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Old 11-18-2015, 10:26 AM   #1
chuck stevens
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Default here we go again... does anyone make 3 7/8

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.
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Old 11-18-2015, 12:38 PM   #2
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Default Re: here we go again... does anyone make 3 7/8

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?
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Old 11-18-2015, 03:24 PM   #3
Ol' Ron
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Default Re: here we go again... does anyone make 3 7/8

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.
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Old 11-18-2015, 05:47 PM   #4
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Default Re: here we go again... does anyone make 3 7/8

Check with EGGE machine they could easily do the pin offset you need on a special order what you need s not that unusual.
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Old 11-18-2015, 06:26 PM   #5
Bruce Lancaster
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Default Re: here we go again... does anyone make 3 7/8

And I think special order will be required, since probably no has made a 1/8 stroker since the '49 Merc crank came out!
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Old 11-18-2015, 07:11 PM   #6
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Default Re: here we go again... does anyone make 3 7/8

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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.
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Old 11-18-2015, 07:59 PM   #7
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Default Re: here we go again... does anyone make 3 7/8

Quote:
Originally Posted by tubman View Post
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
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Old 11-18-2015, 08:04 PM   #8
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Default Re: here we go again... does anyone make 3 7/8

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

Last edited by tubman; 11-18-2015 at 08:28 PM.
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Old 11-18-2015, 08:24 PM   #9
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Default Re: here we go again... does anyone make 3 7/8

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.
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Old 11-18-2015, 08:38 PM   #10
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Default Re: here we go again... does anyone make 3 7/8

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Originally Posted by tubman View Post
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
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Old 11-18-2015, 08:42 PM   #11
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Default Re: here we go again... does anyone make 3 7/8

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Originally Posted by Walt Dupont--Me. View Post
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.
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Old 11-18-2015, 08:48 PM   #12
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Default Re: here we go again... does anyone make 3 7/8

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Originally Posted by tubman View Post
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
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Old 11-18-2015, 09:02 PM   #13
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Default Re: here we go again... does anyone make 3 7/8

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
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Old 11-18-2015, 09:57 PM   #14
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Default Re: here we go again... does anyone make 3 7/8

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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
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Old 11-19-2015, 07:05 AM   #15
chuck stevens
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Default Re: here we go again... does anyone make 3 7/8

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.
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Old 11-19-2015, 07:12 AM   #16
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Default Re: here we go again... does anyone make 3 7/8

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.
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Old 11-19-2015, 07:19 AM   #17
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Default Re: here we go again... does anyone make 3 7/8

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.
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Old 11-19-2015, 07:24 AM   #18
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Default Re: here we go again... does anyone make 3 7/8

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
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Old 11-19-2015, 08:14 AM   #19
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Default Re: here we go again... does anyone make 3 7/8

Quote:
Originally Posted by chuck stevens View Post
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.

Last edited by Bored&Stroked; 11-19-2015 at 08:42 AM.
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Old 11-19-2015, 08:52 AM   #20
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Default Re: here we go again... does anyone make 3 7/8

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.
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Old 11-19-2015, 09:00 AM   #21
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Default Re: here we go again... does anyone make 3 7/8

Bored and Stroked does with cad what I attempt with laborious hand die grinding. The
method I use takes, on average,6 hours per head but exactly matches the comb
chamber to the piston crown. His statement is so correct.....'in many cases the head
head doesn't match the piston anyway'. After milling the heads I actually apply a thin
coating of grease to the piston crowns and valve heads then rotate the motor by
hand. The points of contact are readily apparent and are then carefully relieved.
Each head is hung on 4 studs to insure repeatability. The optimum .040 clearance
is sort of built in when the head gasket is in place at time of assembly.
Charlie ny
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Old 11-19-2015, 09:07 AM   #22
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Default Re: here we go again... does anyone make 3 7/8

Quote:
Originally Posted by Ol' Ron View Post
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.
Probably a good idea to have some 'wiggle room' for rod stretch in a high rpm race situation. What type of rod were/are you using?
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Old 11-19-2015, 09:26 AM   #23
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Default Re: here we go again... does anyone make 3 7/8

I'm going to make one last stab at trying to get you guys to understand me.

"JWL" said "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".

This does not have to be so. Depending on the side of the journal on which the "OFFSET" is ground, it can move the centerline of the rod pin either farther away from the crankshaft centerline or closer to it. The amount to be removed from the 8BA crankshaft to use the earlier rods is .140. Half of this is .070. Since a 1/8" stroke results in a .0625 increase in stroke, which is less than this amount, "OFFSET" grinding the crank and biasing the "OFFSET" to the side of the rod pin away from the crankshaft centerline will accommodate this increase in stroke. I imagine one could obtain optimal piston crown to combustion chamber clearance using this method after some careful measuring.
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Old 11-19-2015, 09:34 AM   #24
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Default Re: here we go again... does anyone make 3 7/8

"This does not have to be so."

