3 7/8 Stroker

[tubman]

I didn't really want to get into this, but we have got to quit hi-jacking the OP's thread. To you doubters; take a good look at the image Ronnie posted. I think it says it all.

[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.

[40cpe]

You can't believe everything you read

[Ol' Ron]

The 4" crank is done the same way to get 4 1/8. This was done back in the 40's befor the Merc crank came on board,

[flatjack9]

Originally Posted by tubman

I didn't really want to get into this, but we have got to quit hi-jacking the OP's thread. To you doubters; take a good look at the image Ronnie posted. I think it says it all.

I think it says that you still don't get it. What you don't understand is that the stroker uses a 2.139" journal as opposed to a 1.99" journal. This moves the outer edge of the throw further away from the centerline of the crank. The position of the top of the journal at TDC is not in the same place for each crank. The 2.139 is about .070 closer to the top of the block. Draw a 1.99" journal in that drawing with the same centerline as the 2.139" journal and maybe you will see the difference.

[JSeery]

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Originally Posted by Ol' Ron

The 4" crank is done the same way to get 4 1/8. This was done back in the 40's befor the Merc crank came on board,

So were there pistons on the market for this setup?

[flatjack9]

Yes, I believe I have a set.

[JSeery]

Originally Posted by flatjack9

Yes, I believe I have a set.

Very interesting learned something new today!!! Thanks flatjack

[flatjack9]

This was a popular modification before the introduction of the 4" Merc crank.

[scooder]

The key here tubman is the center of the Rod throw is (in an 1/8" stoked crank) 1/16" farther away from the center of the main bearing journal. So the stroke is now increased by 1/8". 1/16" farther down the bore, and ends up 1/16" farther up.
The 4" Merc crank has its center 1/8" farther from the main bearing center to give us a total stroke increase of 1/4".
Get it?
If you put Ford pistons on a Merc crank, the piston pops up out of the block by 1/8". If you put stock stroke Ford pistons on a 3 7/8" stroke crank, the piston will pop up 1/16". These popup numbers are assuming that the stock pistons with a stock crank Will have the crown edge dead level with the deck face. Those that have played with these engines know that's a bad assumption, but it used here for clarity.
The same applies if you stroke a Merc 4" crank to 4 1/8".
Hope this makes sence. I know it falls into the same "how's that then?" area as grinding material off a cam lobe to make the valve lift higher.
Martin.

[Fordestes]

Speed o Motive used to sell the kit. What about Egge for the pistons?

[flatjack9]

Speed O Motive made a lot of stroked cranks. I have one of their 4 1/2" cranks. Obviously they had to weld up the journals to get that kind of stroke.

[Fordestes]

Speed o Motive were the go to people when I had dark hair.
fordestes,

[Ol' Ron]

We used Jahns pistons, very popular and inexpensive back in the 40's and 50's, but kinda heavy causing rod stretch, which also caused pistons to hit heads. Needed allot of piston clearance back then.

[tubman]

Originally Posted by scooder

The key here tubman is the center of the Rod throw is (in an 1/8" stoked crank) 1/16" farther away from the center of the main bearing journal. So the stroke is now increased by 1/8". 1/16" farther down the bore, and ends up 1/16" farther up.
.

I quite agree with you, but what is important is not the where the CENTER of the throw is, but the distance from the TOP of the throw to the deck at TDC. When you move the center of the journal down (at BDC) 1/16" the entire stroke increase occurs at the bottom of the stroke.

To all of you guys : I only wanted to confirm that a 3 7/8 stroke flathead is possible, and probably the only cheap way to get any kind of a stroke increase before 1949. I have never built one, but the posts by Ronnie and Ole' Ron seem to confirm my position. I never said anything about pistons, just reground late cranks and early rods. Stock pistons probably could have been used by trimming .030" off the top. We used to mill our heads more than that to get more compression, so maybe the .030 wasn't even required. Who knows? Like I said, I never built one. However, if any of you want to send me set of good early rods and $1000, I'd like to try. (I have the required late parts.)

At least we got this away from the original thread. I sure hope he finds the solution to his problem!

[flatjack9]

I think you might have failed your geometry class. If the stroke increases 1/16" at the bottom, it will be 1/16th greater at the top. Can't be anything else.

[Ronnie]

Does this seem easier to understand? The distance between center lines changes. You could set up to also DE stroke the same amount.

R

[tubman]

Originally Posted by flatjack9

I think you might have failed your geometry class. If the stroke increases 1/16" at the bottom, it will be 1/16th greater at the top. Can't be anything else.

I did very well in all of my math classes, all the way through graduate school, thank you. You have to understand that the rod journal is OFFSET ground, not concentrically. This causes the stroke increase. Take a good hard look at Ronnie's fine diagram (above). You can obviously see that the outside surface of the rod journal is the same distance from the crank C/L as before the regrind, while the bottom surface is 1/16" further away from the from the crank C/L.

What I am saying is that at TDC the top of the journal is the same distance down from the deck as stock. At BDC, the distance from the upper surface of the journal to the deck is 1/8" more than stock.

[JSeery]

Rod length is determined from center to center on the rod ends and stroke is determined by center of the crank throw to the block cylinder head surface at TDC and BDC. The distance you are referring to is really not relevant to anything.

[tubman]

Originally Posted by JSeery

Rod length is determined from center to center on the rod ends and stroke is determined by center of the crank throw to the block cylinder head surface at TDC and BDC. The distance you are referring to is really not relevant to anything.

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.