# Conversion of mm BTDC to degrees BTDC

• posted
Anybody want to take a crack at it? (If you don't know for what BTDC is
an acronym, don't bother trying to answer.)
My bike's points open at 0.8 mm BTDC, and I want to measure that with a
degree wheel and strobe light. I trust dymanic timing before static,
because the points are at the end of a four-gear drive train. Lash, you
know.
If anyone knows the formula, or can tell me where to find it, please let
me know. Thanks.
• posted
Many moons ago, I remember doing the same thing. The .8 mm btdc is actually measured at the top of the piston, through the spark plug hole. I had to calculate the degrees based on rod length and stroke. Do you have that info? If so I'll calculate it for you.
• posted
I'm going to jump out here and take a stab at this Assumption that the 0.8mm is piston displacement from TDC. Draw a circle that represents the stroke of the engine, the Diameter of the circle is the stroke. Draw a vertical line through the center of the circle. at a point 0.8mm down this diameter line draw a perpendicular line. So now you have a diagram of what your doing. The cord of the circle intersects the circle at the timing point. now do the math.
• posted
Crap! I looked closer at the page, an it looks pretty much for Vespas. Greg
• posted
Thanks Steve. I know the stroke, but not the rod length. I'll see if I can find it without taking the engine apart.
• posted
Thanks Greg, but that calculator won't let me input my motor's dimensions. Somewhere in the code for that calculator is the algorithm I want, but I don't know how to dig it out.
• posted
Yes, just put a dial indicator in through the spark plug hole and mark the flywheel at .8mm BTDC... then use the strobe light to verify that the spark is occuring when your marks line up. David
• posted
Attachments in text only NGs are a real no-no.
Ted
• posted
"David Courtney" wrote in news:c79rfj\$1bj63\$ snipped-for-privacy@ID-70857.news.uni-berlin.de:
Note on using this method. With any curved surface, there becomes a point before, and after the apex, where the indication movement will be too small to be registered by the indicator. This dead area could be up to a degree or more. Using the indicator, move the crank until the indicator quits moving, and mark the crank. Then roll the crank until the indicator just starts to move and mark the crank again. the middle of these two points is the actual apex.
• posted
It is a whole lot easier to go the other way, here are some equations with example data for degrees to mm, bang the equations into your favorite spreadsheet or calculator and work backwards. If you want I'll email you the excel spreadsheet I did this in
100 S stroke
200 RL rod length
9 theta Crank angle before top dead center
7.821723252 CPX(theta) Crank position x =(S/2)*sin(theta)
49.38441703 CPY(theta) Crank position y =(S/2)*cos(theta)
249.2314101 PPX(theta) Piston position x =CPY(theta)+SQRT(RL^2-CPX(theta)^2)
0.768589885 DBTDC Displacement below top dead center =RL + S/2 - PPX(theta)
Solving the equations for theta in terms of BTDC is more complicated than I am will to do if there is an easier way.
Carl Boyd
• posted
Draw a triangle, with the three corners representing the wrist pin, the rod journal, and the crank centerline. Label the sides as follows:
a=connecting rod b=crank throw c=distance from crank centerline to wrist pin centerline
At TDC c = a + b
at .8mm BTDC c = a + b - .8mm
cos A = ( b^2 + c^2 - a^2 ) / ( 2 bc ) cos B = ( a^2 + c^2 - b^2 ) / ( 2 ac ) cos C = ( a^2 + b^2 - c^2 ) / ( 2 ab )
Given the lengths of the three sides of a triangle, we can find the cosines of the three angles.
Then this follows:
A = arccos[ ( b^2 + c^2 - a2 ) /
( 2 bc ) ] B = arccos[ ( a^2 + c^2 - b2 ) / ( 2 ac ) ] C = arccos[ ( a^2 + b^2 - c2 ) / ( 2 ab ) ]
A is the degrees BTDC.
Enjoy,

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.