So I have spent a lot of time since 1974 learning about 216/235/261 and the GMC's. I have them figured out. But I would like to learn about the mopar flatheads. Where do I begin? for starters what are the preferred engines and what vehicles are they in? Im thinking the 265 Chryslers have to be like the 302 GMC's???. Please advise

There are 2 "families" with similar design, with staggered bore pitch and staged main bearing sizes like the 235 & GMC.
Small: "Ply-Do" (Plymouth, Dodge) engines are 218 (a.k.a. 217) with a 23" long cylinder head. Replaced in cars by the Slant 6 in 1960, continued in some trucks.
Large: "Spitfire" (Dodge truck, DeSoto, Chrysler) 237, 251 and 265" with a 25" long cylinder head. The 265 was the Windsor (low price model) engine until the 301" "polyspheric" V8 in 1955.
All very durable.
Some data on my site here: http://victorylibrary.com/mopar/m-table-c.htm#Chrysler

There is an even larger L6 series (413), but only big trucks.

hey thank you for the info. I know you and I butted heads on the HAMB a while ago so I hope there are no hard feelings.

So back to the Chryslers: are the backs of the blocks compatable with modern day trans? Im thinking OD trans of some sort. I see these as very similar to the 261's. I love those engines. So If a guy were to put one of these in an early (30's for instance) car or pick up you would have something similar to a 261 in an early chevy. Am I on the right track? If I am, where would a guy find said Spitfire - what years I mean?? Did my dads 51 chrysler Windsor have a 251 or 255 and I didnt know it??

I thought there was a 258 as well. These were my dad's favorite engine group. As a kid we had lots of them. Even our irrigation wells ran them. I have a 413 from a Dodge truck. Was going to power a Bonniville car but it is too big for the vintage classes.
There was speed equipment made for them and they did very well. So did Hudson and Studebaker until the magazines and after market selected their favorites.

he Windsor used the 265 AFAIK through 1954.

All sidevalve engines ha the same disadvantages (vs. OHV): low maximum compression ratio and poor breathing.
They have 9 ports (235 has 7): 3 siamese intakes, 6 individual exhausts, which allow for a true tuned exhaust system and benefits from (relatively) high overlap.
All flatheads can be improved using the same methods: head mill, tight quench, deck the block, relieve the deck, offset gring the rod journal to increase stroke, bigger valves, regrind cam, bigger springs.

The big truck (Industrial) 375 and 413 were 12 port. They came with dual carbs and dual exhaust. Mine has a special "high compression head" maybe 7.5 or 8 to one with a tag that says "use only with special fuel or at high altitude". If I had a 375 I could revive the Bonneville toy plan. Or at least a 375 crank. There is a thread on here somewhere about the recovery of that big engine. It used to have pictures attached.

I remember the pics you sent me of the huge inline 413 Beater. I always have liked flathead and would like to make some parts for them.

I think the MOPAR and Ford flathead sixes are becoming more popular again. Hudson, Pontiac, Olds, Packard, Studebaker ....there are some great engines out there that would make great projects. If affordable speed equipment could be made I think it would sell. Eddie Edmunds had the right idea.

The Chryslers (both 23 & 25) used rod length to adjust deck height vs. stroke, to use the same piston for different size engines.
This suggests that the longer strokes (230, 251, 265) can get 1/8" pop-up (or at least higher deck and tighter quench) by simply using the rod from the next shorter stroke (218, 237, 251 respectively).
Yes, it would require milling a dome relief in the head, and there may not be enough metal for safety. Alternatively, a small improvement by taking some off the dome itself. How to tell? Drill a hole in a junk chamber and see where you break through.

Anyone heard of this?

Yes, that is a viable method to determine the casting thickness.

Considering the low cylinder pressure, what thickness would you accept as safe?

