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.

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?

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.

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.

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!

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

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.

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

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

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

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.

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

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

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

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.

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

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

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

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

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

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.

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.

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.

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

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

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).

Attachment 246874

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.

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.

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

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

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?

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.

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.

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

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.

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

Remember, he wants to have a 3 7/8" stroke crank. Thus JSeery's diagram.

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.

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?

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

Or you could de-stroke it. That has been done to meet class requirements in racing.

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

Exactly my thought. Lots of hoop jumping and machine work for what? A couple extra cubes?

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.

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.

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...

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.

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!

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.

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

Redoming tool, by Ron: here and next page of that post!
http://www.fordbarn.com/forum/showth...ghlight=cutter

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.

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

btt

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

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.