why are some 502 cranks 4.140 and some 4.150?

As the title says it seems some companies sell a crank with a 4.140 stroke and another will be 4.150 .Is something to do with piston availability? Maybe the compression height ? I come up with a compression height of 1.450 for a 4.150 stroke 6.800 rod length .What would the compression height be for a 4.140 stroke ?1.440 Somehow I am guessing this goes back to the days of offset grinding factory crankshafts .Just curious about this and thinking about a 502 for my afr 280 heads .Thanks Doug

4.14 is rounded up:) , actually is 501.31. 4.15 is a true 502 @ 502.53

manofmerc wrote:As the title says it seems some companies sell a crank with a 4.140 stroke and another will be 4.150 .Is something to do with piston availability? Maybe the compression height ? I come up with a compression height of 1.450 for a 4.150 stroke 6.800 rod length .What would the compression height be for a 4.140 stroke ?1.440 Somehow I am guessing this goes back to the days of offset grinding factory crankshafts .Just curious about this and thinking about a 502 for my afr 280 heads .Thanks Doug

4.140" is usually an offset ground 460 crankshaft and 4.150" is what most new crankshaft suppliers offer.

rmcomprandy wrote:

manofmerc wrote:As the title says it seems some companies sell a crank with a 4.140 stroke and another will be 4.150 .Is something to do with piston availability? Maybe the compression height ? I come up with a compression height of 1.450 for a 4.150 stroke 6.800 rod length .What would the compression height be for a 4.140 stroke ?1.440 Somehow I am guessing this goes back to the days of offset grinding factory crankshafts .Just curious about this and thinking about a 502 for my afr 280 heads .Thanks Doug

4.140" is usually an offset ground 460 crankshaft and 4.150" is what most new crankshaft suppliers offer.

I think that the max you can get on an offset grind is 4.145 but in order for that to happen the core crank has to be nearly perfect to end up with a STD rod bearing on all of the throws and that's why most grinders shoot for 4.140. I've never tried to do an offset grind like that but it seems like it would take a long time to do correctly I'll have to ask Adney about it sometime.

It seems eagle has a 4.140 and scat theirs is 4.150 .I guessed it was from the days of offset grinding .Thanks Doug

The change is only .005 at the deck , so a 1.450 piston would be .005 lower in the block with a 4.140 stroke versus a 4.150 stroke. Or you could order a 1.455 c/h piston, or better yet measure the "actual" block height minus half the stroke and minus the rod length to get "zero" deck on the piston.

Eagle and RPM use 4.140. Scat uses 4.150. There is no particular reason for it.

I prefer 4.300 and 4.500 anyhow.

Mark O'Neal wrote:Eagle and RPM use 4.140. Scat uses 4.150. There is no particular reason for it.

I prefer 4.300 and 4.500 anyhow.

The 521 Scat kit I purchased is a 4.3 with 6.8 rods. It seems like a lot of builders like the 4.3 stroker including Jon Kasse. .02

Gregaust wrote:4.14 is rounded up:) , actually is 501.31. 4.15 is a true 502 @ 502.53

502.53 is over 502.5 and is, therefore, rounded up to 503.....

Also,
I believe Lunati makes a 4.16
Sheldon G.

STR-LGL-70 wrote:Also,
I believe Lunati makes a 4.16
Sheldon G.

Yes indeed.

4.140" Eagle
4.150" Scat
4.160" Lunati

manofmerc wrote:As the title says it seems some companies sell a crank with a 4.140 stroke and another will be 4.150 .Is something to do with piston availability? Maybe the compression height ? I come up with a compression height of 1.450 for a 4.150 stroke 6.800 rod length .What would the compression height be for a 4.140 stroke ?1.440 Somehow I am guessing this goes back to the days of offset grinding factory crankshafts .Just curious about this and thinking about a 502 for my afr 280 heads .Thanks Doug

Yes, it is rooted in the original offset stroking of the OEM cast crankshafts.   The BBF rod journal is 2.500” std and the BBC rod journal is 2.200” std. That’s a difference of 0.300”, and so mathematically the maximum offset stroke (BBF std-to-BBC std) would be 3.85” + 0.300” = 4.15”.  But in order to grind the rod journals a full 360* all the way around it’s necessary to offset grind a little less in the real world so as to touch all the rod journal material being ground.  Ergo, 4.14”-4.145”.  Then when purpose-made aftermarket cranks came about many went to the mathematically correct 4.15”.  At least that’s the distilled/simplified answer.  