Sorry, yes it does! An increase in stroke will increase the distance between the centerline of the piston pin and the centerline of the crank throw (and crank centerline) by definition. To maintain the same piston position you have to move the piston pin location or use shorter rods.
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Last edited by JSeery; 11-19-2015 at 09:43 AM.
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Old 11-19-2015, 09:46 AM   #25
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Default Re: here we go again... does anyone make 3 7/8

Quote:
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"This does not have to be so."

Sorry, yes it does! An increase in stroke will increase the distance between the centerline of the piston pin and the centerline of the crank throw (and crank centerline) by definition.
Sorry to disagree with you, but do your diagram over taking the offset off the other side of the pin and see where the rod pin centerline goes.
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Old 11-19-2015, 09:54 AM   #26
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Default Re: here we go again... does anyone make 3 7/8

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Remember, he wants to have a 3 7/8" stroke crank. Thus JSeery's diagram.
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Old 11-19-2015, 09:57 AM   #27
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Default Re: here we go again... does anyone make 3 7/8

Well, sorry tubman, not sure where your confusion is, but this is a well established process, don't think everyone is wrong on this one. Sure you can de-stroke the crank by offset grinding toward the crank centerline, but this will decrease the overall stroke. The objective was to increase the stroke.
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Old 11-19-2015, 10:05 AM   #28
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Default Re: here we go again... does anyone make 3 7/8

OK, I finally see it. While what I am suggesting would work fine, what I'd end up with is a small journal 8BA crank with a 3 3/4 stroke. Doable, but why bother?
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Old 11-19-2015, 10:07 AM   #29
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Default Re: here we go again... does anyone make 3 7/8

Quote:
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OK, I finally see it. While what I am suggesting would work fine, what I'd end up with is a small journal 8BA crank with a 3 3/4 stroke. Doable, but why bother?
Or you could de-stroke it. That has been done to meet class requirements in racing.
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Old 11-19-2015, 10:08 AM   #30
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Default Re: here we go again... does anyone make 3 7/8

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OK, I finally see it. While what I am suggesting would work fine, what I'd end up with is a small journal 8BA crank with a 3 3/4 stroke. Doable, but why bother?
Exactly my thought. Lots of hoop jumping and machine work for what? A couple extra cubes?
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Old 11-19-2015, 10:09 AM   #31
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Default Re: here we go again... does anyone make 3 7/8

In case you missed it, the subject was 3 7/8 stroke. My comments were aligned with the original subject. Now, if we just want to argue about something let's discuss the wonderful job our president and congress are doing. But, not on this site. There is plenty of extraneous stuff already.
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Old 11-19-2015, 10:29 AM   #32
chuck stevens
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Default Re: here we go again... does anyone make 3 7/8

Bored & stroked, and Ol Ron, this has me pretty excided. The standing rule for flatheads has been- anything you do to increase compression hurts flow, and vise-a-versa. By take some off the head it should help flow in the transfer area. Thanks Chuck S.
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Old 11-19-2015, 12:27 PM   #33
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Default Re: here we go again... does anyone make 3 7/8

The 3 7/8 stroker mod was largely obsoleted when the new Merc crank in '49 allowed a cheap over the counter 4", and then 4 1/8 with the same labor as making the 3 7/8...
but stock car racer types with stroke limits frequently cut the big journal cranks concentrically (no change on stroke) in order to slightly cut friction, add a bit of safety, and lower weight of moving parts by using the small journal floaters. Just another old-timey mod trying to get the most with stock Ford parts and restrictive rules...
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Old 11-19-2015, 01:03 PM   #34
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Default Re: here we go again... does anyone make 3 7/8

What kind of tooling is needed to re-dome the head to match the piston dome? I have access to a mill but very little machining experience. Are there companies that make custom tooling? Or is there something available off the shelf? CNC and CAD are out of my reach-need old school.
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Old 11-19-2015, 01:31 PM   #35
Bruce Lancaster
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Default Re: here we go again... does anyone make 3 7/8

Search dome or doming or maybe fly-cutter under Ol' Ron's name...he posted how to make the cutter!
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Old 11-19-2015, 02:16 PM   #36
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Default Re: here we go again... does anyone make 3 7/8

Chuck,
If you do a search on here the 3 7/8 piston question has come up before. I stated in that thread that using stock Ford 3 3/4 pistons and stock Ford uncut heads, as in OEM, will work. From the combinations I've measured, it'll just bolt together. I've dry assembled a 99A block, 8CM crank (4"), OEM 3 3/4 pistons, stock head gasket and uncut 81A heads. The pistons literally just kissed the heads. Now remove the 4" crank, swap in a 3 7/8" crank now you've got 1/16" clearance over the pistons.
The dome on the pistons I had and the dome in the 81A heads matched perfectly. These heads could also stand cutting 0.010" to give 0.050" clearance without cutting into the dome. The dome on these here heads is kinds counter bored into the head, so a 0.010" cut won't actually contact the dome.This would give you a nice uniform quench area, nice. This combination obviously ups the compression ratio aswell, as the combustion chamber now has another 1/16" of piston Sat in it. I think this would be a lovely combination. Higher compression with stock heads with there relatively deep transfer area.
Martin.
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Old 11-19-2015, 03:14 PM   #37
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Default Re: here we go again... does anyone make 3 7/8

