Maximum Bore + Stroke

Flathead

Flathead

Well-known member
On the Model A engine how much, if any, can the stroke be increased? Mainly wondering about space in the crankcase to swing a bigger crank. Would the maximun bore be about 1/8th inch oversize? I am familiar with the V8's but not so much the A. Thanks. :D
 
My last engine was bored .125 over which is the max bore. If I use it again it will have to be sleeved.

Mike
 
Maximum bore probably varies from block to block, depending on water jacket proximity, any core shifting, etc.

Have heard 0.060" overbore is common, 0.080 is "possible", and 0.100" is probably the absolute max IF you have enough meat in the water-jackets. That's still a bit less than 1/8"...

Don't know about stroking... you might achieve 4-3/8" stroke by grinding the rod journals off-center...

Watcha thinking about doing with this engine ?
 
My inserted bearing engine with just under 50,000 miles is bored .125" over. Zero problems during those miles. The next step would be sleeves if ever re-built. I do not have any problems with it running hot...I added a thermostat to get the temp up.

I've built a few others at this same bore with no problems.
Dave in MN
www.durableperformance.net
 
Flathead,

I have an old race engine, 1950-1960, it's a B, with off set ground rods to work
with A rods. The block is .125 over to .135 over, V/8 60 main bearings and a
reworked V/8 oil pump.

Dudley
 
Flathead said:
On the Model A engine how much, if any, can the stroke be increased? Mainly wondering about space in the crankcase to swing a bigger crank. Would the maximun bore be about 1/8th inch oversize? I am familiar with the V8's but not so much the A. Thanks. :D
Click to expand...

I think they have been bored out to 4.062. 4.000' is actually pretty common. The bigger you bore causes headgasket sealing problems between cylinders. Some have bore the cylinders slightly offset. In a recent issue of S O S S there were photos of engine that is bored into the water jackets and is going to be sleeved oversize. The "B" has much larger bearings so the oil pan has clearance dimples in the pan and this cures one of the problems with stroker cranks. I have read where the use of 12 point nuts on the rods also helps with the clearance problem. I have one, a "B", that is ground offset .156 and that gives a total stroke of 4 .562 or 4 9/16". This is a modification that can't be seen so the PC police won't know. Of course, I must admit, my car only goes 1/4 mile at a time.
 
fordgarage said:
My last engine was 0.125 oversize on the bore. I would offset grind a B crank to decrease the stroke not increase it, if I was going to change it. Think about it..
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I agree with you decrease the stroke. More stroke will give you more torque but less RPM. You do not need more torque, you need more RPMS.
My best running A engine was bored to 4.030.
I have been working on a destroked A engine we will see if it works out.
 
I just finished boring a engine to put sleeves in. I always use the thick wall sleeves that are .125 thou thick each side. This means I have to bore the block to 4.125 to have the sleeve a .002 thou interference fit. I do this all the time and have never hit water. I would say the block could be bored at least to 4.250 with no problems.
 
Thanks for all the responses, I was thinking about a torquey touring motor as opposed to higher RPM. Putting dimples in the pan is not a problem, but how much clearance is there between the rods and the camshaft for a longer stroke?
 
Flathead said:
Thanks for all the responses, I was thinking about a torquey touring motor as opposed to higher RPM. Putting dimples in the pan is not a problem, but how much clearance is there between the rods and the camshaft for a longer stroke?
Click to expand...

I used "Skokie" A rods and had no clearance problems. The engine does have a lot of torque and goes pretty fast 1/4 of a mile at a time.
 
What gasket are you guys using for .125" bore? I'm having a hard time finding a B head gasket for that bore size, and the A head gasket I used before is no longer available.
Chuck
 
Just plain Bill is right, he's "been there, done that". Back in the day many racers bored them to 4 1/16" with no problems but I recommend 4" to be the maximum. A 'B' crank can be offset ground to give 3/8" stroke, clearance between the rods and the crank, and between the rods and the pan get real close, so be careful. Guys used to use Pontiac rods, they had to be modified to clear the cam and often the cam was ground to clear them too. The old cam grinders knew right where to grind them.
 
Just thinking out loud here... stock stroke ( 4-1/4") is already fairly long and torquey; if you're looking for a touring engine with a little extra "grunt", how about a touring cam and perhaps 5.5:1 "police head"... ?

Sounds like "stroking" a Model A might be a complicated and delicate art...
 
You're right, a touring cam and 5.5 or 6 head is what I will probably do. However there is that "no substitute for cubic inches" rule that is still valid, especially with a Flathead. Just trying to do my homework on this.
 
if you look at the hamb and a guy with then screen name crazydaddyo. you will see that he has had one or two cranks break after doing this offset grinding.
i think you are really just making things more complicated for yourself. just put in a mitchell or volvo overdrive and call it a day.
then its just bolt ons without sacraficing reliability
just my .02
tk
 
kelley's restoration said:
if you look at the hamb and a guy with then screen name crazydaddyo. you will see that he has had one or two cranks break after doing this offset grinding.
i think you are really just making things more complicated for yourself. just put in a mitchell or volvo overdrive and call it a day.
then its just bolt ons without sacraficing reliability
just my .02
tk
Click to expand...

Those particular cranks were off set to the max and after much discussion it was never clear as to the radius on the rod throws. The radius and finish in the radius are critical. I know of one with a 1/8" off set with 20,000 miles on it. I do use a SBC dampener on my engines.
 
kelley's restoration said:
just put in a mitchell or volvo overdrive and call it a day. hen its just bolt ons without sacraficing reliability
just my .02
tk
Click to expand...


An overdrive won't help ya if your engine doesn't have the oats to turn it. Remember the old adages, "There is no substitute for cubic inches". "Speed cost money! How fast do you want to go? :D
 
Dave,
How is your engine with inserts? Is it a much stronger bottom end? Issues with bearing failure eliminated?

Thanks, Steve
 
kelley's restoration said:
if you look at the hamb and a guy with then screen name crazydaddyo. you will see that he has had one or two cranks break after doing this offset grinding.
i think you are really just making things more complicated for yourself. just put in a mitchell or volvo overdrive and call it a day.
then its just bolt ons without sacraficing reliability
just my .02
tk
Click to expand...


Yes, I have broken two cranks. They both were B cranks ground to A mains and rods. Rod journal offset ground .187" for a total stroke of 3/8". Fillet radii were too small, but even .125" - .187" radii are hard to get with this big of an offset grind. Bore was 4".

As Bill has said, If you are only expecting your engine to go short distances fast, stroking it is advantages. But if your building a touring engine, then changing the stroke is a waste of money.

Head, Cam, And carb are your best "bang" for the buck.

I prefer the T-5 trans. but I will be banned from this sight if I talk any more about "non-stock" alterations. :rolleyes:

.
 
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