I'm impressed. I usually apply A. C. Clarke's line,
"Any sufficiently advanced technology is
indistinguishable from magic" to electronic type
wizardry, but that's got to be one slick milling
machine. Thanks for the edification.
Greg
--- JES_VFR
wrote:
> At 12:52 PM 10/31/2006, you wrote:
>
> >John,
> >I'm not sure I understand what you mean by the
> >following:
> >
> >"Offset grinding the crank to get the additional
> 1mm
> >of radius on the journal bearing center..."
> >
> >The simplified(?) picture in my mind involves
> shaving
> >metal off of one side of the throw shafts (if
> that's
> >the right terminology, the shaft section that the
> con
> >rods connect to - I'm a technical geek, but not
> much
> >of a gear head) gradually tapering the grind to
> keep
> >the shaft round. This would shift the center of the
> >throw shaft out by half the distance of the maximum
> >thickness of metal removed. If that makes sense,
> and
> >is actually what you propose, then I'm amazed that
> it
> >can be done at all, let alone be regarded as a
> >relatively safe approach.
> >
> >I don't see how any other metal shaving anywere on
> an
> >existing the crank shaft would do anything to
> change
> >the stroke. Though I am willing to be enlightened.
> >
> >Greg
>
> You have got the idea on the first pass Greg.
> This process is done all the time in the performance
> Automotive
> industry. The bearing surface that the connecting
> rod turns around is
> called the rod journal.
> When an engine is rebuilt and the crank refinished,
> these journals
> are often turned down to a smaller diameter inorder
> to provide a
> fresh bearing surface for the rod bearing to mate
> with. There is
> usually enough material on the rod journals for a
> couple of light
> grinding and polishing passes for rebuilds. The
> grinding tool is
> setup to grind all the journals to the same size and
> ensure that all
> of their centers are the same distance from the
> centerline of the crank.
>
> The only thing that offset grinding does move these
> centers outward
> from the crank centerline half the increase in the
> stroke and remove
> more material from the inside of the rod journal
> than the outside.
> If it is done correctly, using a tested crank or a
> new blank then
> there should be no concern.
> Just to give you an example.
> Take the 5.0l v8 that has powered ford cars and
> trucks since the late
> 60's (back when it was called the 302).
> All the way up to its end in the late 90's the bore
> was 4.00 and the
> stroke was 3.00. Now back in the late 80's when the
> EFI mustangs were
> really taking off, it was common to do a stroker
> rebuild that would
> increase the motor to 331 cubes.
> That was accomplished by offset grinding the stock
> crank journals by
> .125, which when doubled increased the stroke to
> 3.25.
> These engines could make lots of power and run for
> hundreds of
> thousands of miles, I know I helped install several
> in mustangs and
> tbirds in this area. The best part is nothing
> visible on the outside
> of the motor had to be changed for this stroked
> motor to run properly.
> Now there were guys that did not think that was
> enough of an increase
> and would offset grind it to .225 which yielded 352
> with a stock
> bore. The crank was still not the issue here, it was
> the length of
> the rods and the fire deck height of the block.
> These two dimensions
> caused a spacial conflict between the piston pin and
> piston ring
> grooves. These motors quickly had issues with
> burning oil and poor
> ring sealing, but the cranks were never and issue.
>
> Now, I'm sure that there is much less space in any
> modern motorcycle
> engine since they are so compact, but even a little
> bit of stroke
> increase would be useful.
>
> >
>
> JohnS
> A Dragon Ascending
> "Forging my body in the Fires of my Will"
>
>
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