Redoming tool, by Ron: here and next page of that post!
https://www.fordbarn.com/forum/showth...ghlight=cutter
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Old 11-19-2015, 03:20 PM   #38
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Default Re: here we go again... does anyone make 3 7/8

I used 29A Rods with floating bearings on a 4" crank. I like using EAB heads on big inch engines. The give all the CR you need for the street, In my case the 280 in my truck has EAB heads Milled .040", which is a ttad too much, but as Charley NY said alittle work with a die grinder gives the deaired clearance. However I also increase the angle of the exit of the transfer area to improve flow. You don't have to remove much materal just brake the edge. My compression measures 8.2. Engine also has an EAB cam with valve rotators, 2GC carb and a distributor by Richard.
I just got the truck regestered yeaterday and have 3 miles on it. now I have to get it inspected.
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Old 11-19-2015, 08:24 PM   #39
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Default Re: here we go again... does anyone make 3 7/8

btt
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Old 11-20-2015, 02:34 AM   #40
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I think I've said it before, incase I haven't, that sounds like a real nice street engine Ron.
Good luck with the inspection, looking forward to some real street use feedback.
Martin.
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Old 11-20-2015, 06:05 AM   #41
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Default Re: here we go again... does anyone make 3 7/8

Here is one Jim Khougaz did in 1956 at CT-Automotive.
I started a thread here about pistons and was advised to use stock Ford pin height.
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Old 11-20-2015, 06:51 AM   #42
chuck stevens
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Default Re: here we go again... does anyone make 3 7/8

Jim I was 6 years old when this crank was turned, proof that almost anything you can think up has already been done, but it's still fun, thanks Chuck S.


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Originally Posted by OHV DeLuxe View Post
Here is one Jim Khougaz did in 1956 at CT-Automotive.
I started a thread here about pistons and was advised to use stock Ford pin height.
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Old 11-20-2015, 08:47 AM   #43
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Default Re: here we go again... does anyone make 3 7/8

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Originally Posted by GreenMonster48 View Post
Exactly my thought. Lots of hoop jumping and machine work for what? A couple extra cubes?
Actually no extra cubes - same stroke it started out with, just smaller journals and early rods.
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Old 11-20-2015, 01:31 PM   #44
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Default Re: here we go again... does anyone make 3 7/8

Gentlemen, thanks for your attentn to the Ford Barn, really helpful and enjoyable for guys that dont get out much.The 3 7/8 cranks seems like a fantastic archeological exercise to me! No offense to anyone's age! From my limited understanding, there was a window between '46-48 where the cranks had stabilized at the common, later, main bearing size (2.4990) with the larger rod journals (2.1390) but still only 3 3/4" stroke. The 1/16 offset grind and the small journal rods would give a theoretical displacement of +/- 247c.i. were pistons made with the pin relocated closer to the pistons" top". Main and rod bearings would be stock parts too. Blocks as far back as '42 seem to accept this rotating assembly. Correct so far? So for a few years did engine builders use pop up heads, custom made pistons, or both and who made what? Probably a small h.p. difference but torque you could feel in the pants? We're cranks stroked by really ingenious racers before the large journal version was available using really thick bearings in the early rods? In one discussion i had, using stock pistons, early rods and the offset crank for the displacement increase was possible but the pin location required pop up heads and the top ring could exit the deck surface in some cases. Apparently not? In another discussion, it was theorized that Ford was about to make the bearing changes but the war postponed it until Mercs could be sold for more money and make up the cost of changeover. I like the idea that the engineers would give a leg up to racers who chose Ford running gear, with more wins adding to the reputation of the brand. I have been intrigued by the idea of building an engine like this as I am cursed with an over active imagination, strong appreciation of nostalgia, and almost no sense of practicality. Any elaboration would be most interesting to myself, and I'm sure others. I don't want to call anyone "old" around here, but perhaps some experienced builders can add some clarity to this concept, having "been there, done that". Thanks! -Chris
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Old 11-20-2015, 04:22 PM   #45
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Default Re: here we go again... does anyone make 3 7/8

Easy interchange would run '39-48, with the later cranks easily transplanted into those with either floater or locked rod setups. Prewar engines need the later seal parts for later cranks.
All the '39-53 cranks will go into the same mains, 221 cranks are the small journal type for 91A and 21A rods, 239 and 255 cranks have the large rod journals.
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Old 11-20-2015, 07:39 PM   #46
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So that crank has been stored in that box since 1956 and being a small journal crank it must be warped and unusable.
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Old 11-20-2015, 08:20 PM   #47
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Default Re: here we go again... does anyone make 3 7/8

Chris,
There's a lot of Ford pistons out there with 5/16" from the crown edge to top of first ring. These won't reach the top with an 1/8" stroke. Also all the OEM Ford pistons have the crown edge quite a bit lower than the deck. These both make the use of them with the 1/8" stroke ideal.
Martin.
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