Lets Talk Flathead Modifications

Lets Talk Flathead Modifications (Page 1)

Ive copied the last few entries in the Ball&Ball Woes discussion and started a new one. Flathead inline information is hard to come by and flow of information whether it comes from the briggs 5 hp racer or the Hudson 7x builder is valuable stuff.

The ohv builders take alot for granted that just isnt a possibility with the flatties. On the other hand, flathead inlines possess some traits that that are seldom exploited or refined to compete with the ohvs

The discussion had started as follows:


Sounds like you have a heavily modified flathead. Would you care to share some of the details and performance with us?

Active Inliner posted 08-21-2001 05:11 PM


First, dont anybody jump out and bore your B&B carbs out to those dimentions. The step up jet is wayyyyy too big. Even though the tractor would run up and down the road like top fuel dragster, its too much fuel under the type of load a tractor pull inflicts on an engine. I buzzed down the track until the load popped up the step up jet – then billowing black soot rolled out, and the rpms stopped going up. I backed out to let the step up seat and hopefully gain my vacuum back, this may work on a drag strip – it wont on a pulling track. The main jet wasnt enough and the step up jet was too much. Ideally, the step up jet would pick up the load and additional fuel needed to sustain the engine. Mine is simply too rich.

Luckily, a new step up jet for these carbs comes with an overhaul kit from The Carburetor Shop in Eldon Missouri.

Now, as for modifications, nothing real outlandish has been done to the engine. It was noticed that is full potential should be gained at 5000 rpm, but the crank didnt seem to want to stand that.

The crank was fully cross drilled from the mains to each adjacent rod and from rod to rod between mains. An oil groove was cut in the main & rod bearings to allow continuous oil flow to the crank at all points. We are running stock stroke and had the entire rotating assembly balanced out to run 6000 rpm.

Another important modification to get this rpm is to plug all the oil bleeds from the rods. Depending on what type of chrysler engine it is, some have holes in the yoke to spray the cam, others have in addition ports drilled through the rod beams to lubricate the wrist pins and spray the piston head. Be sure and plug these up to conserve all the oil and associated cooling in the rod bearing. Almost all the engines we tore down had some degree of damage to 2 and 5 rods, attributed to poor oil distribution and the excessive bleeding of oil from the adjacent rod nearest the source oil port.

The rest is pretty vanilla in terms of hardware. 58 head cut .090, stock valve diameters, homemade split exhaust, and a custom grind on the cam 270 duration seat to seat with a .425 lift and 110 separation angle and centerline.

The valves were left stock because of the shrouding that would occur if they got nearer to the head relief at full lift. We could have ground this shrouding away for better flow, but that costs CCs in a situation that doesnt allow many compression boosts. Perhaps if we stroke it, these CCs can be made up with cylinder displacement. We run 9.5 compression and can burn 93 octane pump gas.

Thats about it, nothing very elaborate – but we can turn some wicked sustained rpms. Two years of some very hard, winding service without a hitch. We needed some more induction, just need to get it harnessed!

Active Inliner posted 08-21-2001 08:24 PM


Hey Hudson, would one cylinder be considered in-line?

Ive had a lot of experiance with flathead one cylinder engines. We had a lot of restrictions on our rules so we had to get inovative.

One thing to improve breathing without disturbing the combustion chamber is to move the head off center towards the valves. By either necking down the head bolts or sloting the head bolt holes in the head. The more tne better, but in the Briggs, in order to still have enough gasket surface 0.070 to 0.080 was all we could go and it made a significant improvement.

Another thing we did was to reduce the head diameter on the valves, thus putting the margin near the edge of the valve. This seemed to work well also to increase flow.

These two things helped us get suspended from racing and asked to never return.

Active Inliner posted 08-21-2001 10:01 PM


I hear you Frank, and the briggs is the defining flathead to me. I dont race them, but one has to admire their tenacity and durability! Not to mention their outright punch for such a little package.

