Top Dead Center question
- Thread starter cjc56
- Start date
Ole Don
Well-known member
If you put your thumb on the edge of the piston and the top of the block, you can feel pretty close to TDC. To get really close, use a strap bolted to the block with a bolt threaded through, and turn the piston up to about 1/4 inch from the top, then turn the other way to the same, and divide the two.
I have my heads off and would like to set my pointer to exactly TDC. MY pointer has gotten bent in the pass, so I figured since I have to chance I would do it right. I do have the proper magnetic machinist gauge to do this with. I only need to know where TDC is in the stroke. From what I can see there is approx 4-6 degrees of dwell at the top.
Just for information I will pass on the answer that I found to my question. TDC is the center of the piston's dwell period. Here is a good site to read about this: http://www.iskycams.com/degreeing.html . Since most of us set these older engines by ear, i guess TDC being off a few degrees won't matter. But I have heard on this forum and other that some are running these engine several degrees off the suggested timing. It makes me wonder if maybe their pointers are off.
dmsfrr
Well-known member
Yes, making sure the pointer is straight is a good idea. After almost 60 yrs, having a bent pointer would seem pretty likely.
Setting the timing to stock is like a safe base-line to make any personal adjustments from based on; stock or optional engine parts, driving conditions, available fuel grade, altitude, etc. etc.
I've seen several folks suggest setting the timing on a y-block with a few extra degrees of advance, and I'll go along with them.
Setting the timing to stock is like a safe base-line to make any personal adjustments from based on; stock or optional engine parts, driving conditions, available fuel grade, altitude, etc. etc.
I've seen several folks suggest setting the timing on a y-block with a few extra degrees of advance, and I'll go along with them.
Y-Blockhead
Well-known member
The "best" way to find TDC is with a mechanical stop as Ole Don suggests. Turn engine so it hits the 'stop' (as described in Ole Don's post, mark damper, turn ~360° opposite way until piston hits 'stop', mark damper, divide the space between your marks by two.
If your head was on you could use a tool similar to this; http://www.amazon.com/Competition-Cams-4795-Center-Bolt-Style/dp/B000CIOA0Q/?tag=atomicindus08-20
Yes I agree that the best way to find top dead center in relation to the damper is to use a stop for the piston and turn it back and forth, making a mark. But the real question is finding when the piston is in the middle of it's dwell at the top of the stroke and then lining up the pointer with the mark you made on the damper. The dwell period at the top of the stroke can be 2-4 degrees. Since the pointers on these engines are so weak they can be easily moved, so if they are lined up with the damper when the piston first reaches dwell (instead of in the center of the dwell period , which is correct), your timing mark could be several degrees off
dmsfrr
Well-known member
"...lined up with the damper..."
If it's a T-Bird crankshaft pulley the timing marks aren't on the damper ring. Pictures below.
The timing marks should be on the back lip of the pulley, or are they worn off?
If the timing pointer you have doesn't reach the back lip of the belt pulley it isn't the correct one anyway.
Make a chalk mark on the crank pulley that lines up with the tip of the timing pointer when the #1 piston stops moving on the way up. Then make another chalk mark on the crank pulley that lines up with the tip of the timing pointer when the piston starts down.
Then turn the crank back until the spot in the exact middle of those two chalk marks on the pulley lines up with the tip on the pointer.
The tip of the pointer AND the "0" / TDC mark on the crank pulley should now be lined up, or something is incorrect.
If your pointer and pulley are too far apart to get accurate markings fasten a stiff piece of wire to something solid and put an end of it right next to the pulley to be more helpful for placing the chalk marks.
If you want to get even more accurate there are stick-on 'degree wheels' marked off in 360 increments that can be placed on the crank pulley. But that's usually necessary only for racing purposes.
If it's a T-Bird crankshaft pulley the timing marks aren't on the damper ring. Pictures below.
The timing marks should be on the back lip of the pulley, or are they worn off?
If the timing pointer you have doesn't reach the back lip of the belt pulley it isn't the correct one anyway.
Make a chalk mark on the crank pulley that lines up with the tip of the timing pointer when the #1 piston stops moving on the way up. Then make another chalk mark on the crank pulley that lines up with the tip of the timing pointer when the piston starts down.
Then turn the crank back until the spot in the exact middle of those two chalk marks on the pulley lines up with the tip on the pointer.
The tip of the pointer AND the "0" / TDC mark on the crank pulley should now be lined up, or something is incorrect.
If your pointer and pulley are too far apart to get accurate markings fasten a stiff piece of wire to something solid and put an end of it right next to the pulley to be more helpful for placing the chalk marks.
