC, but probably up to a few degrees off unless you are exceptionally careful and observant as the flat spot is fairly large (in degrees). If you are looking for a more accurate way (I learned from this forum) the following has worked well for me:

1.) Set your crankshaft to some bit before TDC, mark the crank pulley and measure how far down in the cylinder the piston is with the dial indicator.
2.) Rotate the crank until it is past TDC and the depth of the piston is the same on the indicator. Mark this point on the pulley.
3.) Using a degree wheel find the point exactly halfway between the marks and that will be TDC.

Mark carefully as the error in putting the marks on the wheel will also create errors in finding TDC. For me this was a method that gave me more confidence than trying to establish where in the flat spot TDC was.

[18Fiatsandcounting]

Thanks, A.J., I like your method and agree it is more accurate. But, if you think about it, my original post about what I do isn't that different. I don't try to find TDC by turning the crankshaft by hand (timing belt off, plugs removed) and seeing (or feeling) where the piston is at the very top of its stroke. Rather, I find the points on either side of TDC where initial movement is seen, or I can start to feel the resistance of the piston rings as the piston starts to move downward. "True" TDC is then in the middle of those points.

I realize that there will be error in virtually any method, short of taking the crankshaft out and putting it on a very precise measuring setup to define a mark on the pulley where the #1 and #4 journals are at the top of their stroke, and then using an equally accurate measurement to define a mark on the engine exterior somewhere.

But, I'm also of the mind that there is enough slop in the ignition system and camshaft alignment so that being within a few degrees of TDC is probably close enough. I set my ignition timing by where the engine seems to perform best, and I use the recommended setting (10 BTDC for my engines) only as a starting point. I also aim for timing the engine at 3000 rpm, aiming for around 36 oBTDC, since I'm more concerned with the performance at speed than when it's idling. Other than emissions and idle smoothness, the ignition advance at idle is fairly unimportant.

-Bryan

[SteinOnkel]

So when the marks line up, cylinder 1 is just about to start the intake stroke, correct?

Man, you got it! Yes, when the marks line up (the stars are aligned), cylinder #1 has just opened its intake valve and the next 180* degrees of crankshaft (not camshaft) rotation will draw in the air fuel mixture.

*90 on a four banger.

[geospider]

maybe too simplified...but...:
crank at the TDC marks on the pointer
cams lined up with their pointers and
the rotor pointing to #4 on the Dizzy

geo

[18Fiatsandcounting]

So when the marks line up, cylinder 1 is just about to start the intake stroke, correct?

Man, you got it! Yes, when the marks line up (the stars are aligned), cylinder #1 has just opened its intake valve and the next 180* degrees of crankshaft (not camshaft) rotation will draw in the air fuel mixture.

*90 on a four banger.

Actually it's even more complicated than that, and we could spend hours discussing this over a beer (or several). The intake and exhaust valves don't open exactly at TDC or BDC and my '69 has 26/66 66/26 intake and exhaust cams, which means they open 26 oBTDC and close 66 oABDC.

An example for a random cylinder: 26 degrees before the piston reaches TDC, the intake valve starts to open, and it stays open (intake stroke) from that point and it is fully closed when the crank is at 66 degrees after it passes BDC. The crankshaft continues from that point back up to TDC, and this is the compression stroke. But shortly before it reaches TDC, the spark plug fires, and the gases start to expand and then push the piston back down on the combustion stroke. At 66 degrees before BDC on the combustion stroke, the exhaust valve opens and the burnt gases are expelled into the exhaust manifold after the piston passes BDC and is now traveling upward (the exhaust stroke). At 26 degrees before TDC (again), the intake starts to open and the cycle starts over. However, the exhaust valve doesn't close until 26 degrees ATDC, so there is a short period time (26+26 or 52 degrees of crankshaft rotation) when both the intake and exhaust valves are slightly open. The reason for this is that the exhaust gases rushing down the exhaust manifold "pull" on the new intake charge coming in, and this increases the volumetric efficiency of the intake.

The downside of this cam timing is that it tends to result in a "lumpy" idle, which is why performance engines don't idle smoothly and some can't even maintain an idle below 2000 rpm depending on how wild your cams are. Another downside is that some of the incoming fuel makes its way out the exhaust without being combusted, and this increases hydrocarbon emissions and slightly lowers gas mileage. Later model Fiats went to 5/55 55/5 cams which smoothed the idle, reduced emissions, and increased gas mileage. However, this reduced ultimate horsepower which is one reason why the 1438 cc engine cranked out 96 HP while larger versions usually only made 80 to 90 HP. There are other reasons, of course.