You are exactly right about the valve arrangement and scooting the head over on the briggs. The 58 head already has that licked over the earlier heads for the little chryslers. The valve pockets are just enough around the stock valve diameters to flow good, but close enough for a good compression. Instead of the long air port over the top of the piston, it has a deeper, shorter air pocket that saves compression but flows better. Sometimes bigger isnt always better, especially when you consider the limitations of the simesed intake runners.

Our valves are setup just like yours in the briggs, barely grabbing the edge of a 3 angle job with plenty of meat in the valve head to keep the edge from rolling up. It may not be textbook, it was the machinists doings – but it worked! (he had built a briggs or two himself)

To diverge from the mopar a little, my preoccupation with Hudson flatheads stems from the fact that their valves are canted towards the cylinder at an angle, they have 12 large ports, and you can run a higher compression with the ability to breathe better than the big three flatties.

To put that in Briggs perspective, wouldnt you love to regrind and reseat your valves right up to the edge of the cylinder at an angle and fill up all that backspace? Ohhhh Boy!

Hey Hudson, sounds like you have a great little motor. Info/tricks for Mopar flatties is hard to come by & even harder to extract out of people so thanks for the goodies on yours. My father dirt raced a 250 Dodge in late 60s that had a skimmed head & zoomies with the biggest carb he bolt on. He always knew when it was time for a set of bearings as he could hear the pistons hit the head at an honest 6000rpm!!!! The rods must have been screaming for a break. At the end of season meeting they decided to try & break its back with a 225 slant in the shed for next season, broke the gearbox & sold the motor to another driver who got another season out of it!!!!!!!!! Anyway I have a 230 Dodge thats going to get the works for my 37 coupe & Im interested to learn more.

Glad you could get in on this flathead talk! I wonder if you could ask your dad which rods gave problems, or if he had done the same oil port modifications to his engine?

Ive seen our 230 hit 6000 with the single carb, but never held it any length of time to know if we could go that far and sustain it (for one reason the single carb wouldnt let us!). Right now somewhere around 5000 is all I want to hold, as our cam really isnt giving us anything beyond that. I really dont think the valve springs will take the 6000 as when dad hit it, it stuttered a little and began leaning out, the exhaust was very very hot.

Ive speced out the bigger series mopar flatties (3.438 bore), and like what I see! There is a 265 version out there, used in ag machinery and industrial versions. I think a few were used in very heavy dodge trucks during 56-60 also. The cranks from these engines will drop into any of the 3.438 bore blocks without modifications. These bigger engines have 2.125 rod journals compared to the 2.00 journals of the smaller engines of 3.25 bore.

The stroke of the 265 is 4.750, .125 larger than the little 230. With an offset grind on the bigger crank to shave it down to the 2.00 journal, the stroke can be increased to 5.00 gaining a total cid of 279 for the cheap, stroked flatty. Of course, there is room for more, as this mod will leave the outer clearances of rod to crankcase the same as before. But, youve just gone from a $400 mod to a $1200 situation.

The rod lengths in these engines, both series, differs in length according to stroke. The longer the stroke, the shorter the rod. Its possible to increase your torque output at lower rpms by simply swapping rods with a shorter stroke engine and shaving off the piston head a smidge. A very good rod length/stroke ratio is around 1.75:1. If you check out the rod lengths vs. stroke in most flatties – youll find something close to this number, thats why they seem to lug so smoothly at lower rpms than their bigger cid ohv cousins, who consequently run ratios around 1.55 with not much room to improve.

In the tractor world, Minneapolis Moline tractors built pre 55 all had stock rod ratios of 2:1, this is the ultimate rod ratio – my stock 40 hp Moline 4 cly. can show up a 66 hp IH 6 cly. on the pto any day of the week grinding corn @ 1000 rpm less at the flywheel.

I can hear those gears turning in your heads even now!

[This message has been edited by Hudson (edited 08-23-2001).]

[This message has been edited by Hudson (edited 08-30-2001).]

