3.75 versus 4 inch crank

[Krylon32]

I'm just starting a new French FH motor and it came with the French 3.75 rotating assembly. I really don't want to bore it but but is it worth the effort to add a 4 inch crank to increase the displacement from 239 to 255? I'm thinking Isky Max 1 with the usual internals. Also does the French block have hard valve seats? I get mixed responses.

[KiWinUS]

Put a 4.25 crank in it since you have to buy a crank anyway. You won’t regret it. Your obviously go to spend some Jack so get the best bang for you buck.

[deuce lover]

French blocks that I have seen and know of do not have hardened valve seats.

[deuce_roadster]

Yeah, what KiWinUS said. The crank/pistons are the most cost effective bang for the buck you can get. I have never had a 4.25 crank. Do the rods miss everything or do you have to relieve places on the block?
I just put a NOS 59 series engine in my 40 coupe, swapped out the 3.75 crank for a 4 inch one and metric ring Ross pistons. Did not bore the block, using the stock cast iron heads Has PLENTY of power and will certainly outlive me. An all out race engine isn't the same as what you want to drive on the street. I doubt you could tell much difference if I bored some of the life out of the block.

[19Fordy]

Gary, Keep what you have using that French crank and build yourself a nice dependable cool running flatty using the Max 1 cam. I don't think that you will notice any big difference in performance by going to 255 cubes alone. One change mandates another change. I bet you could build a nice "snappy" 239 without using the stock rotating assembly, assuming it's in good condition.

[Krylon32]

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19Fordy: The rotating assembly is all new, only thing I don't know yet is the quality of the French pistons, I did notice in the pictures the seller sent me they included Perfect Circle rings. Deuce Roadster: Any arm longer than 4 inches requires relieving the block for clearance according to the Tardel book.

[tubman]

I don't know what this prejudice against boring is all about. An 1/8" overbore will give you 258 ci (3 MORE than a Merc crank with a stock bore) and lower piston speed (which is always good). The pistons are about the same price as a set of stock pistons, which is a helluva lot less than a Merc crank and any required preparation. I have never done a French block, but from everything I hear and read, they are better than a garden variety 8BA, which will take a couple of overbores past 3 5/16" easily. Plus, you can always do a hone or two in between.

How many re-rings are going to have to be done to a carefully assembled, meticulously maintained engine that gets a couple of thousand miles a year on average? By the time the bores are worn out in a couple of hundred years, the guy currently owning it will probably be able to whip up a new block on his home 3D printer in 15 minutes.

Every time I see a post about "saving the engine for a future rebore", I have to wonder. Maybe for some of the 3 3/8+.030 or 3 7/16" (yep, some guys did it) bore engines out there, but for a stock bore? I don't see it.

[deuce_roadster]

Tubman, To each his own, if you like taking it out to the max, go for it. My old Ford cars the way I drive them (mainly on tours) would get ZERO benefit of boring out the block. I have plenty of power now with replaceable parts.
If I want to be pushed back in the seat, My 55 brand X with built 327, tremec 5 speed and 9" rear end will satisfy any of those urges.

[JSeery]

On the bores, if you don't go to wild you can always sleeve it in the far, far future if required. So, I'm in the category of why not go to the larger overbore? It will provide you more low-end torque and allow you to run higher rear end gears. Seems like a win-win to me.

[Mart]

The French motors I have have valve seat inserts. I don't know if by saying HARDENED valve seats you are making some sort of differentiation, but the ones I have do have valve seat inserts.

I'd say they are as hard or harder to cut as the ones in the 99 Merc block I'm currently working on.

Mart.

[Ggmac]

I agree with tubman . Unless you plan on racing or driving an extremely heavy car or one with accessories, AC , ps , big alternator , etc stay stock and reliable . But that's me . If I want horsepower I'll use a overhead valve or cam motor . Maybe something as new as a 394 olds .

[JSeery]

Originally Posted by Ggmac

I agree with tubman.

Think you missed on that one! Tubman is in the overbore it camp.

[deuce lover]

Originally Posted by Mart

The French motors I have have valve seat inserts. I don't know if by saying HARDENED valve seats you are making some sort of differentiation, but the ones I have do have valve seat inserts.

I'd say they are as hard or harder to cut as the ones in the 99 Merc block I'm currently working on.

Mart.

Mart, Is your motor like this one?
https://www.fordbarn.com/forum/showt...rench+Flathead


I was talking about the later Military ones(like this ) with the strange oil port castings on the back. They don't have removable seat inserts,at least the ones I have seen.

[tubman]

Gentlemen - You have to realize the intent of the O/P, which is : "is it worth the effort to add a 4 inch crank to increase the displacement from 239 to 255?" I am just pointing out that if you're looking to increase your displacement, boring the engine out is a more than reasonable alternative to using a 4" Mercury crankshaft. Not only do you get 3 more cubic inches and less piston speed, it is MUCH less expensive. You could also update to better pistons with the metric rings everyone is so excited about. The only possible downside on increasing the displacement in this way is compromising the future rebuildibility of the engine. It is my personal opinion, that by going to 3 5/16" , that compromise is so small as to be almost nil. You can disagree with that if you want, but there are so many 3 5/16" and up bore engines out there that I am completely comfortable with my position.

With all due respect "deuce_roadster", the O/P came here asking about an increase in displacement, not whether a stock displacement engine provides adequate performance on tours. I went from a rather tired 239 in my '51, which provided "adequate performance" to a warmed over '51 Mercury; while it wasn't necessary, I am much happier with the car now. One last comment : I do not consider a 1/8" overbore in a 239" block "taking it to the max".

