The Ford Barn

bugsiegel · 12-26-2011, 10:00 AM

I reading Flathead Facts and on page 20 and 21 the book describes inserting a wire between the head and the piston to feel the piston reach its highest point, which will be marked on a pulley. then it says to rotate the engine the opposite direction and make a second mark which will be on a different location and also marked. The difference between is TDC.
I'm going to say the obvoius so that it's covered in the question. All the above is done on the combustion stroke.
So my question is why don't the two marks align with each other? is the difference the play in the push rod to the piston in relation to the direction of rotation?
and then if different marks are achieved in different directions shouldn't the more important mark be the one that represents the direction of the engine while running?
Go easy on me , my engine experience is a hand full of two stroke engines.
I do understand the basics of two strokes vrs four strokes

Thank in advance
Paul

Jack E/NJ · 12-26-2011, 10:22 AM

Paul>>>So my question is why don't the two marks align with each other?>>>

In theory, both the combustion & exhaust stroke TDC pulley marks should align. In practice, due to 'play' as you suggest and the difficulty in discerning vertical piston movement near TDC, the midpoint between the two marks is 'assumed' to be the true TDC.

Jack E/NJ

40cpe · 12-26-2011, 10:44 AM

The object inserted into the spark plug hole prevents the piston from reaching TDC. When you reverse the rotation and bring it back to the highest position you still haven't reached actual TDC, so the point between the two marks is the actual point the piston would have traveled to if it had not been restricted.

bugsiegel · 12-26-2011, 10:44 AM

Thank you Jack

36tbird · 12-26-2011, 02:27 PM

What is the relationship between the cranshaft slot keyway and TDC?

George/Maine · 12-26-2011, 02:51 PM

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I was reading about finding TDC in old book.
If you get the pistons at equal distance from deck even.
That would be #2 and #3 one is going down and other going up.
That the way it was done.Try it.It doesn,t care about the valves.

V8 Bob · 12-26-2011, 03:14 PM

Quote:

Originally Posted by bugsiegel

I reading Flathead Facts and on page 20 and 21 the book describes inserting a wire between the head and the piston to feel the piston reach its highest point, which will be marked on a pulley. then it says to rotate the engine the opposite direction and make a second mark which will be on a different location and also marked. The difference between is TDC.
I'm going to say the obvoius so that it's covered in the question. All the above is done on the combustion stroke.
So my question is why don't the two marks align with each other? is the difference the play in the push rod to the piston in relation to the direction of rotation?
and then if different marks are achieved in different directions shouldn't the more important mark be the one that represents the direction of the engine while running?
Go easy on me , my engine experience is a hand full of two stroke engines.
I do understand the basics of two strokes vrs four strokes

Thank in advance
Paul

Keep in mind JWL was finding TDC on pre-8BA flatheads that do not have pointers or marks on the pulley. The '48 and up 8RT/8BA/8CM all have pointers on the front covers and notches or bumps on the pulleys, and when aligned, are 2 degrees BTDC, NOT TDC. JFYI

Junknshit · 12-26-2011, 03:17 PM

Do it like JWL, Jack and 40 cpe say, mark accurate and measure accurate and you will have it. If you want to check it for true, dead nuts on, pull the head and use a dial indicator.

Frank Miller · 12-26-2011, 04:08 PM

If you use the dial indicator use the same methoud of measuring a given point in different directions. The rocking of the rod and piston Can make measuring at exact TDC difficult.