Hello Hudson: Long rods are a definite advantage. But I think a lot of people dont understand why. And I dont understand that 1.75 ratio being the ideal, I tend to think the longer the better. My thinking on the rod lenght is that the longer the rod, the better the angle you have while trying to push the crank down (some kind of physics thing). Along these lines, this is why moving the piston pin over in the direction of rotation helps power because when you do this you get a better (less) angle on the rod to crank. I think many older engines had the pins offset the other way to quiet down piston knock (rattling). And these engines, if they got flat top or symetrical domes,could be reversed in the bores to offset the pins in the power gaining position.

The torrential rain we had this morning has nixed my plans, so what better to do than talk engine!

The 1.75 is not ideal, but the reasoning behind calling it ideal is that the greatest increase in torque is attained at this ratio. In other words, if the torque increase due to rod length was represented in a curve, 1.75:1 would be a short plateau at the end of a steep rise. The apex would be at 2:1 with less relative rise than up to 1.75. Any increase beyond 2:1 being very small and impractical with most engines.

Why does this become so important? Its part mechanical physics in relation to rod flex and power transmission from reciprocating to rotating, part hydrodyamics in relation to cylinder filling and scavenging, and part pyrotechnics in using the combustion gases to their best potential.

The basic physics of the matter: the longer the rod in relation to stroke, a lesser angle is present between the centerline of the cylinder and the centerline of the rod beam at any point in its travel: as it deviates from vertical at TDC and BDC. This characteristic allows more direct transmission of power to the crankpin, via the longitudinal length of the rod. In shorter ratio engines, more of this transmission is transferred to the lateral section of the rod, using up power by attempting to force the rod to bend rather than push the crankpin around.

Another enormous benefit is the increase in piston dwell near TDC and BDC. As the ratio increases the amount of crank degrees that the piston dwells near the terminal ends of the reciprocating path will increase also. In mechanical perspective, this allows the crankpin to change the direction of the reciprocating mass under less resistance. The crank also rotates more degrees past tdc before piston direction changes and reduces the relative piston speed at these two points. Put these together and you have a rod acting more directly against a crank that is already past TDC by more degrees per unit of piston travel and moving the piston slower at critical points in its travel.

Hydrodynamics? Gases and fluids share the same phyical properties with the exception of compressability. We express fluid/gas flow in CFM. We often catch on to the volume, but overlook the function of time in this expression of measure. To increase the amount of piston dwell at BDC during the intake stroke, is to allow a more complete filling of the cylinder at peak demand by allowing more time for the cylinder to fill at its maximum volume. In addition, to increase piston dwell at TDC during the exhaust stroke, is to maintain the cylinders minimum volume longer for a more complete expulsion of gases through the exhaust valve. Although these properties do not change the cfm capacity of a group of cylinders, they definately change how effective an engine employs its theoretical capacity. This will effect the VE% in the classic formula ((CIDxRPM)/3456)VE%=CFM.

Now, we have exhausted the spent gases more completely, taken a denser charge into the cylinder – lets examine the pyro part of longer ratios. Again, we are dwelling at TDC longer with the wider ratio. This allows the combusting gases to reach more of their potential in a smaller space over a greater amount of time, and crank degrees. This really conserves the maximum force of the combustion at is most advantageous point against the piston head. Power is really only transmitted at its maximum at this point as the change in cylinder volume during the power stroke takes up the potential of the explosion rapidly.

The benefits of this are enormous, just ask the sbc builders how much improvement they get from just .300 of rod length. The first measurement I take on any engine is deck height above the mains centerline. The stroke is then determined based on available rod length and piston combos to effectively use the available height.

Offsetting the wrist pin center in the direction of crank rotation will accomplish the TDC effects of the rod ratio, but speeds up the BDC end of things. In high compression and forced induction engines, the increase in lateral force on the rod, due to the exaggerated angle during its upward stroke, can bite you on the touche. But, all things considered, if I had to build and engine with a ratio less than 1.55 – I would seriously consider offsetting the wrist pin. Offsetting the wrist pin makes it important to degree your cam closely, as the crank position at TDC will change – altering everything from cam phase to timing marks by a smidge.