[Mart]

Sorry, deuce, (not calling you out on this) but my evidence differs. My roadster definitely has them because I struggled to cut them. The one I can access in my shed has them (I just went outside and looked). I have a third one but can't get to that today. Maybe there is a cut off year for inserts. I know the motor in my roadster has a 1964 cast date on it. I think, but would have to check, the other 2 are later.

They have the extra casting features to accommodate the governor at the rear, but not the big square boss like on the (halibrand?) ones.

They are the 4" motors from the SUMB military trucks.

mart.

[Mart]

My roadster goes really well (check the video).

It has a 3-5/16 bore and a 4" crank, twin Holley 2100s and a Schneider cam.

Mart.

[Bored&Stroked]

Asking any general flathead question about "what is best", "what is the max", etc - you'll get more answers than you know whats! LOL

I've built them in many different sizes - including bone stock. I'm currently gathering all the pieces to build a 307 cubic inch race style engine (injected) - but might go up to a 320 or so.

It all comes down to what do you really want and how much time/money are you going to put into it? Also, if you've "always wanted" a high-performance, higher horsepower flathead - is it not the time to build one? How many MORE are you going to build after this one? And yes, the bigger you go, the more expensive it will be . . . just like any other engine platform. But the ole' saying "there is no replacement for displacement" isn't really that far off the mark.

Here are a few of my opinions - consider it going to the 'Fortune Teller' - keep what you want, throw the rest out . . .

1) Great Street Combination - 276 cubic inches: My favorite street combination for reasonable dollars is a 4" stroke crankshaft and a 3 5/16 bore - with Ross pistons and metric rings - using stock 49-53 rods and insert bearings. Also, I'd probably run a Potvin 3/8 cam in it. Others might like the L100 or maybe the Isky 1007B grind. At 276 cubic inches it is a great combination, easy to do and been done thousands of times.

My favorite manifold for this (currently available) is the Edelbrock Slingshot - with a couple 97's or even the Holley 2100 or 2110s. Also, you can run the generator/fan in the stock position on an early Ford - sure makes things easy and nice . . .

Here is the exact combo - blurry video, but you'll get the sound. Exact combination listed above:

https://www.youtube.com/watch?v=9akeT4Ua9lU

2) Bigger displacement - 284 cubic inches: In the old days I'd offset grind a 4" Merc crank to 4 1/8" stroke, put some 21A rods in it with full-floater bearings and go to 284 cubic inches. This is another great combination - using the same bore. This requires no special block relieving for the rods . . . easy peasy. This is what I have in my 32 Cabriolet now - but my particular engine is really a race-style engine --> huge ports, big roller cam, big valves, etc. - but yah know what, it is pretty damn FUN - and it sounds like a fricking BEAST! I built it to experiment with some ideas . . . and I love it.

You can run the same manifold/carb setup as up above, though I'd run the bigger Holley 2110's - maybe even try the Big97's - and port the manifold. Mine currently has only two Stromberg 48's - is quite under-carbed at the moment:

https://www.youtube.com/watch?v=Ehu5bnSqamo

3) Getting a bit Bigger - 293 cubic inches: If you have to buy a crankshaft and rods, then go with a 4.250 stroke crank, a set of H-Beam rods and the same 3 5/16 bore. You'll still have the great metric rings and you'll now have 293 cubic inch motor. Depending on the weight of the car, the rear end ratio, etc. - you could run a Potvin 3/8 or maybe step up to a Potvin 425 cam.

Once you get to these larger cubic inches, you're going to really need either 3 carbs, or maybe two Big97's and a ported manifold.

Valves and Port Work: I've not started with a French block, but I hear they do need a lot of porting work on the intake side - especially in the bowls. It makes sense to do this work and potentially go to bigger valves if you want to spend a few more bucks. I like to run 1.72" intakes (with a lot of port/relief work) - but it does get a lot more expensive once you have to start pulling valve seats out, putting bigger ones in, etc..

There is nothing wrong with even a stock 239 as far as moving a light hotrod down the road. My 32 cabriolet had a 59AB in it for the first two years I drove it - bone stock except for the magneto I threw on it. It had a lot more getup and go than one would imagine . . . just a bit too boring for me. I like a snotty flathead, one that makes a visual as well as a hotrod sound statement. Works for me every time . . . and for every guy who I've built one for - the smile factor is always there. Again - if you're wanting to be a bit conservative, go for the 276 incher . . . if you want to build a bigger one, then get a bit bigger wallet out and go on up . . .

[deuce_roadster]

I don't feel picked on or anything. The 2 engines I have chosen not to take out to 3 5/16 or more were NOS BRAND NEW, not French blocks, 59 series Ford blocks.. I have bored out tired engines with massive taper, I have nothing against that. I sold my French block and never got around to building it. But as I said before everyone should do what they want with their own stuff. I am only stating what I felt would give me what I wanted and perhaps might for him.

To give the OP more valid advice one would need to know what kind of use this engine is to be used for, what kind of vehicle (car, pickup), trans, gearing (drive on freeway or not) etc. Fortunately he has a new block and I only offered an opinion of what was right for me with my new blocks.


I would also say that on a used block, rust and core shift should determine what the Max bore is for that particular block, not some arbitrary number. This is why sonic testing is so important in a thorough evaluation of a used block. I won't leave less then .100 wall thickness at the thinnest spot on a bore.

[flatjack9]

I would personally prefer the 4" crank. Longer stroke, more torque. More torque, more driveable. Horsepower is relevantly less important for a street engine than torque.

[Ol' Ron]

like the 258ci engines, with Egge 3 ring pistons. Algle mill the heads, use an EAB cam and you have a very inexpensive engine that will outperform any of the above for the first 100 feet., and ge t 20 MPG 2gC carb and a chevy dist. Now if you worried about rebuilding it, Egge has .020/.030 oversize pistons, That;s what is in my 280