Jack E/NJ · 12-26-2011, 04:10 PM

Lou>>>What is the relationship between the cranshaft slot keyway and TDC?>>>

Well, you went and made me run out into a cold garage to try to find out. But I couldn't see the keyway without pulling the crank nut and washer. And I just don't feel like doing it. It's almost 60 degrees down there, why don't you do it and report back? 8^)

Jack E/NJ

bugsiegel · 12-26-2011, 08:08 PM

I'm just reading a book and trying to familiarize my self with the Flathead engine. One is being built for me as I don't have the knowledge or company around me to do it my self. I had known about using a dial indicator with the head off but didn't quite understand the head-on method explained in the book.
I posted the thread because I didn't understand measuring both directions then centering those two points. Makes better sense now.
For the curious I bought a 49 F1 and the engine needs a complete rebuild. The block is in the machine shop being cleaned and then tested. if it passes the test I have to start making decisions soon.
Thanks guys, more questions as I read on.

PeteVS · 12-26-2011, 08:27 PM

There's a problem with finding Top Dead Center by finding it AT top dead center. When the piston is AT top dead center, the crankshaft can rotate several degrees in either direction with either not moving the piston at all, or, moving it so slightly that perceiving the movement would be next to impossible with crude instruments. Seems to me, that the best way to ESTABLISH where the crankshaft is at top dead center is to have the head off, fasten a rigid bar between two studs and have a bolt thru the bar to stop the piston some distance below the uppermost point. Have a pointer down by the pulley and put a mark on the pulley when the crank is rotated in one direction and the piston comes to stop against the bolt. Then, rotate the crank in the opposite direction and put another mark on the pulley when the piston hits the bolt. Then, put a third mark half way between the other two marks. THAT will be crankshaft position AT top dead center.

Shadetree · 12-26-2011, 09:09 PM

So my question is why don't the two marks align with each other? ~Bugsi

When the piston is AT top dead center, the crankshaft can rotate several degrees in either direction with either not moving the piston at all~PeteVS

Pete answered this question very well I thought.
Shadetree

lee34 · 12-26-2011, 10:36 PM

Bugsi The marks will be in line if the stops for piston are in same position each time you check it .If your stops are near 12 o clock you will move your crank several degrees with out moving the piston up or down any,Set your stops at 5 to 10 minutes btc and check making sure your stops are the same each time.The crank has to stop same on compression stroke as exhaust stroke , The piston assembly does not know which stroke its is on.You can't get accurate reading in degrees if using near tdc as a stop.You have the proper method just needs tweaking a little.PeteVS remarks very good. my 2cents lee34

Frank Miller · 12-26-2011, 11:44 PM

For the record the ratio of rod length to stroke determines how long the piston stays at the top before changing direction.

Mart · 12-27-2011, 05:33 AM

PeteVS has stated the correct and classic way to accurately locate a tdc mark.

If the heads are on, as the top of the piston is not directly accessible, another method is possible, that uses the same principal.

If you thread a suitable object down the plug hole so that it can limit piston travel before it gets to tdc, the same can be achieved. A cable tie is suitable for this method. Thread the cable tie, big end first, down the plug hole so it sits sort of mid piston. Bring the piston up till the tie acts as a stop, mark the pulley. Then without moving the tie, rotate almost one full turn in the opposite direction and bring the piston up again, mark the pulley. As PeteVS says, a new mark half way between is the true TDC.

It's the same as the method Pete VS stated but using the head and a cable tie as the stop.

A few years ago I used a long length of welding wire as a "wand" to get a reassonable indication of tdc. I had about 2" of wire poked through the plug hole, and about 2' sticking out, bent, so as the piston travelled past TDC the wand rose and then fell. As there was a 12:1 ratio, the movement at the end showed TDC pretty well, Easily good enough to allow me to make a mark on the pulley and use it for setting up the engine initially. In hindsight, if I were to do it again I would stop the wand 1" short of the highest point in both directions and split the difference as in the other two methods.

Mart.