[This message has been edited by Hudson (edited 08-24-2001).]

[This message has been edited by Hudson (edited 08-24-2001).]

The motor my father ran was bone stock internally & nothing really broke, more wore out really bad. His experience with them was he saw cranks break,piston crowns break away bores wear out unusually fast. In our part of the world we also recieved Australian cast engine parts. Castings never seemed to have the same strength/durability as the U.S. cast equivalent. The other problem he had was exhaust seats coming loose under prolonged load. I spoke to another guy who circut raced a 39 Ply coupe with a 250 in the 60s & he had the same problem. To fix the problem it came down to cooling. Make & drill acurately your distribution tube, this helps to keep the valves/seats cooler & stable. Secondly he put a larger pully on the water pump to slow it down. These engine were never designed to rev so high for so long & the coolant at high rpm simply never got the chance to cool down. Remember the fan blade becomes effectively worthless at high rpm so the motor relies on air flow entering the rad. not the fan drawing air in. All of this makes a difference to high rpm reliability. On the subject of bottom ends, nobody really played with them to much because of the crank breakage problems. A guy my father raced against did lie his 250 dodge on its side to get the center of gravity lower & dry sumped it- didnt make a difference to h/p & he had trouble with reliability so flagged it – 35 yrs ago it was a case of build it/race it/fix it, all home made of course. Just a note – technology has come a long way & the theories & practices have our made our jobs a lot easier than the trial by fire the likes of my father had to contend with. Ive learnt a couple of tricks & hope I can pass some on – lets keep it going.

Im going on a little family outing this weekend, Ill ask my Dad about the cooling tube. He did most all the work on the engine while I looked over the bigger ones laying in the shop floor. You might say I was the technical advisor. I paid attention to the components, he put them together. I do seem to recall him mentioning the water tube though. Pop has a few tricks up his sleeve as well.

You are right about the previous generation going through the trial by fire. I try and listen to as much of that as possible to either avoid the same mistakes, or recognize it before something breaks.

After my weekend excursion with the Sr.Hudson, he pointed out a few errors in my cid posts. I would like to make the neccessary corrections and also do that in the original posts as well.

First, the bore of the bigger engines is not 3.5, its actually 3.438. I had made my notes based upon what could be as my dads material is as manufactured. I didnt have the mfgs spec data and used my incorrect bore info to deduce the stroke of the 265 engine. Its actually 4.750! I know this disappoints you *laughing*.

Why did I error in my measurement? Because the chrysler engines are actually a dry sleeve design, with a very thick sleeve. If you really clean your block deck good, you can see it, but it is a very good press fit. Dont automaticly assume that you can run without this sleeve, as it is very good hardness and the block is very soft.

The larger bore 4.750 stroke, a 250 crank offset ground to the small 218 rod, is my favorite compromise in relation to rod ratio, numerous parts sources, and cid. One could offset grind the 4.75 large journal crank out to 5.00, but the piston speed really robs Hp at this stroke in the higher rpm levels. If you were going for a low rpm torquer, the stroked 5.00 would be good, but with a loss in rod ratio that may not make up the extra cubes.

[This message has been edited by Hudson (edited 08-30-2001).]

Let me recommend a book to you fellas. In high performance applications, you need to start with the best components. Nothing against the production car engines, but the truck series engines were stepped up, heavier versions that had the good parts as stock items. Consequently, you can often find these heavy applications in better condition when you find them.

The book is DODGE Pickups: History and Restoration Guide 1918-1971 by Don Bunn and Tom Brownell. It is published by Motorbooks a link to them on the web:

This is an excellent engine reference as the authors actually compare the engines, year by year, to the auto counterpart. Listing the modifications in detail and what models they were used.

The last chapter is an excellent guide for rebuilding the L6, regardless of application. They even delve into modifications for those so inclined.

To clear the muddy water regarding cid, block families, and applications: lets summarize according to this book (as my dad quickly pointed out to me!)