There are some modern H-D and Indian flat-top pistons that might prove useful for someone building a modified flathead.
All the pins are too small, but a new pin bushing fixes that.
Bore sizes:
Chief 3.25"
H-D UL 3.3125"
H-D UH 3.422"
H-D XL Evo 1200, FL 1340 (same bore, different compression distances) 3.498"
Corvair 140 3.375"
Corvair 145, 164 (same bore, different compression distances) 3.4375"

Given the lower cylinder pressure, I can only make a comparison and comment on what I feel a safe range of chamber thickness may be. Having said that, we used .125"(1/8") of thickness to be the minimum thickness for a cylinder wall on the major thrust side in our V8 race engines. A cylinder has both thrust loading and some compressive pressures on them, while a chamber would only have compression pushing against it. So I would feel that .150" thick for the chamber would be a safe starting point and maybe work your way down to less if you think you need to. However, the chamber is a source of high heat, and machining the chamber thinner may make it create more heat. So I would be interested to see what thickness the stock chamber is to begin with and see how far from my guess of .150" it is from that. But throw a turbo on there where higher chamber temps can be expected and you may want to have the chamber thicker.

Some interesting cutaway photos from a flatty Ford V thing:
HAMB circa December '08

Does the inline 'ply-do' head have the same contours with a 'quench' side situated very close to the piston (at TDC) opposite a hump on the combustion chamber-valve side?


I'd be worried that thinning the quench side would weaken the shoulder where it transitions up into the combustion chamber.

Seems a fully custom piston running proud of the block on the quench side and popping up into the combustion chamber might be a better strategy - with as little machining on the head as necessary.

I remember reading about a Bonneville record holder running about 6.5 to 1 in a Merc flathead in a vintage sedan class. His thought was why increase the compression in an engine that had trouble breathing already. Of course he had miles to get it to speed.

Ken Kloth....

The flathead always has a Hobson's choice, striking a balance between breathing efficiency (large X-section passage from the valves to the bore) and static CR.
The best compromise depends not only on what the engine is used for (maximum torque vs. peak power are different), but the basic architecture including bore to stroke ratio. Unlike OHV, a small bore + long stroke engine has an advantage because the small piston makes the total chamber area narrower without reducing valve size, and the long stroke adds displacement and compression without shrinking the transfer area.
AFAIK the power king of all flathead production is the last H-D factory KR in 1969:
2.745" bore by 3.8125" stroke (the basic engine was introduced in 1929) = 739 cc, 45.12". The best ones produced 58 hp on pump gas at 7,500 RPM from 5.5:1 CR and really big cams.
In a 292" Ford flathead (seriously big, bored & stroked) that would be 375 hp. Missed it by over 100...

I believe current state of the art Junior Dragster engines have surpassed the KR's legacy.

Even back in the 60s when we were racing 1/4 midgets we all started with the same Continental engine. Stock class had to run them right out of the box. Then came a modified class, B modified, A modifies, and AA modified that ran racing fuel. The AAs could hit 45-50 mph on a 20th of a mile track and were pretty much in a constant drift. A lot can be achieved with a little single cylinder flathead. I have no Idea what the Jr. Dragsters do. I bet they are safer than those little roundy round cars were.

My daughter's Jr. Dragster ran 8.90s in the 1/8 on Elky. all out of a 5hp briggs.

There is some serious work done for the Kohler garden tractor engines.

If for example, you reverse calculate the power based on the 7.9 second eighth mile capability of a 500 lb junior and extrapolate that into a 12.3 second quarter mile time then calculate the power to do that on his $4800 3 3/4" bore single you'll get roughly 50 hp from a 25 ci engine , or TWO HP PER CUBIC INCH.

http://www.stoutracingengines.com/jrdrag.html

That's correct! For decades, the near max you could ever get out of a N/A Flathead in the automotive versions was around .8 HP/CI. So the Jr. Dragster racing technology has greatly advanced the Flathead into the 21st century. I've spoken to several guys on the Bonneville site that have a very knowledgeable background in Jr. Dragster and are applying that to their land speed racer currently. Interesting to see what they discover by crossing over to the automotive version with this technology....

I can't find anything on the B&S cylinder that doesn't look exactly like a 1940 Harley. No pictures of the chamber...

Advances are mostly in cam profiles and chamber/transfer slot shape and design. The higher end B&S stuff is all billet and CNC machined, and doesn't resemble a stock production cylinder head in those areas. That racing genre is still highly competitive and somewhat proprietary since it's still relatively new. So you won't likely find much in the way of pics or detailed description about head design or cam specs.