12-26-2011, 10:00 AM	#1
bugsiegel Senior Member Join Date: Dec 2011 Posts: 248	layman question about TDC I reading Flathead Facts and on page 20 and 21 the book describes inserting a wire between the head and the piston to feel the piston reach its highest point, which will be marked on a pulley. then it says to rotate the engine the opposite direction and make a second mark which will be on a different location and also marked. The difference between is TDC. I'm going to say the obvoius so that it's covered in the question. All the above is done on the combustion stroke. So my question is why don't the two marks align with each other? is the difference the play in the push rod to the piston in relation to the direction of rotation? and then if different marks are achieved in different directions shouldn't the more important mark be the one that represents the direction of the engine while running? Go easy on me , my engine experience is a hand full of two stroke engines. I do understand the basics of two strokes vrs four strokes Thank in advance Paul

12-26-2011, 10:44 AM	#4
bugsiegel Senior Member Join Date: Dec 2011 Posts: 248	Re: layman question about TDC Thank you Jack

12-26-2011, 02:27 PM	#5
36tbird Senior Member Join Date: May 2010 Location: nw SanAntonio, TX Posts: 939	Re: layman question about TDC What is the relationship between the cranshaft slot keyway and TDC?

12-26-2011, 02:51 PM	#6
George/Maine Senior Member Join Date: May 2010 Location: Mid coast Maine Posts: 1,878	Re: layman question about TDC Sponsored Links (Register now to hide all advertisements) I was reading about finding TDC in old book. If you get the pistons at equal distance from deck even. That would be #2 and #3 one is going down and other going up. That the way it was done.Try it.It doesn,t care about the valves.

12-26-2011, 03:17 PM	#8
Junknshit Banned Join Date: Oct 2011 Location: Wash. Posts: 233	Re: layman question about TDC Do it like JWL, Jack and 40 cpe say, mark accurate and measure accurate and you will have it. If you want to check it for true, dead nuts on, pull the head and use a dial indicator.

12-26-2011, 10:22 AM	#2
Jack E/NJ Senior Member Join Date: May 2010 Posts: 2,259	Re: layman question about TDC Paul>>>So my question is why don't the two marks align with each other?>>> In theory, both the combustion & exhaust stroke TDC pulley marks should align. In practice, due to 'play' as you suggest and the difficulty in discerning vertical piston movement near TDC, the midpoint between the two marks is 'assumed' to be the true TDC. Jack E/NJ

12-26-2011, 10:44 AM	#3
40cpe Senior Member Join Date: May 2010 Location: Star, MS Posts: 4,120	Re: layman question about TDC The object inserted into the spark plug hole prevents the piston from reaching TDC. When you reverse the rotation and bring it back to the highest position you still haven't reached actual TDC, so the point between the two marks is the actual point the piston would have traveled to if it had not been restricted.

12-26-2011, 04:08 PM	#9
Frank Miller Senior Member Join Date: May 2010 Location: Auburn, MA Posts: 2,106	Re: layman question about TDC If you use the dial indicator use the same methoud of measuring a given point in different directions. The rocking of the rod and piston Can make measuring at exact TDC difficult. __________________ “The technique of infamy is to start two lies at once and get people arguing heatedly over which is true.” ~ Ezra Pound

12-26-2011, 04:10 PM	#10
Jack E/NJ Senior Member Join Date: May 2010 Posts: 2,259	Re: layman question about TDC Lou>>>What is the relationship between the cranshaft slot keyway and TDC?>>> Well, you went and made me run out into a cold garage to try to find out. But I couldn't see the keyway without pulling the crank nut and washer. And I just don't feel like doing it. It's almost 60 degrees down there, why don't you do it and report back? 8^) Jack E/NJ

12-26-2011, 08:08 PM	#11
bugsiegel Senior Member Join Date: Dec 2011 Posts: 248	Re: layman question about TDC I'm just reading a book and trying to familiarize my self with the Flathead engine. One is being built for me as I don't have the knowledge or company around me to do it my self. I had known about using a dial indicator with the head off but didn't quite understand the head-on method explained in the book. I posted the thread because I didn't understand measuring both directions then centering those two points. Makes better sense now. For the curious I bought a 49 F1 and the engine needs a complete rebuild. The block is in the machine shop being cleaned and then tested. if it passes the test I have to start making decisions soon. Thanks guys, more questions as I read on.