More than any of us wants to know, perhaps... I think Geo's response is much more digestible....!

-Bryan

[geospider]

I am a simple man.
When putting all of this part back together; went quite well.
I do have a penchant for making the simple things difficult

geo

[SteinOnkel]

Man, you got it! Yes, when the marks line up (the stars are aligned), cylinder #1 has just opened its intake valve and the next 180* degrees of crankshaft (not camshaft) rotation will draw in the air fuel mixture.

*90 on a four banger.

Actually it's even more complicated than that, and we could spend hours discussing this over a beer (or several). The intake and exhaust valves don't open exactly at TDC or BDC and my '69 has 26/66 66/26 intake and exhaust cams, which means they open 26 oBTDC and close 66 oABDC.

An example for a random cylinder: 26 degrees before the piston reaches TDC, the intake valve starts to open, and it stays open (intake stroke) from that point and it is fully closed when the crank is at 66 degrees after it passes BDC. The crankshaft continues from that point back up to TDC, and this is the compression stroke. But shortly before it reaches TDC, the spark plug fires, and the gases start to expand and then push the piston back down on the combustion stroke. At 66 degrees before BDC on the combustion stroke, the exhaust valve opens and the burnt gases are expelled into the exhaust manifold after the piston passes BDC and is now traveling upward (the exhaust stroke). At 26 degrees before TDC (again), the intake starts to open and the cycle starts over. However, the exhaust valve doesn't close until 26 degrees ATDC, so there is a short period time (26+26 or 52 degrees of crankshaft rotation) when both the intake and exhaust valves are slightly open. The reason for this is that the exhaust gases rushing down the exhaust manifold "pull" on the new intake charge coming in, and this increases the volumetric efficiency of the intake.

The downside of this cam timing is that it tends to result in a "lumpy" idle, which is why performance engines don't idle smoothly and some can't even maintain an idle below 2000 rpm depending on how wild your cams are. Another downside is that some of the incoming fuel makes its way out the exhaust without being combusted, and this increases hydrocarbon emissions and slightly lowers gas mileage. Later model Fiats went to 5/55 55/5 cams which smoothed the idle, reduced emissions, and increased gas mileage. However, this reduced ultimate horsepower which is one reason why the 1438 cc engine cranked out 96 HP while larger versions usually only made 80 to 90 HP. There are other reasons, of course.

More than any of us wants to know, perhaps... I think Geo's response is much more digestible....!

-Bryan

Fair point.

This is the beauty of modern engines. Even with simple on/off intake camshaft VVT (like my VW has) you can slam the intake cam open under load and leave it closed at idle. That way you get the best of both worlds.

If I wasn't so lazy I could use a map with a targeted value as well, theory see attached image. Hard to do without a dyno though. And don't even get me started on VVL or Hydunai's new valvetrain. That is the bee's knees.

[SteinOnkel]

*90 on a four banger.

Actually it's even more complicated than that, and we could spend hours discussing this over a beer (or several). The intake and exhaust valves don't open exactly at TDC or BDC and my '69 has 26/66 66/26 intake and exhaust cams, which means they open 26 oBTDC and close 66 oABDC.

An example for a random cylinder: 26 degrees before the piston reaches TDC, the intake valve starts to open, and it stays open (intake stroke) from that point and it is fully closed when the crank is at 66 degrees after it passes BDC. The crankshaft continues from that point back up to TDC, and this is the compression stroke. But shortly before it reaches TDC, the spark plug fires, and the gases start to expand and then push the piston back down on the combustion stroke. At 66 degrees before BDC on the combustion stroke, the exhaust valve opens and the burnt gases are expelled into the exhaust manifold after the piston passes BDC and is now traveling upward (the exhaust stroke). At 26 degrees before TDC (again), the intake starts to open and the cycle starts over. However, the exhaust valve doesn't close until 26 degrees ATDC, so there is a short period time (26+26 or 52 degrees of crankshaft rotation) when both the intake and exhaust valves are slightly open. The reason for this is that the exhaust gases rushing down the exhaust manifold "pull" on the new intake charge coming in, and this increases the volumetric efficiency of the intake.

The downside of this cam timing is that it tends to result in a "lumpy" idle, which is why performance engines don't idle smoothly and some can't even maintain an idle below 2000 rpm depending on how wild your cams are. Another downside is that some of the incoming fuel makes its way out the exhaust without being combusted, and this increases hydrocarbon emissions and slightly lowers gas mileage. Later model Fiats went to 5/55 55/5 cams which smoothed the idle, reduced emissions, and increased gas mileage. However, this reduced ultimate horsepower which is one reason why the 1438 cc engine cranked out 96 HP while larger versions usually only made 80 to 90 HP. There are other reasons, of course.

