Hi Dick, glad you asked me that, because I think I know the answer.
The 24 stud Flathead started life with a 3-1/16 x 3-3/4 bore and stroke,
but within a year or so Ford started making the same block with a 3-3/16
bore, and ran both sizes (221 and 239 cubes) for various lines of cars,
trucks, military and industrial equipment, up until '46. '46 onwards
3-3/16 was std. bore for this block, until the end of the Flathead line
in '53. Ford cars kept the 3-3/4" crank from '32 to '53, but as you know
the '49 to '53 Merc was given a 4" crank for another 16 or so cubes and
10 or so horsepower (think the last Flathead Mercs had the hp rating
raised).
Many combinations of bore and stroke can be used in all 24 stud blocks,
even the 3-1/16 because allegedly, they are all identical as to the
intentions of the factory (identical as to cylinder wall thickness I
mean, the '49-'53 blocks have a different bellhousing arrangement and
other features, but all 24 stud cranks are interchangeable, and all 24
stud blocks can use the same maximum cubic inches). Oil pans are
available in a number of patterns but early and late blocks' pans do not
interchange because of the bellhousing details. All '32 - '53 front
timing covers do interchange, and there are a half dozen or more
patterns for front-mounted or side-mounted (right-angle drive)
ignitions.
The original cylinder walls on all Flatheads are very thick, and maximum
practical bore was 3-7/16, but although this worked, it was risky, and
most people stopped at 3-3/8. This bore is still commonly used today,
but here again due to age and rust in the water jackets, lots of
operators today think 3-5/16 is plenty. There is a legend that certain
models of 24 stud Flathead blocks have thicker walls than others, and
people give big bucks for such blocks when they turn up, but authorities
I respect say this is purely a myth. There are minor variations due to
core shifts, but no extra-thick-wall blocks. The real issue with
Flathead blocks today is that probably 2/3 of the surviving units are
cracked too badly to use. But the 1/3 we can use still represent, I'm
sure, many hundreds if not thousands in this great land, it's just a
matter of finding them, cleaning the muck out of them, and magnafluxing
them, but the latter is no small matter of investing time and effort
with a 1/3 chance of success. But once you have a sound one, if you
avoid blowups, you might race it 10 or 15 years and it will hold
together for you. Some Bonneville fellas I know report this.
Maximum stroke with a stock Merc crank is 4-1/8, by offset-grinding the
crankpin and using a '38-'45 Ford 21A conrod (STILL available new or
crack-free used!) which has a smaller big-end. The early crank had a 2"
crankpin but later they went to 2.139" which allows this offset grinding
to get a longer stroke. It's not clear why the factory enlarged the
crankpin because time has shown the 2" crankpin to be plenty strong even
for 90% nitro racing.
Strokes over 4-1/8 require a welded Merc stroker crank or a big-bucks
billet crank. Strokes up to 4-1/2 have always been used, and still are
today, but require a lot of work to get clearance for everything because
it gets extremely tight in there. 4-1/4 is easier to do and more common
in the long strokers. Today, some racers prefer aftermarket racing
con-rods and some are still using stock rods.
The 3-7/16 x 4-1/2, I think, goes right to the SCTA limit for vintage
Flatheads and Arduns of 325 inches.
My new 290 Ardun has a 3-5/16 + .030 bore and 4-1/8 offset ground Merc
crank. Ardun heads can be used on all 24 stud Flathead blocks that are
not "relieved". The factory relieved some, but apparently not many.
The new French Flatheads are relieved, hence no good for Arduns.
The above info, partly, comes from Ford Service literature I have and I
welcome any corrections from Flathead experts, but I doubt there will be
much. Hope this is of interest to you and a few others on the List.
Some on it knew all this when I was a boy! Cheers Bill
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