here we go again... does anyone make 3 7/8

[chuck stevens]

I have a nice set of early rods and 2or3 8BA 3 3/4" ford cranks, they need to be put to use. I have found a excellent mechine shop that knows what I'm talking about, soooo!! Now all I need is the pistons. If any of you experianced flathead guys have any ideas or different approches to this 3 7/8 stroke build please speak up, thanks Chuck S.

[JSeery]

Ok, not following the connection between 3 3/4 cranks and 3 7/8 stroke. Are you refering to an off-set grind on the throws?

[Ol' Ron]

This was common back in the 40's to stroke a ford large journal crank to the early size and use the 21A rods. Janns made pistons for these, as well as others.

[Ronnieroadster]

Check with EGGE machine they could easily do the pin offset you need on a special order what you need s not that unusual.

[Bruce Lancaster]

And I think special order will be required, since probably no has made a 1/8 stroker since the '49 Merc crank came out!

[tubman]

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I don't really like to get into this again, but when you consider what's involved, it looks like a "no-brainer" to me. The early rod journals are 1.990", while the 8BA's are 2.139". It looks to me that if the proper offset grind were performed on the crankshaft, a 3 7/8 stroker with standard Ford pistons would be a natural. Do the math.

[Walt Dupont--Me.]

Originally Posted by tubman

I don't really like to get into this again, but when you consider what's involved, it looks like a "no-brainer" to me. The early rod journals are 1.990", while the 8BA's are 2.139". It looks to me that if the proper offset grind were performed on the crankshaft, a 3 7/8 stroker with standard Ford pistons would be a natural. Do the math.

I don't get, how can you use 3-3/4 pistons with a 3-7/8 stroke with out the pistons sticking out a 1/16? and use a 7in. rod. Walt

[tubman]

Walt,

Think about it. A stock Ford piston on a small journal rod will be no higher in the bore at the top of the stroke than a stock setup. With a proper offset ground crank, it will go down 1/8" lower in the cylinder.

Tubman

[Alaska Jim]

I think you may be able to use the 255 merc. pistons, but am not sure. was always going to check into this. my brother has one of these 3 -7/8 cranks.

[Bored&Stroked]

Originally Posted by tubman

Walt,

Think about it. A stock Ford piston on a small journal rod will be no higher in the bore at the top of the stroke that a stock setup. With a proper offset ground crank, it will go down 1/8" lower in the cylinder.

Tubman

I think you're missing something . . . he wants to achieve a 3 7/8 stroke from an 8BA crank. The way that he accomplishes this is to have the rod journals offset ground to the earlier 91A/21A journal size. This offset grind will give him a stroke of 3 7/8. Now that you have that stroke and are using a standard 7" rod, you need pistons with the right compression height (pin location to the deck).

He is now half-way between using a normal 3 3/4 piston, versus a 4.00 Merc piston - needs the compression height to be in the middle of the two - which calls for custom/special pistons.

Compression Heights:

3 3/4 Stroke: 1.561 (Standard stroke - 32 to 48)
3 7/8 Stroke: 1.4985 (no standard Ford piston is available for this)
4 00 Stroke: 1.436 (Mercury 255 engine - 49 - 53)
4 1/8 Stroke: 1.374 (no standard Ford pistons is available for this)

Each of the above strokes requires that the piston pin be in the location as noted - such that the piston crown is at the correct height at the deck.

Hope this helps.

B&S

[Pete]

Originally Posted by Walt Dupont--Me.

I don't get, how can you use 3-3/4 pistons with a 3-7/8 stroke with out the pistons sticking out a 1/16? and use a 7in. rod. Walt

In the old days, most pistons as ordered, did not come to the top of the block (deck) by varying ammounts.
It was common to use 4 1/8 pistons with a 4 1/4 crank.
The edge of the piston was then flush with the deck.
The heads were cut to fit.
Today you should always specify when ordering special pistons that they come flush to the deck. You could end up shy on compression otherwise.

[Walt Dupont--Me.]

