3 7/8 Stroker

[JSeery]

Originally Posted by tubman

It was absolutely relevant in answering Jack. He was saying the the stroke increase is split equally at the top and bottom. I think I have adequately shown that, when ground with the proper offset, it is all at the bottom.

Again; look at Ronnie's diagram. Now imagine it upside down. Given a stock rod, and the proper piston, the crown of the piston will be 1/8 inch lower at the bottom of the stroke, while flush with the deck at TDC.

Not true, there is something that you are not understanding here. Stroke is measured from the center of the crank throw which moves with an off-set grind. It requires a different piston with the pin relocated 1/2 of the stroke change. Sure the piston will be flush with the deck and 1/8 lower at the bottom of the stroke. That is because the crank center and the piston pin have both been moved 1/16 inch upward at TDC.

[scooder]

Sorry tub,
The center IS the key. I'm not sure how I could explain it better.
As the center of the pin moves farther away from the crank main bearing center. The rod and piston must also move in the same direction as the new center.
Ronnies picture shows what happens to the center. Imagine a rod and piston assembled on each circle in his pic, you should then see that the one assembled on the smaller circle,will end up higher at its top. Turn the crank 180 degrees, the one assembled on the smaller circle will now be lower.
I hope that's clearer.
Martin.

[Ronnie]

Wrap your mind around this. Center lines x 2 again determine stroke nothing else!!!! Not diameters not top of throw not bottom of throw.
Stroke changes require piston pin height change or con rod length change.
Offset grinding does change the throw diameter,but that isn't what changes the stroke, it's the change in center line. I hope this helps Tub.

R

[tubman]

Originally Posted by Ronnie

Wrap your mind around this. Center lines x 2 again determine stroke nothing else!!!! Not diameters not top of throw not bottom of throw.
Stroke changes require piston pin height change or con rod length change.
Offset grinding does change the throw diameter,but that isn't what changes the stroke, it's the change in center line. I hope this helps Tub.

R

You gentlemen are misunderstanding me . Offset grinding is what changes the throw diameter, which changes the centerline of the journal. I totally agree that changing the centerline causes the change in stroke. We have all been saying the same thing here, except using different terms. I say the offset grinding changes the stroke; you say the change in centerline changes the stroke. I believe my statement is more correct, because it is the offset grind that causes the centerline to change. Can we all agree that a 3 7/8" stoker can be created with an offset grind on a late 3 3/4" crank along with using early rods? That's all I was trying to say. If anyone disagrees with that, we can keep arguing, but otherwise we have been going back and forth over terminology and cause and effect, and it is time for this discussion to end.

[40cpe]

Originally Posted by George/Maine

I guess if you want some ideas how you get 3 7/8"
Take a 4" and say .030 under size reduce stroke to 3 .970
Then use 239 pistons drops from deck 1/8" now if you can take .100 off deck.
You still have 3.970 stroke but with the top cleaned off it looks like 3 7/8"
If you wanted to use a bad engine this could be done.

Before this subject is closed, are there any comments on the above statements?

Since the radius stays the same, does turning a crank reduce the stroke?

Will using a 239 piston not put the top 1/8" ABOVE the deck?

How does a 4" stroke LOOK like a 2 7/8?

Maybe I'm misreading the whole post.

[V8COOPMAN]

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Originally Posted by 40cpe

Before this subject is closed, are there any comments on the above statements?

Since the radius stays the same, does turning a crank reduce the stroke?

Will using a 239 piston not put the top 1/8" ABOVE the deck?

How does a 4" stroke LOOK like a 2 7/8?

Maybe I'm misreading the whole post.

Normal "turning" of a crank does not reduce the stroke. It merely reduces the overall diameter of the throws, and that reduction is made-up again with thicker replacement bearing shells.

"Offset grinding" of a throw (with regard to the centerline of the throw) changes the stroke. Depending on which side of the throw centerline that the crank is offset-ground, the stroke can conceivably be increased, or decreased.

A 239 piston used with a stock 4" crank will put the top 1/8" above the deck.

2-7/8 should have read 3-7/8". DD

[JSeery]

One issue would be taking any material off the deck. The flathead decks are think to begin with so this would be a very bad idea. The difference in the piston to deck height would be 1/16, not 1/8. And, you don't destroke a 4 inch crank, you off-set grind a 3 3/4 crank to get the 3 7/8. As this is not done anymore continuing this debate serves little constructive purpose. There may still be a few people off-set grinding a 4 inch crank to get increased stroke, but in any case, any change in stroke requires a matching piston pin location change (or change in rod length).

[Kahuna]

DD & JSeery have it exactly correct

[scooder]

Tub,
I fulley agree that you can offset grind a Ford 3 3/4" crank to achieve a 3 7/8" stroker. I'm pretty sure no-one will disagree with that, it was done quite a lot before the 49 Merc crank showed up, that pretty much killed the 3 7/8" job.
It is possible to go in opposite offset and get a 3 7/8" stroke from a 4" Merc crank, but as I said earlier, you'd have to be insane.
The disagreement here is that you clearly said "the entire stroke increase happens at the bottom of the stroke" this statement is incorrect. As we have tried to explain to you, the stroke increase is half at the bottom of the stroke and half at the top.
That is the geometric fact.
I really hope you understand this.
Martin.

