[Dirty Biker]

Originally Posted by cobalt327

Cranks aren't usually going to have errors measured in degrees, rather in minutes and seconds of degrees. A minute of arc is = 1/60 of a degree, etc..

How can you know for sure without measuring tho?
No offense cobalt327, I am wondering how you know this for sure is all.

[XNTRCI-T]

And the answer is... .0034" over 6 degrees of rotation. Tomorrow I'm going figure out what 1 degree of rotation produces (at TDC).

This revelation causes me to question even further how some shade-tree mechanic can claim he can time a motor by sticking a pencil down the spark plug hole and finding "top dead center."

[Dirty Biker]

Originally Posted by XNTRCI-T

And the answer is... .0034" over 6 degrees of rotation. Tomorrow I'm going figure out what 1 degree of rotation produces (at TDC).

This revelation causes me to question even further how some shade-tree mechanic can claim he can time a motor by sticking a pencil down the spark plug hole and finding "top dead center."

I just went through a similar thing, and got a math lesson on law of cosines and how to calculate the length of the hypotenuse with Pythagorean Theorem... I wanted to find the angle of the crank when the rod and crank centerline are at 90 degrees because that is the point of highest piston speed. Now it looks like I gotta work that down to figure out what one degree of crank rotation atdc equals in terms of piston travel depending on stroke and rod length? I wish I didn't sleep so much in math class in school so that I might already know some this stuff. How can it be easily solved? Thanks

[XNTRCI-T]

DB, I think I can figure that out with AutoCAD. I'm not a math person... I have no idea how to figure this stuff out with math. I am a visual learner, so I need a picture. Fortunately, I've learned how to use AutoCAD in my job, so I think I can figure it out for you by drawing scale pictures and measuring things with AutoCAD's built-in tools. You've given me something to do tomorrow morning in an otherwise boring job!!

[Dirty Biker]

Originally Posted by XNTRCI-T

DB, I think I can figure that out with AutoCAD. I'm not a math person... I have no idea how to figure this stuff out with math. I am a visual learner, so I need a picture. Fortunately, I've learned how to use AutoCAD in my job, so I think I can figure it out for you by drawing scale pictures and measuring things with AutoCAD's built-in tools. You've given me something to do tomorrow morning in an otherwise boring job!!

I like boring jobs for that reason sometimes! Sounds like you may have a good job, not so easy to come by these days.. I would love to try software like you get to use!

[Duntov]

Originally Posted by cobalt327

Cranks aren't usually going to have errors measured in degrees, rather in minutes and seconds of degrees. A minute of arc is = 1/60 of a degree, etc..

12 or 15 minutes is a lot though. Seen some new Eagle 4340 cranks up to about five years ago that took all of .010" to index, and some nearing .020" before truing up. Battle damaged Bryant crank having to go from 2" to nearly 1.888" < Thank God that's where we were going.

Gm forgings, 4577s, 1182s, ect, even after wearing out original build, and needing to be turned have a tendency to be better than all but the high dollar stuff, but have seen some that were stopped by a thrown rod that were more than 30 minutes out. And some which have been turned two or three times out half that much. What kind of Flintstone machine is doing that?
According to the oldest, best custom grinder I know a 2442 crank averages better than most. Yeah a full degree is a very damaged or possibly Chinese crank.. NAFTA GATT......Ross Perot, Clinton, Ralph Nader.

[4 Jaw Chuck]

Seriously though guys, indexing being out slightly is only going to cause a slight harmonic that I doubt anyone could feel or find without some very sophisticated equipment...the balancer is going to hide anything caused by indexing being out slightly.

Not to mention a V8 is inherently un-balanced anyway!

[XNTRCI-T]

OK Dirty Biker (and anyone else paying attention), here's what I came up with. Using a 4" stroke (2" crank throw) and a 6" rod, piston travel from TDC to 1* ATDC is .0005". Position of the crank throw when the rod is 90* to the crank throw is 71.56* ATDC. Piston travel from 71.56* to 72.56* is .0362" (intuitively, that seems low to me, but that's what AutoCAD says).

OK math wizes, how'd I do?

[airboat]

One engine builder explained to me that the piston at TDC is the only constant on every engine. he said while he blueprints many engines so that every cylinder is as close to the same as the other, Deck clearance, crank indexing, CC, CR, header tube differences, Intake mixture diferencec etc... variances can actually make a engine have a stronger? or broader HP/TQ curve.. Like cyl #1 has peak torque at 4200 rpm and #2 has peak torque at 4400 rpm etc.

[OneMoreTime]

Something we all have to realize is that tolerances do gang up on us..So if the crank is out of index and then the cam drive is a bit out and then maybe the cam was not cut right on the money and so on we can wind up with an engine that is a slug and we are wondering why it is..

there are some machine shops that can and will take the time to see that the crank is right on as far as index, stroke, journal size and straight..for the high dollar race guys they order a crank as an unground forging and go from there to build a crank. All of this makes a difference if one is building a race motor that is running on the bleeding edge for compression. this attention to detail really shows up in an engine that is zero decked when you do your assembly and all the pistons come up at various heights. It also comes up when you check out the cam timing on cylinders other than number one and they are all over the place. So do what you will knowing this..

Sam

[Dirty Biker]

Originally Posted by XNTRCI-T

OK Dirty Biker (and anyone else paying attention), here's what I came up with. Using a 4" stroke (2" crank throw) and a 6" rod, piston travel from TDC to 1* ATDC is .0005". Position of the crank throw when the rod is 90* to the crank throw is 71.56* ATDC. Piston travel from 71.56* to 72.56* is .0362" (intuitively, that seems low to me, but that's what AutoCAD says).

OK math wizes, how'd I do?

That seems about right to me, I think the math is good.
One degree doesn't seem like very much does it?

[Duntov]

Originally Posted by OneMoreTime

Something we all have to realize is that tolerances do gang up on us..So if the crank is out of index and then the cam drive is a bit out and then maybe the cam was not cut right on the money and so on we can wind up with an engine that is a slug and we are wondering why it is

Sam

The sentiment expressed above by OMT is partly what prompted me to make the original post. My crank grinder was telling me about an 1182 crank he did for a dirt customer. Plan was pretty standard; cut to SJ and set it on 3.5". Said first journal, no problem... Other three ended up causing him to have to back up and go -.010". He said that was unusual for mildly worn STD crank, but not rare at all for one which had been previously turned.

So hypothetically, had this crank been polished and installed as a 3.48.... then the cam installed and degreed to the #1 cylinder, it would have had 6 or maybe 7 cylinders??, at least to some degree, <<(no pun intended) out of sync with the cam??
I ask him a similar question... He grinned and said "Half of 'um run in the backfield for some reason or another".. <you had to be there<