Crankshaft offset

R

russcc

Well-known member
The crankshaft in the flathead is offset .265 from center. I assume the camshaft centerline is also offset the same amount. What was the purpose of the offset ? Thank you Fordbarners.
 
Thank you Mark. In a nutshell, the purpose of the offset was to improve the rod's angle of attack to the crankshaft on the power stroke.
 
Mark, I'm surprised at you, posting that grossly inaccurate link from Street Rodder. I never even got to the "rods angle of attack" before closing it down in fear of contaminating my brain with such nonsensical drivel. You of all poeple should know that the hacks who write for those magazines do a bare minimum of research and get paid by the word. They typically know squatt about the subject they write about.
 
There is a thread on the HAMB dealing with crankshaft and camshaft offset, and no, they are not offset the same amount.
 
WOW Mark you have thrown down the "Gauntlet" here !!!
let the games begin ...................or for ever shut up !!!
I love it !!!!
Cheers
Tony
 
I wouldn't touch this with a 10 foot pole, I had sone strange idea it had something to do with the thrust?? Might be the same reason they offset the pistons??
 
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ford38v8 said:
Mark, I'm surprised at you, posting that grossly inaccurate link from Street Rodder. I never even got to the "rods angle of attack" before closing it down in fear of contaminating my brain with such nonsensical drivel. You of all poeple should know that the hacks who write for those magazines do a bare minimum of research and get paid by the word. They typically know squatt about the subject they write about.
Click to expand...

Then explain why?
 
The HAMB tread list some information on this from a Ford Engineering Training Manuel that at least sounds factual.

Per Bruce Lancaster on the HAMB: To quote the Ford engineering text,
"The principal object of this [Desaxe] offset is to diminish the obliquity of the connecting rod during the power stroke, so that the mean thrust of the piston on the cylinder walls during this stroke when the bearing loads are greatest, is reduced."

http://www.jalopyjournal.com/forum/...-asymmetrical-why.457580/page-2#post-10750147
 
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Wow! Away from the computer for a few hours and look at all the smoking brains! I really like that discussion on the HAMB! Not being a wizard at math or building engines, but still, I see an obvious mistake in the 1932 Ford drawing that I didn't see ironed out in the HAMB discussion:

The pistons in the 1932 drawing are apparently at the same height according to the bore angle radius, a feature of that drawing that is in disagreement with the crank offset. Notice that that radius line showing bore angle 45º is the only indication of height, as the drawing is a demonstration of internal components, excluding the block itself. I think the engine builders here are more able than I to quote the difference in deck height to accommodate the offset.

No wonder there were contradictions between drawings if a Ford drawing can't agree with itself! I can see now that the hack who wrote the Street Rodder article didn't do too badly in his "angle of attack" reference after all! ...But yes, I do still stand by my opinions on magazine hack writers. I think most of us here and on the HAMB have found many discrepancies in magazine articles authored by staff and freelance writers, to the point of distrust in all such articles.
 
I do not understand the reference to "deck height difference"?? Obviously the deck height is the same from passenger to driver side on our flatheads. How else could we use the same rods and pistons in each bank of the engine?
 
Excellent question from John. Any opinions on that ? Thanks for the education. It seems we all agree the crankshaft and camshaft are offset, but we do not seem to agree on the purpose, Although the angle of attack seems the most logical, as I believe it is considered in other reciprocating engines.
 
John, I'm at a loss here. I'm not real strong on flatheads, but I'm pretty strong at math. If the crank center is offset, and the same dimensions are used for rods and pistons for both sides of engine, and if the tops of the pistons travel to 'deck height' on both sides, then how can that be possible unless the deck heights are different ????

Maybe I need a definition of deck height. If it is measured from actual crank center, it's a screwball measurement that could be argued to be the same for both sides. If it's from the intersection of the right and left cylinder bores, like most engines, then the decks need to be at slightly different heights.
Of course, this leads to a series of questions. Did Ford compensate for all this with valve timing and ignition timing? Are cams ground with different timing, right and left? Theoretically, I believe they should be. How about the distributor cam lobes? Again, I believe right and left timing lobes should be slightly different.
This is more math than I want to think about. My point is, if you establish a plane at the top, 45 degrees from cylinder center-lines, perhaps the intake surface, and measure down to a common reference on the decks, then the decks have to be at slightly different heights. (Or... is the intake surface REALLY at 45 degrees to the cylinder bores? Or... are the cylinder bores REALLY at 90 degrees?)
I really wonder if Ford really worked this all out, or did they just 'shine-it-on' because the math difference is pretty small. The drawings that I've seen don't really have enough info. How about the cam grinders here - can you see a difference, right and left banks?
 
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bobH said:
John, I'm at a loss here. I'm not real strong on flatheads, but I'm pretty strong at math. If the crank center is offset, and the same dimensions are used for rods and pistons for both sides of engine, and if the tops of the pistons travel to 'deck height' on both sides, then how can that be possible unless the deck heights are different ????
Click to expand...

Hi bobh; It's just semantics. The deck heights are the same relative to the centerline of the crankshaft. Craig.
 
Deck height is always measured from the centerline of the crankshaft, has to be. Anything else would be a theoretical point in space that could not be measured.
 
bobH said:
John, I'm at a loss here. I'm not real strong on flatheads, but I'm pretty strong at math. If the crank center is offset, and the same dimensions are used for rods and pistons for both sides of engine, and if the tops of the pistons travel to 'deck height' on both sides, then how can that be possible unless the deck heights are different ????

Maybe I need a definition of deck height. If it is measured from actual crank center, it's a screwball measurement that could be argued to be the same for both sides. If it's from the intersection of the right and left cylinder bores, like most engines, then the decks need to be at slightly different heights.
Of course, this leads to a series of questions. Did Ford compensate for all this with valve timing and ignition timing? Are cams ground with different timing, right and left? Theoretically, I believe they should be. How about the distributor cam lobes? Again, I believe right and left timing lobes should be slightly different.
This is more math than I want to think about. My point is, if you establish a plane at the top, 45 degrees from cylinder center-lines, perhaps the intake surface, and measure down to a common reference on the decks, then the decks have to be at slightly different heights. (Or... is the intake surface REALLY at 45 degrees to the cylinder bores? Or... are the cylinder bores REALLY at 90 degrees?)
I really wonder if Ford really worked this all out, or did they just 'shine-it-on' because the math difference is pretty small. The drawings that I've seen don't really have enough info. How about the cam grinders here - can you see a difference, right and left banks?
Click to expand...

One thing to consider is that the shift of the cylinder center-lines will cause the rod to not be perpendicular with the piston at TDC or BDC as it would be in a conventional layout. I think it is that affect that allows the deck heights to be equal.
From what I see on the drawing the CL's of the bores are shifted .168 to the drivers side of the block using the CL of the crank for the datum point, and further, the camshaft CL is also shifted .0242 to the drivers side. I don't know enough about flathead valve pockets but I will assume that the different angles, 49.36° pass. and 52.09° drivers side were necessitated by the .0242 cam shift so that the path to cylinders on both banks be equal.
Regardless, I do not see the reason for the .0242 shift in any case.
The cam shift leads me to a question and I apologize in advance if this hijacks the thread. With the different valve angularity from side to side in the flathead an astute cam grinder will certainly have recognized the need for adjusted timing events for right and left banks. Or did they? Is it possible some thought this through and some overlooked it? Are cams for early valve angle engines different from the later f/h engines where I believe there was symmetry in valve angles? Not to single any particular performance cam out, but is a cam ground on masters that may have been designed for the 49.39° and 52.09° angles corrected for the later flathead?
 
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