Originally Posted by tubman

Walt,

Think about it. A stock Ford piston on a small journal rod will be no higher in the bore at the top of the stroke than a stock setup. With a proper offset ground crank, it will go down 1/8" lower in the cylinder.

Tubman

Well Tubman, some one else will have to explain it, I don't care what size the journal is, it's still 3-7/8 stroke, the 3-3/4 piston is made to go the top of the bore, if you use it with a 3-7/8 stroke it's going over the top 1/16 the wrist pin hole has to be up higher in the piston. JWL, jump in. Walt

[oldford2]

Okay,
So Chuck asked a question about 3 3/4, 3 7/8 stroke. Before we all jump in he needs to clarify if it is 3 7/8 stroke and how he obtained it

[Walt Dupont--Me.]

Originally Posted by oldford2

Okay,
So Chuck asked a question about 3 3/4, 3 7/8 stroke. Before we all jump in he needs to clarify if it is 3 7/8 stroke and how he obtained it

Well, Chuck says he like to go 3-7/8 stroke, he asked what to do about pistons, I guess he could use the 3-3/4 pistons but there going to go up higher, maybe he could have the heads domed to get some clearance. Walt

[chuck stevens]

Thanks everyone, I was afraid this would open a can of worms. Walt has an idea, use 3 3/4 pistons and trim the domed area of the heads to get some clearance. This wouldn't effect the compression, and might help flow. Now I'm wondering how much material is in the head. This would work as long as the top ring is not too close to the top of the piston hummmm.
I'm trying to use up some of my old flathead stuff, thanks again, Chuck S.

[JWL]

I think what might be causing confusion, on the part of some people, is that the diameter of the crank pin has NOTHING to do with the length of stroke. If the bearings are assembled to properly fit the shaft diameters the actual stroke length is controlled by the centerline distance of the main journal to rod journal. "OFFSET" grinding moves the centerline of the rod pin away from the main line requiring shorter compression height pistons to avoid the pistons extending above the block deck.

However, Pete makes a good point about most piston sets not reaching the deck. Some pistons were actually de-compressioned, when made in bore oversizes, so the original compression ratio could be maintained. Matching the head dome to a piston which extends slightly above deck would be a good combination in most cases.

[JSeery]

The top ring is down quit a bit on most pistons, this is what allows the block to be relived. Happen to have a piston laying on my computer desk and the top edge of the piston to the top compressing ring upper groove is ~.20 inches.

[Walt Dupont--Me.]

JWL, your a life saver, I learned to be an auto machines all my life but never learned to talk or spell, it just stuck in my head never learned how to get out. Walt

[Bored&Stroked]

Originally Posted by chuck stevens

Thanks everyone, I was afraid this would open a can of worms. Walt has an idea, use 3 3/4 pistons and trim the domed area of the heads to get some clearance. This wouldn't effect the compression, and might help flow. Now I'm wondering how much material is in the head. This would work as long as the top ring is not too close to the top of the piston hummmm.
I'm trying to use up some of my old flathead stuff, thanks again, Chuck S.

You're only talking the removal of less than 1/16 of material - so unless your heads have been reworked before, don't see an issue. The added benefit is that you could "tune" the squish area over the piston to get exactly what you need for best performance (which is about .040).

Most piston/gasket/head combinations (out of the box) have a lot more squish than is needed. I have a 3D CAD model that I made to tune my dome height (using a CNC machine center and a 1/4" ball mill to machine the contour).

Flathead3DModel.jpg

Once I have the crank and a rod and piston in the block (to accurately measure from), then I add my compressed head-gasket thickness (usually about .050) and I can then determine how much clearance I need above the piston. I use these numbers in my 3D CAD model to setup the dome location and have the domes cut to exactly match what I need.

Also, you can use 'clay' in the head domes to figure out the dome shape - in many cases the head doesn't match the pistons anyway.

[Ol' Ron]

When I built the Bville engine I used a 3 3/4 stroke piston on a 4" crank. The Heads had been domed by Grancor for this. Had very good flow with 9.2 CR. I used .060" piston to head clearance to protect against rod stretch at Hi revs.