Engine Rebuild ???

[55 Fargo]

Hi all, just got off the phone with an engine rebuild place, was asking some questions.

First ? I have a 251 which has been rebuilt once in 1956, stamp says Using the "Chrysler Method". My ? was, can this engine be rebuilt again, or would the cylinders need to be sleeved.

His answer was that it shouldn't be a problem.

2nd ?, what can my current engine handle, as far as rpm is concerned, the engine is a 218 long block, not rebuilt, oil pressure is 40 lbs idle, 55 lbs at 50 mph, compression is 100 average, engine does not smoke.

His answer, the Chrysler Flathead 6s had an inherent flaw in there oiling system, running over 2500 rpm sustained will kill the engine.

I am not sure if I buy into the last statement, not sure he knows these engines all too well, he knew the 251 ci engines, but when I mentioned the 218s and the 228s, he didn't think you could get parts for them, but I know for a fact you still can.

I would like to pull this 251 out of the car it's in, and strip it down, have the block cleaned, and possibly get the machining done ,and then put here back together myself,

I have the head off the engine now, it's currently seized, there is no sludge in the valve chamber, so that may be a good sign.

I seem to be afraid to drive my 47 with the 218 over 50 mph, I don't find the engine that noisy at this speed, I am afraid of blowing her up, being that she has miles, and is not rebuilt.

You guys with unrebuilt engines, what spEeds/rpm do you guys cruise at....................Fred

[oldmopar]

I usually drive my around 50-55 when i go over 60 the engine seems to be working to hard oir pressure is around 40 and temp is always good. I have no way to know if the engine has been rebuilt it may have as when I changed the head gasket a few months the cylinders looked good no ridge of scoring.

I had a 47 bus coupe I sold a while back that had less the 40,000 miles (yes I should have kept but needed the money) and it drove about the same as the P23 speed wise

Ed

[De Soto Frank]

Fred,

I believe Chrysler furnished pistons in the following oversizes:

.010", .020", .030", .040", .060".

I've heard of guys taking flatheads out to .080", but that's making the cylinder walls mighty thin. If your block's bores aren't "clean" by .060" over-bore, then I'd plan on sleeving or finding another block.

Sustained revs: during the flathead era, Chrysler Corp. had an engineering standard that all engines must be capable of running at full-power/output for a minimum of 50 continuous hours on the test stand.

For a 1940's -'50s vintage 218/230/251, that would be running at 3,600 RPM for 50 hours minimum.

Personally, I don't like winding my old MoPars up past 3,000 RPM.

In my '41 De Soto, with 4.1 rear & 6.50 x 16 tires, 3,000 RPM should be moving the car alone at 63 MPH, according to Tod Fitch's "How fast should it go?" calculator.

http://www.ply33.com/Misc/speed.html

This agrees pretty well with what I've measured with a tune-up tach wired-up to my car, and actually driving down the road.

At any rate, for an engine with a stroke between 4" and 4-3/4", and (modern) full-pressure oiling and insert-type bearings (post-1935 flathead MoPars), I would suggest a 3,000 RPM limit for sustained driving... this is approximately 83% of "red-line" (3,600 RPM).

If you've got any bearing knocks or low oil-pressure, then I would lower that ceiling to 2,500 RPM, or less...

I don't know about an "inherent oilling flaw"... Chrysler had pressurized oiling to all bearings from day-one in 1924... Hudson, Chevy and Buick were still using poured-bearings and splash lubrication into the 1950's...

If flatty MoPar engines did have an inherent flaw, my bet would be insufficient end-gaps on the piston rings: factory specifies and end-gap between .007" and .012"; most 1950 & later sources recommend an end-gap based on .004" per inch of cylinder bore: so a 3-1/4" bore engine should have minimum end-gaps of .013"...

When most guys tear-down a flathead MoPar engine, they find the top ring in several pieces, suggesting that the end=gap was set-up too closely, and when the engine got up to operating temp, or perhaps if overheated, the gap would close-up entirely, the ring would keep trying to expand, buckle, then break.

I have heard enough flatty MoPars with bearing knocks to suspect that either oil filters were no changed enough, and/or the old single-weight non-detergent oils just did not provide adequate lubrication.

Jon Robinson at De Sotoland, and our own James Douglass favor MoPar's Full-Flow oiling system, which is ID'd by the oil-filter being mounted to the block with a cast housing, using no external lines. This system sends ALL the oil from the pump through the filter before it goes on through the engine.

Previous oiling systems used a by-pass filter, with 3/16" or 1/4" lines, which filtered only SOME of the oil from the pump.

The 251 Chrysler six was an excellent engine, having a parallel existence as an industrial engine into the early 1970's...

I'd suggest driving your 218 while rebuilding the 251... Jon Robinson usually gets around 100,000 miles between rebuilds with his '50 De Soto...

I would look into having the crank, flywheel/clutch, and reciprocating stuff all balanced by a reliable shop; if you're flinging all that weight around at 3,000 RPM, having it critically balanced will increase the longevity of your engine.

Good luck !

De Soto Frank

[james curl]

Also the big block engines do not have offset rods like the small block engines do. I would think a straight-line power flow perpendicular to the crankshaft centerline would be better than the small block offset rods.

[RobertKB]

On my ' 53 Plymouth I have 96,000+ miles. It had rings and a valve grind about 20,000 miles ago. Bearings are still original as at the time of the new rings it had 55+ lbs oil pressure and when plastic gauged they were within specs. I drive this car at 50-55 MPH without any concerns. Oil pressure is still the same. I pay attention to the engine but do not overly baby it. When there is a problem I imagine it will let me know but until that time I just drive and enjoy it.

[steveplym]

The old engine I had in my car before rebuilding the original 218 would smoke like a chimney on deceleration, but ran really good at highway speeds. Didn't have to do much to it, but add a quart of oil every 100 miles.

In my opinion I would get the 251 rebuilt while you are still using this motor. Then you can have it all cleaned up and ready to install. Then pull your current motor and clean the engine compartment really good and paint it. Drop the new engine in and away you go.

Makes it much better as if you find a snag in the 251 and it takes longer to rebuild for whatever reason, you can still enjoy driving your '47.