All these compression height variances in this thread seem to presume a 10.300” deck height in order to attain zero deck, but remember we are not stuck with a 10.300” deck and for most carbs & gas builds where the builder wants quench we really have about a 0.050” variance to work with (ie, 10.275”-10.325”) and stil be able to screw an engine together without much if any unique machining steps.  Frankly as far as the build perspective being discussed here (1.45” for zero deck) I wish someone offered a 1.47-ish c/h shelf piston given that 90% of the blocks produced have a ~10.325” deck height, but I guess that might risk liability issues from the ignoramus people who use them in a C9VE block and destroy their own engine then blame the piston manufacturer.  But if I did another round of crate engine builds I’d probably get a special run of 80-125 pistons with a pin location specifically for the engine program for 1973-up engine block applications.

DaveMcLain wrote:
I think that the max you can get on an offset grind is 4.145 but in order for that to happen the core crank has to be nearly perfect to end up with a STD rod bearing on all of the throws and that's why most grinders shoot for 4.140.  I've never tried to do an offset grind like that but it seems like it would take a long time to do correctly I'll have to ask Adney about it sometime.

As I’m sure you know this assumes the crankshaft is ground to BBC standard 2.200”, whereas grinding to a BBC undersize dimension enables further offset stroking.  The BBC rod bearing is made to 0.060” under; the first commercially available stroker kit of which I am aware was offered by Lunati and I think it was 4.18” stroke (+ 6.535” OEM BBC rod + 1.67” c/h Taylor piston).

I have a combo over here that will use a 4.25” offset stroked OEM crank (SBC 2.100”), just need to get the crank ground.  4.25” was another popular stroke back in the days of almost nothing but custom offset ground cast cranks for the Ford.

Thanks Paul for your explanation and clearing up the compression height for the pistons .Both my d1 blocks have been decked and I am at ,008-.007 on both .This is a 460 crank and flat top forged pistons .If and when I decide to go the 502 route I would probably need a custom piston since most stroker kits don't come with valve reliefs for AFRs and I would want a dish to get my compression down to 10.5 -11.1 .I know as well most guys go for a 4.300 stroke . I have a 460 crank sitting around and I know a really good crank grinder that is familiar with offset grinding these fords so now you know the rest of the story .Thanks for the words of wisdom .Doug

Colt Macara wrote:

Mark O'Neal wrote:Eagle and RPM use 4.140. Scat uses 4.150. There is no particular reason for it.

I prefer 4.300 and 4.500 anyhow.

The 521 Scat kit I purchased is a 4.3 with 6.8 rods. It seems like a lot of builders like the 4.3 stroker including Jon Kasse. .02

If you're gonna date a fat girl, date a fat girl.

Mark O'Neal wrote:

Colt Macara wrote:

Mark O'Neal wrote:Eagle and RPM use 4.140. Scat uses 4.150. There is no particular reason for it.

I prefer 4.300 and 4.500 anyhow.

The 521 Scat kit I purchased is a 4.3 with 6.8 rods. It seems like a lot of builders like the 4.3 stroker including Jon Kasse. .02

If you're gonna date a fat girl, date a fat girl.

Okay, that's where I draw the line.... I have NEVER dated a fat girl...... at least not that I remember.

ceah...that old "there is no substitute for cubic inches" line needs some clarification.....

Mark O'Neal wrote:ceah...that old "there is no substitute for cubic inches" line needs some clarification.....

Clarification?..... meaning 557, 598, 632??? There probably is a 700 roaming around the woods out there getting ready to come off of somebody's' engine stand. Ha!

Colt Macara wrote:

Mark O'Neal wrote:ceah...that old "there is no substitute for cubic inches" line needs some clarification.....

Clarification?..... meaning 557, 598, 632??? There probably is a 700 roaming around the woods out there getting ready to come off of somebody's' engine stand. Ha!

We were discussing specifications within a different set of parameters. More like the comparison between Kate Beckinsale and Fat Amy.

Therefore the axiom "no substitution for cubic inches" needs to be amended...or at least be approached with greater specificity.

Mark O'Neal wrote:

Colt Macara wrote:

Mark O'Neal wrote:ceah...that old "there is no substitute for cubic inches" line needs some clarification.....

Clarification?..... meaning 557, 598, 632??? There probably is a 700 roaming around the woods out there getting ready to come off of somebody's' engine stand. Ha!

We were discussing specifications within a different set of parameters. More like the comparison between Kate Beckinsale and Fat Amy.

Therefore the axiom "no substitution for cubic inches" needs to be amended...or at least be approached with greater specificity.

Do you mean like..... if "Adders" are brought into the equation? Their could still be a misunderstanding like....is a 'Blower', a 'Supercharger', or just a 'good time'? There could be a substitution for "Cubic Inches", just like "even fat girls can be fun!"...... even though I don't have any real experience with either. I'm just supposing......

I think I will trust your judgement. It has been my experience that most large cubic in females have an extremely irritable tuneup.