[George/Maine]

I really think it was measured wrong. 3 3/4 or 4" my guess. Flathead knock
I was playing with grinding a 4" crank down .030 with little off set really not worth it but if you had a bad engine and making it to sell as a crate engine for $$$$
With 239 pistons would drop piston 1/8"
Now don't know how much you can take off deck but say .100
All looks good to measure stock looks like 3 7/8 but really not its only .030 less then 4"
There is to many problem with this engine says rod hitting I dough it.
I see no need for dome pistons with high compression head and a hot cam to make things worse.

[George/Maine]

With enough weld you can stroke it to what you want.
Set the grinder to what stroke you want and grind to your bearing size.

[scooder]

George,
I wouldn't cut anything off the deck. Already a bit thin up there.
Not sure any good flathead builder would cut the deck unless for whatever reason he absolutely had to. Unless it's the thinnest of shaves.
I can't remember the actual thickness, but sure it's around 0.125" or so.
I agree with the measured wrong, it's got 8BA rods, so it's gonna be 3 3/4" or 4", 3 7/8" would require early 2" journal rods.
Martin.

[flatjack9]

Originally Posted by scooder

Tub,
I fulley agree that you can offset grind a Ford 3 3/4" crank to achieve a 3 7/8" stroker. I'm pretty sure no-one will disagree with that, it was done quite a lot before the 49 Merc crank showed up, that pretty much killed the 3 7/8" job.
It is possible to go in opposite offset and get a 3 7/8" stroke from a 4" Merc crank, but as I said earlier, you'd have to be insane.
The disagreement here is that you clearly said "the entire stroke increase happens at the bottom of the stroke" this statement is incorrect. As we have tried to explain to you, the stroke increase is half at the bottom of the stroke and half at the top.
That is the geometric fact.
I really hope you understand this.
Martin.

Amen.

[ford38v8]

I wonder what kind of stroke would be possible if you combined this discussion with the offset main discussion?

[Tim Ayers]

This is an interesting topic. I agree with some in that a 3 7/8" stroke has been replaced by the 4" or more stroke cranks. I'm interested in those that have de-stroked their engines in order to fall into different race classes.

I believe Barney Navarro did this and I'm sure others as well.

I would assume the de-stroking process work in opposite of the stroking process.

Plus, I'd assume there are other advantages of having a large bore, short stroke engine as well.

[tubman]

Again, a clarification. Given this setup and the proper pistons (they were available back then), the piston occupies the same space in the cylinder at TDC that it does with a stock stroke. At BDC the piston is 1/8" deeper in the cylinder than stock. In my opinion, that means to me that the stroke increase occurs at the bottom of the stroke. This is surely just a matter of semantics. Look at it this way. If you have an unknown 8BA type engine and want to measure the stroke, you will get no where unless you turn a cylinder to BDC and measure to the top of the piston. Again, this is a matter of semantics and we will have to agree to disagree.

Also, I may have mis-spoke : What I meant to say is that the entire stroke increase occurs at the bottom of the cylinder. With the exception of engines with pop-up pistons, I believe this to be a true statement. I, of course realize that stroking an engine requires the coordination of the proper crank, rods, and pistons. It takes the whole system. Perhaps "stroke" was a poor choice of words for that statement.

[V8COOPMAN]

Originally Posted by Tim Ayers

This is an interesting topic.

Plus, I'd assume there are other advantages of having a large bore, short stroke engine as well.

Well, as for an engine with great breathing capabilities like a 301 Chevy (4" bore/3" stroke) for instance, it should be able to rev to the moon. DD

[Ronnie]

Can't help it. Hope not many engine builders stumble upon this thread.
They sure would have some interesting comments wouldn't they about those old fart flathead guy's.

[Mike B]

Originally Posted by tubman

Again, a clarification. Given this setup and the proper pistons (they were available back then), the piston occupies the same space in the cylinder at TDC that it does with a stock stroke. At BDC the piston is 1/8" deeper in the cylinder than stock. In my opinion, that means to me that the stroke increase occurs at the bottom of the stroke. This is surely just a matter of semantics. Look at it this way. If you have an unknown 8BA type engine and want to measure the stroke, you will get no where unless you turn a cylinder to BDC and measure to the top of the piston. Again, this is a matter of semantics and we will have to agree to disagree.

Also, I may have mis-spoke : What I meant to say is that the entire stroke increase occurs at the bottom of the cylinder. With the exception of engines with pop-up pistons, I believe this to be a true statement. I, of course realize that stroking an engine requires the coordination of the proper crank, rods, and pistons. It takes the whole system. Perhaps "stroke" was a poor choice of words for that statement.

If you still feel the increase of stroke occurs at BDC, then ask yourself "why a shorter piston/relocated pin, is needed to lower the piston head inside the bore"?

[V8COOPMAN]

Originally Posted by Mike B

If you still feel the increase of stroke occurs at BDC, then ask yourself "why a shorter piston/relocated pin, is needed to lower the piston head inside the bore"?

Touche! DD