Thread: way fast flattie

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Well hold your horses a bit! You can get plenty of low rpm torque with a 4" crank, but(!) unless you can find someone who will relieve the block for you (cut about 1/8" down into the block around the valves using the head gasket outline, the higher compression heads over about 8 1/2:1 will actually choke off breathing because the space between the valves and the bore will act like a dam to the flow. Also do you have a block? I bought three and gave up when they all had some kind of crack in one place or another. There are two problems with boring, the first is that the engine may overheat due to thin cylinder walls and then crack from uneven heating or worse maybe one or more of the cores shifted in the original mold and one side of the cylinder will be much thinner than the other. There is a great article in the July 2004 Street Rodder mag. on page 182 showing how to relieve the block. However in the September 2004 issue of Street Rodder there is a picture on page 216 showing the effect of core shift in that the rightmost cylinder is offset so that boring will make one side much thinner than the other. There is a way to use ultrasonic testing for this, but my humble opinion is that you should STROKE the engine and bore it only the least amount to cleanup the cylinders. I can recall reading Hot Rod in the 1950s when it seemed all the flatheads were bored out to 3 3/8" and stroked with an offset Merc crank to 4 1/8", but I do not know how long those engines lasted. First you need to find a good block and if/when you do that is great but you may have to hunt or buy one of those blocks from French military surplus (not cheap). You can buy a stroker crank from Speedway but I have been trying to sell one to someone with a good block for some time. I have a 4" Merc crank that needs a 0.010 cleanup grind with rebuildable rods which I will sell to you for $225 plus shipping. First get back to me and tell me you have a good block. From my memories of a stock 256 cu. in. Merc, even with low compression heads and about 3.78:1 gearing you will really have an excellent low rpm torque motor. I will take some flak for saying this but it seems to me that up to about 2000 rpm a 4" stroke flathead has more torque than a stock SBC 350, but of course above 2000 rpm there is no comparison and the flathead runs out of torque around 4500 rpm. On another topic there used to be thousands of cams available for the flathead but after many tests the Iskederian cams won the shakeout and probably with the Isky 3/4 cam and a 4" stroker crank with relieving and multiple carbs you can get about 250 H.P., maybe even 280 H.P. with a carefully constructed engine, but likely with a stroker crank, Isky cam and dual carbs you will get about 150 H.P. with plenty of low rpm torque. Well I am coasting along on memories and book reading except for the part about how hard it is to find a good flathead block. If you do find one try not to bore it too much and stroke it instead. Let me know if you want the crank and rods. Note the engine displacement increases with the SQUARE of the bore due to the pi-r-squared effect and only linearly with the stroke, BUT (!) the probablility of cracking the block also goes up dramatically with excessive boring! Still if I could have found a good block that is what I would have built, but now I am in kindergarten learning about the SBC 350.


Postscript: Suedeplymouth mentioned the Ford/Merc block. There were several. I am rusty on my restorer facts but in the early '39-'41 blocks the Fords had only 3 1/16" bore compared to the Merc 3 3/16" bore but after WWII both Merc and Ford had 3 3/16" bore and 3 3/4" stroke. Then the '49-'52 Fords stayed at 3 3/16" x 3 3/4" with a lighter sheet metal bell housing while the '49-'52 Mercs stayed at 3 3/16" bore but increased the stroke to 4"; it is these cranks that can be used in Ford or Merc blocks back to '39, ie any 24 head stud engine. The earlier flatheads only had 21 head studs and the very early ones did not have insert bearings but I do not know the first year for insert bearings. Only a restorer would try to rebuild a '32-'34 flathead anyway (at great cost) so you are more likely to find one of the '49-'53 Ford "8BA" blocks or the '46-'48 "59AB" engines. If you find a block, count the head studs, I would only try to rod a 24 stud block and leave the 21 stud engines for purist restorers.
(24 studs on each side) I would agree with suedeplymouth about the 4-barrel intake, it is tricky to balance the dual or trip setups, although if you don't mind relocating the alternator, a triple carb setup with progressive linkage using the center carb mainly and the other two for pedal-to-the-metal could look great AND give better economy. However, four barrel carbs were supposedly easier to tune and manifolds for them should still be available.

Don Shillady
Retired Scientist/teen rodder