Piston travel

S.A. Baker

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Does anyone know of a chart or formula that correlates piston travel in thousandths of an inch to crankshaft rotation in degrees? Last time I used these methods was in the seventies...so if there was a formula I forgot it long ago. Counting flywheel teeth or fractions of a tooth is too damn imprecise for my liking......Thanks
 

S.A. Baker

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Find out how many thousandths piston travel equates to 28 degs of crankshaft revolution. Timing with a dial indicator has to be more precise than counting flywheel teeth and fractions thereof! Flywheel has no marks... except a TDC mark that I made.
 

Jiggz

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If you post the year and model and size of your motor maybe someone who already has a marking on their flywheels can give you an accurate measurement of the 28, 30 and 32 degree marks from the TDC marking. Although I understand the physical measure on the flywheel is not linear but spherical, it still can easily be converted to linear using a tape measure or a piece of wire string and re-measured with a tape measure.
 

S.A. Baker

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It should be a constant regardless of stroke or flywheel diameter. I'm sure it's something I forgot from the 70's . Engine is a 1975 Chrysler 45 hp. Nordin aluded yesterday that 28 degs. equated to .260 of piston travel. I was just wondering how you figured that. Hell .. maybe it's in the instructions in my old dial indicator set that's in my roller cabinets in a storage unit 60 mi. away!
 
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Jiggz

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Well, if you ever find the formula it would be great if you can share it.
 

S.A. Baker

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I found some stuff on a Brit bike forum....I can relate to that!..but... most of that stuff is just tuned to run wherever it runs best! And they are four strokes! After trying to weed thru the engineering and theories theads ....the math was WAY beyond me! Too many variables involved. Crank offset, rod length,bore and stroke....I'm not THAT bored!
 

Frank Acampora

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Don't know about the new ones, but the old Clymers gave the tool AND the number of thousands of inches to get 36 degrees for both the 2.80 stroke and the 2.875 stroke. Once you get 36 degrees 4 lines to the left of the pointer is 32 degrees.
 

gm280

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A simplistic way to think about it is 180 degrees can equal to the length of the stroke on a two cycle engine from BDC to TDC. So you could take the stoke and divide it by 180 and that should give you the movement in degrees... :noidea:
 

Frank Acampora

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Too simple. That assumes that the piston travels the same distance for each degree. In actuality, it travels much less distance per degree at the top and bottom of the stroke.

The number of inches was something like ..28 and .32 but it was not important to me so I never remembered it. If I ever find my old Clymers, I will post the actual numbers.

The reason for using 36 degrees is because top dead center is rather imprecise. This is due to slop in the con-rod bearings. You can feel a little "play" as the crank passes through TDC. Thus: 36 degrees is a better initial measurement for determining TDC. I don't know why the engineers chose that particular number though.

However, with practice you can get very close to actual TDC, feeling by hand or making a tool out of an old spark plug. From there counting flywheel teeth will be as accurate as any measurement.
 

gm280

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Frank, I realize that it isn't a linear equation, but for simplicity it works. Obviously there is a lot more movement per degree around the 270 degree area then TDC or BDC. But I was trying to make it simple. With a dial indicator and graph paper, you can easily plot the true movement from BDC to TDC. So for each degree there will be different amounts of movement...but should mimic those movements two times in that 180 degree swing. The movement per degree from BDC to the 270 degree arc should be the inverse of 270 degree to TDC. But only if the piston and crank are absolutely lined up on their center lines. Again JMHO!
 

scout-j-m

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I probably could have done this mathematically a few years ago when I was fresh out of college. I had an engineering course called kinematics that dealt with this very thing. We would even do the problems in a nifty solver called Mathcad that we could plot and animate the piston travel allowing you to visualize change in speed of the piston due to the angle/offset of the connecting rod on the crank.
 

scout-j-m

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This came from wikipedia page called "Piston Motion Equations" so take it with a grain of salt. But you could possibly use the equation below to find x and then subtract that value from I + r to get travel from TDC.

Piston_motion_geometry.png
883809a46e0ed320270e5090901c2f66.png
 

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Jiggz

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Or, does anybody has a '75 Chrysler 40 HP? Can you please measure the linear distance from the flywheel's TDC mark to the 28, 30 and 32 markings and post it here.
 

Frank Acampora

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Jiggs: You would be better off counting the flywheel teeth and dividing into 360. This will give the number of degrees per tooth. If I remember correctly 32 degrees is something like 6 1/2 teeth to the left of TDC

Are you certain the engine is a 40? During those early years Chrysler made 20, 25, 35, 45, 50, and 55. I don't remember a 40 until later Force engines.
 

Nordin

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S.A Baker, Frank, Jiggz and all others in this thread.
I do not know the formula of piston travel correlates to crank rotation angle.

From my servicebook that the dealers got from the Swedish general agent that imported and sold Chrysler OB in Sweden in the 60.s and 70.s, one service Bulletin handled about setting the timing without the timingtool.
For 35, 45 and 55Hp singlecarb engine they set the value at WOT as 6,6-7,0 millimeter (0,260-0,276 in) BTDC. They do not say that it is 28, 30 or 32 dgr.

SO that is where I got the value from.

Well tomorrow I can measure with a caliper the distance from timing mark and TDC on my 1974 45Hp Chrysler.
 

S.A. Baker

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Like I said....the math is WAY beyond me! My last math class was in 1967! I'm surprised that the old outboards didn 't have the timing listed in thousandths of piston travel usng a dial indicator. All the old sleds and two stroke bikes of that era did.
 

Nordin

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Have now check the piston travel on my 1974 45Hp Chrysler. Did it with a dial indicator in the top plug hole and depending on the backlasch at TDC and exact of marking line up, the value can be a little different. Smal movement of the flywheel gives big differenst in the travel.

28 dgr 0,218 in (5,55 millimeter)

30 dgr 0,266 in (6,75 millimeter)

32 dgr 0,329 in (8,35 millimeter)
 

wickware

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Is it common for timing to change without broken or worn parts? I have had my 1970 35hp Chrysler since 1984-85 and I have not noticed a reason to question the timing once (but checked it using the teeth method). All of my Hesitating, Stumbles Lack Of Getting Up or WOT has always gone back to Dirty Points.

BUT! On my 1968 and 70 VWs I never failed to Check or Adjust my timing with my almost yearly pitted points. IMO, the lack of not using Original Parts caused some of the pitting. Electronic Ignition Technology has been a Labor Saver but I still have my 2[SUP]nd[/SUP] Set Of Points since the 80s (on the 35hp). They have never left me on the water vs warning me of needs.
 

Silvertip

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Use a piston stop to find TDC. Then make or buy a degree wheel. Mark the block and flywheel for "0" and mark the block with whatever advance marks you need. No math needed. If you own a computer and a drawing program you can make a degree wheel easily. Print it, paste it onto an acrylic sheet, drill the center hole and you are done. You can even download the artwork from the internet. It's just too simple.
 
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