More than any of us wants to know, perhaps... I think Geo's response is much more digestible....!

-Bryan

Fair point.

This is the beauty of modern engines. Even with simple on/off intake camshaft VVT (like my VW has) you can slam the intake cam open under load and leave it closed at idle. That way you get the best of both worlds. The result is that the torque delivery feels almost linear and with the individual throttle bodies it sounds amazing.

If I wasn't so lazy I could use a map with a targeted value as well, theory see attached image. Hard to do without a dyno though. And don't even get me started on VVL or Hydunai's new valvetrain. That is the bee's knees.

[18Fiatsandcounting]

Agreed Steiny. Modern technology has come a long way, but for the Fiat DOHC engine, we're essentially looking back in time to engine design of the late 1950s/early 1960s. Not bad considering we're 60 years or more past that. I do not expect my spiders to behave like a modern car, and nor should they. That's the fun part!

-Bryan

[SteinOnkel]

Agreed Steiny. Modern technology has come a long way, but for the Fiat DOHC engine, we're essentially looking back in time to engine design of the late 1950s/early 1960s. Not bad considering we're 60 years or more past that. I do not expect my spiders to behave like a modern car, and nor should they. That's the fun part!

-Bryan

And let's not forget that the Fiat Twin Cam engine was way ahead of its time. Think about American cars form the era, my goodness. Pushroad V8s and single cam straight sixes. Ancient history by comparison.

I believe the Fiat engine was also the first (or one of the first) mass produced ohv engines where you didn't have to remove the camshaft(s) to adjust the valves. That being said, the VW EA111 engine family did it one better in 1976. Pop the cam cover off, stick an 8mm allen socket into the appropriate adjuster and fine tune it to your heart's content. No shims, no plates, no special tools. They didn't go to DOHC with that block until 1998 (!) though.

I have yet to adjust the valves on my Fiat. Seems like an expensive PITA. And some of them are a bit noisy, if I'm honest.

[RRoller123]

It is really easy to do! Just get the little tool. It only needs a little bit of filing to work on E4. And a little magnetic wand and a small awl prodder or hook tool lifts and pulls the shims out. Measure with a micrometer, need feeler gauges, easy peezy.

[ORFORD2004]

And if you have adjustable cam wheel...

I'm not Guy Croft but that's the way I did it:

I don't see the need for timing marks on the pulley. For timing my cams i have to find the lobe center of the cam and be shure that the crank was at 110 degrees. you need a dial indicator and a degree Wheel for that job.

The way i did it is you find the real TDC. The way to do it is to use the infamous screwdriver method to find TDC.
Then you install your degree wheel on the flywheel and put a piece of wire attach to the engine to 0 degree.

Next is to put the dial indicator in the plug hole. Not much of steel in that area for my magnetic base so i made a steel bracket that use the valve cover bolts to old steady. So you install your dial indicator so you have some pressure on the dial. Set to 0 and turn the crank clockwwise. When you read let's say .002, note the number on the degree Wheel. Continue turning and when you are .002 before the 0 you set, note the number on the degree Wheel. For example if you read 355 and 3. You have to move your pointer 1 degree up to 4 so the reading will be 356 and 4. Equal from the zero. From now the pointer or the degree wheel can't move. Retest to be shure. Now you have your true TDC.

Now you have to find the lobe center of the cam. For that you have to install the dial indicator 90 degrees from the valve tappet and clear the lobe rotation. The trick i found, sorry for the purist was to use electric tape and a piece of steel wire at the end of my dial indicator because my needle was to big to fit on the side of the cam and clearing the lobe. When there is full pressure to the tappet, you install your dial indicator whit some pressure on it. Set to 0 and turn the crank clockwise. when the valve start closing let's say .002, note the number on the degree Wheel. Continue turning and when the valve is .002 before the 0 you set, note the number on the degree Wheel. For example if you read 84 and 134, you take 134-84=50/2=25. So 25+84=109. Your lobe center is at 109 degree and you want 110. If you move to 85 you will read 135 also. So 135-85=50/2=25. So 85+25=110. So you set your degree wheel to 85 degree and using the adjustable pulley turn back the dial indicator to .002 when the crank is at 85 degree. Retest and you should read 85 and 135. Now your lobe center is at 110 degree.
Same thing for the other side.