Inliners International Forums General Discussion Engines My MOPAR flathead

I don't seem much mention of flatheads or MOPARs here, so I'll start a thread on the one I'm working on.

I have a 42 Plymouth 2 door sedan and I'm putting a motor together for it.

I've started with about a 1950 Dodge 2 ton 251 motor. This one has sodium cooled valves, which may or may not be a benefit. I've had the block gone through, bored, new pistons, decked the block. Valve job. A pretty hot cam reground by Edgerton. Ported, head shaved .100 which should give about 9 to 1 compression.

For breathing, I have a set of tube headers built by Langdon, and an Edmonds aluminum dual 2 bbl manifold intake with water heat.

Not sure what I'll do with the clutch, pressure plate and flywheel, but probably some version of stock. I have a 50's overdrive transmission, disk brake conversion and wire wheels.

It's about all sitting there, new bearings and all, just need some time to start assembling. I'm anxious to get it assembled and hear it run. I doubt it'll be all that fast, but I bet it'll sound cool as can be.

That sounds like a good build. Not sure about the sodium valves. It will go faster than the cops will let you drive it.

I,VE worked on several 251's in my days. they also put them in dodge m37 power wagons and civy power wagons. make sure you install hard exhaust valve seats in the block for unleaded gas. very durable engines. also only premium engines have sodium filled valves..........

i can testify that a well-prepared "long block" Mopar six-cylinder with overdrive in a Plymouth body, such as you envision, will cover the ground in a hurry. i've seen the speedometer needle buried when one of these made 89 miles in 62 minutes on two-lane with five passengers aboard. It was 1957, and that one would run.

i should hope to see your project when you have it up and running. The prospect brings joy to the heart.

God's Peace to you.

d
Inliner #1450

Not sure about the Spitfire engine, but many sodium-cooled exhaust valves have larger stem like 7/16", so there's a weight problem.

From what I've read sodium filled valves were first used in air cooled aircraft engines in order to promote heat transfer 'up' the valve stem through the guide and into the heat-sinking fins of the head.

Their use in hopped up flat-heads is a logical progression because the valve is inverted - so the liquid sodium helps to transfer heat 'down' the stem and into very long guides that are in close proximity to the water jackets in the block.

This is very important when one is raising compression ratio - and therefore the combustion chamber temperature . . .

AllPar Web Site

Everyone knows that heat rises - so promoting heat to dissipate 'down' a valve stem can only be achieved by improving the thermal conductivity of the material - sodium is outstanding in this regard.

Panic - does the additional weight really matter with a 'valve-in-block' setup? The cam is acting directly on the stem and the additional weight/inertia doesn't need to be offset with spring pressure to prevent float (like it does in an overhead valve arrangement) . . . since gravity is assisting in returning the valve onto the seat.

Gene G. I like what you are doing with this build. I just love hop-ups from this period. I think it is going to be a great cruiser . . .

IIRC one of the problem children was the B29, which got hot enough for the magnesium casting to catch fire.

All of these motors had really big lifters, the stem itself would weld to the lobe.
Gravity is far too weak to be of real help, the valves open and close too fast.
They definitely need more spring, and how much spring is reduced by saving every gram you can. Too much spring adds torsional load to the cam (twists over the extreme length) which upsets the timing at # 5 and 6.
A 265" L6 (3-7/16" bore by 4-3/4" stroke)is about the size of my Harley flathead engine: 3-7/16" bore by 5" stroke for 92". I'm using 5/16" OD intake valves to both reduce weight and increase port flow, the valves are over 6" long, 2-1/16" head. The exhaust are 3/8" OD to keep a lot of contact with the guide for heat but smaller at 1-3/4" head.
The cam is dual pattern, with roller tappets.
Intake: 294° @ .010", 270° @ .050", 49-65, .377"
Exhaust: 278° @ .010", 256° @ .050", 60-38, .407"
The numbers look funny, but there isn't much available for these - not made since 1948.

I don't think valve float is an issue, as top speed is probably about 3600 RPM. With the higher lift and shaved head, I might have to do a little clearing around the valves on the head, though. Time will tell. Might be a little die grinder work involved.

Shameless self-promotion:

Sounds interesting. When will it be available?