3.250X4.375 217cid DodgePlymouth 41-54

3.250×4.625 230cid DodgePlymouth 54-59

3.375×4.060 201cid PlymouthDodge 38-39

3.375×4.250 228cid PlymouthDodge to 54

Production Hybrid:Canadian Block+US crank

3.375×4.500 241cid Chrysler early 40s

This runs down the little blocks, and similiar to the old canadian chevy Chrysler apparently produced a thicker block for the cold temps up north with a little more room for improvement.

3.438×4.500 251cid Chrysler 46-51 and Dodge trucks 46-56

3.438×4.750 265cid Chrysler 52-54, Crown Marine, and Heavy Dodge trucks 54-60

The maximum bolt together of this interchangable series is the Crown Marine engine of 265.

Of course, weve already talked about mods from this point on.

[This message has been edited by Hudson (edited 08-30-2001).]

Learning all the time!Just to throw you into confusion heres a couple more.

2.875 bore x 4.063 stroke = 170.4 ci

This was a short motor produced 30s-40s

3.375 bore x 4.063 stroke = 218.06ci

This was a long motor produced 30s,40s,50s

Chrysler seemed to have made a lot different engines to suit a huge array of situations when in my opinion 3 or 4 of them would have covered all the terchange is not quite as easy as you would think.Crank journals,piston pins,oil pump drives,dizzy drives,water pumps,con rods,flywheels,sumps, all varied. There was even an engine that was in between the long & short motors produced for the 35 DeSoto that almost nothing interchanged.This has always confused me as Mopar had such a well rounded package & the economics didnt stack.I think if you can identify a good engine & try to avoid as much unnesicary interchange you should pop out the other end with out modifying everything to fit.

The truck/marine engines are definitely stronger/meatier,lifting my 251 truck crank is almost a 2 man job!

Please dont get disillusioned by this reply

its not a big deal if you have some info to know what your looking at/for.

Over here 1/2 the engines you discribe we never saw & we tended to get the left-overs or get build your owns,this included everything from the front of the car to the back.

[This message has been edited by Flat-Ram (edited 08-30-2001).]

I agree with you Flat Ram, I cannot figure out why Chrysler made so many variations when just a few would have done just as well.

About all I can say is that it allows a person to mix and match to build something to suit the need. I am amazed that these engines had such rod length variations, a rod for each stroke variant. It would have been much cheaper to vary the pistons compression height to accomodate a +/- in stroke. Yet, for each bore series, you have only one piston of the same compression height – regardless of stroke, @1.978 from the wrist pin center.

I have never seen a canadian block, nor heard of one besides the truck book. I suspect them to be long blocks with thicker sleeves/cylinder walls than the US counterparts. The strokes of the Canadian and US long blocks look suspiciously similiar to me.

And I also agree, that really only four of these variations represent the majority of concerns to us, whether restoration or hot rodding a bit. The 217/230 short blocks and the 251/265 long blocks. The resulting differences in crankpin diameter in these two engine series are the source of my glee in concocting hop ups.

Its just nice to know, that within a particular series, a person can up the torque output over the range of the engine by swapping rods from lesser stroked engines.

Lets add a little more confusion to the mix, if thats possible. I was just cruising the Silvolite piston website, Chysler OEM in the 50s, and found a little morsel. Going back to Franks mention of offsetting the wrist pin, the Slant 226 piston is listed as being slightly offset. The 226 slant piston is 3.400 bore, 1.740 comp height, and available up to .060 overbore. The amount of offset is not listed, just mentions offset. It may be possible to set up a flathead engine combination that uses the best bore, rod, stroke combination and utilize this piston as an off the shelf component. A person could at least utilize a 251 rod length on a 265/4.750 stroke and get the advantage of a longer rod and a little wrist pin offset. By the way, a 226 piston will fit the 3.375 bore as the difference in fit is only .025.

The Silvolite link to chrysler 6 pistons:

[This message has been edited by Hudson (edited 08-30-2001).]