12-26-2011, 08:27 PM	#12
PeteVS Senior Member Join Date: May 2010 Location: FP, NJ Posts: 2,811	Re: layman question about TDC There's a problem with finding Top Dead Center by finding it AT top dead center. When the piston is AT top dead center, the crankshaft can rotate several degrees in either direction with either not moving the piston at all, or, moving it so slightly that perceiving the movement would be next to impossible with crude instruments. Seems to me, that the best way to ESTABLISH where the crankshaft is at top dead center is to have the head off, fasten a rigid bar between two studs and have a bolt thru the bar to stop the piston some distance below the uppermost point. Have a pointer down by the pulley and put a mark on the pulley when the crank is rotated in one direction and the piston comes to stop against the bolt. Then, rotate the crank in the opposite direction and put another mark on the pulley when the piston hits the bolt. Then, put a third mark half way between the other two marks. THAT will be crankshaft position AT top dead center. __________________ Don't never get rid of nuthin!

12-26-2011, 09:09 PM	#13
Shadetree Senior Member Join Date: May 2010 Location: Cottageville, WV Posts: 1,535	Re: layman question about TDC So my question is why don't the two marks align with each other? ~Bugsi When the piston is AT top dead center, the crankshaft can rotate several degrees in either direction with either not moving the piston at all~PeteVS Pete answered this question very well I thought. Shadetree __________________ Son, you will never blow an engine up in high gear.

12-26-2011, 10:36 PM	#14
lee34 Senior Member Join Date: Jul 2010 Location: Kentucky Posts: 127	Re: layman question about TDC Bugsi The marks will be in line if the stops for piston are in same position each time you check it .If your stops are near 12 o clock you will move your crank several degrees with out moving the piston up or down any,Set your stops at 5 to 10 minutes btc and check making sure your stops are the same each time.The crank has to stop same on compression stroke as exhaust stroke , The piston assembly does not know which stroke its is on.You can't get accurate reading in degrees if using near tdc as a stop.You have the proper method just needs tweaking a little.PeteVS remarks very good. my 2cents lee34

12-26-2011, 11:44 PM	#15
Frank Miller Senior Member Join Date: May 2010 Location: Auburn, MA Posts: 2,106	Re: layman question about TDC For the record the ratio of rod length to stroke determines how long the piston stays at the top before changing direction. __________________ “The technique of infamy is to start two lies at once and get people arguing heatedly over which is true.” ~ Ezra Pound

12-27-2011, 05:33 AM	#16
Mart Senior Member Join Date: May 2010 Location: Solihull, England. Posts: 9,085	Re: layman question about TDC PeteVS has stated the correct and classic way to accurately locate a tdc mark. If the heads are on, as the top of the piston is not directly accessible, another method is possible, that uses the same principal. If you thread a suitable object down the plug hole so that it can limit piston travel before it gets to tdc, the same can be achieved. A cable tie is suitable for this method. Thread the cable tie, big end first, down the plug hole so it sits sort of mid piston. Bring the piston up till the tie acts as a stop, mark the pulley. Then without moving the tie, rotate almost one full turn in the opposite direction and bring the piston up again, mark the pulley. As PeteVS says, a new mark half way between is the true TDC. It's the same as the method Pete VS stated but using the head and a cable tie as the stop. A few years ago I used a long length of welding wire as a "wand" to get a reassonable indication of tdc. I had about 2" of wire poked through the plug hole, and about 2' sticking out, bent, so as the piston travelled past TDC the wand rose and then fell. As there was a 12:1 ratio, the movement at the end showed TDC pretty well, Easily good enough to allow me to make a mark on the pulley and use it for setting up the engine initially. In hindsight, if I were to do it again I would stop the wand 1" short of the highest point in both directions and split the difference as in the other two methods. Mart.