If you want to get even more accurate there are stick-on 'degree wheels' marked off in 360 increments that can be placed on the crank pulley. But that's usually necessary only for racing purposes.
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willowbilly3
Well-known member
I agree. And even though I've been doing this more than 40 years, I never tune an engine without my dialback timing light. Oh, I can come close timing by ear, but never as close as I think it should be. With the dialback, I can make any adjustment or reading at zero on the balancer and read the timing on the timing light. Especially handy for checking advance curve. It's almost more important to know where your timing is all in at than to know base timing.
dmsfrr
Well-known member
More material for additional comments?
I dug out the timing pointer that was on the (incorrect) engine I pulled out of my '55 T-bird. It's the top pointer in the first picture.
The 'correct' reproduction replacement T-Bird pointer I ordered is at the bottom of that same picture.
Not only is the T-Bird part an inch longer, which I already knew and is correct, but the 'point' is in a different place.
For now I'll have to assume the replacement pointer will match up with the TDC mark on the correct crank pulley I have and TDC on the #1 piston. Since the timing cover, water pump & damper pulley aren't ready to go back on the engine yet.
Even tho the reproduction pointer seems slightly less stiff than the older oem pointer it would be pretty difficult to bend accidentally without it being obvious. It's flat with one 90 degree bend square to its mounting surface.
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I dug out the timing pointer that was on the (incorrect) engine I pulled out of my '55 T-bird. It's the top pointer in the first picture.
The 'correct' reproduction replacement T-Bird pointer I ordered is at the bottom of that same picture.
Not only is the T-Bird part an inch longer, which I already knew and is correct, but the 'point' is in a different place.
For now I'll have to assume the replacement pointer will match up with the TDC mark on the correct crank pulley I have and TDC on the #1 piston. Since the timing cover, water pump & damper pulley aren't ready to go back on the engine yet.
Even tho the reproduction pointer seems slightly less stiff than the older oem pointer it would be pretty difficult to bend accidentally without it being obvious. It's flat with one 90 degree bend square to its mounting surface.
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The pointer that you show in the bottom picture is like the one that I have. And yes, the marks on my "damper" are on the pulley. You are correct that the pointer sticks out over the the pulley(about 3/8in) even when the pulley is completely on the crankshaft. This overhang hides the timing marks on the pulley and makes it difficult to get a reading with a timing light. I do not have a dial back timing light so I will have to make due with what I have. Also the engine is out of the car and I have no way of start up. I would like to bend the pointer back a little so it does not cover the pulley but I am sure that in doing so I will have to reposition the point. This is why I need to know how to find exact TDC of the piston. I will try your advice and see if I can find the center between when the piston stops(up) and starts down. Thanks everyone. If anyone has a suggestion for a better pointer let me know because even though I hope to get this right before I put the engine back in the car, these pointers can be easily bent in the future. I can't imagine trying to correct this with the engine in the car. The pointer needs to be a FIXED point all the time.
dmsfrr
Well-known member
cjc56
My neighbor has two very nice complete drivable T-Birds, a '55 & '57.
I'll find time later today to look at the timing pointers on both his cars, take a photo or two and let you know what I find.
'...your pointer hides the timing marks on the pulley...'
With your engine out it should be almost easy to use any straight-edge and copy / transfer the original marks on the back of the pulley forward onto the next outside lip of the pulley just in front of the pointer. You could use some dots of light colored paint to make the marks easier to see.
My neighbor has two very nice complete drivable T-Birds, a '55 & '57.
I'll find time later today to look at the timing pointers on both his cars, take a photo or two and let you know what I find.
'...your pointer hides the timing marks on the pulley...'
With your engine out it should be almost easy to use any straight-edge and copy / transfer the original marks on the back of the pulley forward onto the next outside lip of the pulley just in front of the pointer. You could use some dots of light colored paint to make the marks easier to see.
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TedEaton
Senior Member
Exact TDC is in the middle of the 'dwell' period that the piston is not moving at the top of the bore.
dmsfrr
Well-known member
cjc56
Yes this is the least of your problems at the moment, but just over a week ago I said I'd get you these other pictures just to help make sure, so here they are.
The darker pointer is on a '57 T-Bird with the original 312 engine, the red pointer is on a rebuilt 292 in a '55 T-Bird and looks to have the same problem you mentioned, covering up the marks on the pulley.
.
Yes this is the least of your problems at the moment, but just over a week ago I said I'd get you these other pictures just to help make sure, so here they are.
The darker pointer is on a '57 T-Bird with the original 312 engine, the red pointer is on a rebuilt 292 in a '55 T-Bird and looks to have the same problem you mentioned, covering up the marks on the pulley.
.
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