Thought Id jump in for a few moments here when you guys started talking about the oddball varieties of flathead MoPar 6;

Its my understanding that in the European Countries, such as England & continental Europe, vehicles are/were taxed on Horsepower or Displacement, thus the Chrysler export models such as the Kew and the Wimbledon, as well as various Plodges and Plysotos seen farther East had tiny engines such as the 170 cid flat-head, and the cars bearing the Chrysler nameplate were really re-badges of the smaller Plymouth & Dodge.

I also recall reading (dont recall where) that when the new generation of engines were designed at Chrylser, around 1932 or 33, there was a goal to share as many internal parts as possible, such as common pistons, bearing shells, etc.

I dont have my shop manuals handy, but I believe the 323.5 straight 8 (34 to50) and the comparable 6 of 33-34 had the same 3 1/4 inch bore, and had the same rod bearings, cam profile, etc.

It also seems that as the engines increased in size at the top (Chrysler), the previous displacements were handed down to the lower priced lines;

the Chrysler 241 was de-stroked to become the Chrysler Royal & De Soto 228, then bored to become the De Soto 236; then De Soto inherited the 250 from Chrysler when Chrysler went to 265.

Similarly, Plymouth inherited displacements from Dodge as Dodge got bigger engines: 230>



As far as I know, all of the small(170) export engines were based off of the 201 Plymouth engine.

Its kind of interesing how certain displacements have a way of coming back; the first Chrysler of 1924, the B-70 had a 201 ci. six, as did the Plymouth 6 of the late 30s (entirely different engine), and the 170 ci diplacement returned in the Valiant in the early 60s, in slanted form.

I suppose this isnt terribly helpful in terms of performance talk… more of a historical rambling gleaned from the foggy recesses of a sometimes faulty memory…

at any rate, Im quite interested in this topic, and would like to learn what can be done with (or to) the larger Chrysler and De Soto sixes and possibly the 323.5 Straight 8, to get more from them.

I just loathe the thought of having to put a later model V-8 w/AT in my 41 DeSoto just to be able to cruise in the left lane.

(and, if it comes to that, you can be assured it will be a MoPar engine)

So Ill sit back now for another while and learn some more…

My 41 De Soto has the original 228 six, and manual trans (no fluid-drive!)and 4.11 rear, and at 88,000 miles, its tired! It is also not happy about being pushed beyond 45-50mph.

So, with an engine rebuild in my future, Id like to know how to get the most out of my old girl.

Im curious, as Ive never seen the 228 before. Is it a long engine? If it is, I think a very good combination exists for you to get into the left lane and run with the big dogs.

I think your gearing may be causing most of your troubles. I am by no means a gearing guru, I just deal with engines. 4:11 is very low for a road hawg, you spend alot of your time over the back edge of your torque curve – heading for peak hp. Acceleration is all about riding your torque curve upwards. Im not saying you dont have the hp to play in the fast lane, but its takes a hard downshift to get back over your torque curve and run at a higher relative rpm to get to your desired mph. I dont care what engine youre talking about, the higher sustained rpm – the shorter life it has.

I was recently treated to a ride in Bob Criswells Dodge Bros. Coupe (Thanks again Bob!). He installed a very tall gear in the coupe and hitched the stock 230 to a t-5 tranny. Peak torque on a stock 230 is somewhere in the 2500-3000 range with peak hp in the low 4000s

His coupe was cruising down the road at 2000 rpm in 5th,@60mph, and just begging for more rpm. A good lean on the pedal, and you could feel the little 230 climbing up the torque band with noticable acceleration. He had all the throttle response you would want with no downshift to get there. He leaned on the pedal and put us in high 70s @2500 in seconds and had more pedal to go.

The ideal setup to me would be to cruise just before the peak of my torque curve, use the curve to help me accelerate without a shift. Then, if If wanted to get froggy, use the rest of my torque range to bang me into peak hp at a significantly higher mph – after a good shift to wake it up!

The seat of your pants feel Bobs coupe has is directly associated with his engine dwelling just at the front side of his torque curve, right below its peak. His engine wants to accelerate