Fred44676

Engine Swaps II

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I have the opposite problem to the earlier Engine Swaps post about the dodge motor.  I have a decent original '41 Plymouth 4-door with a spun bearing.  The motor has been apart for 40 years and is quite rusty.  A couple of years ago, I bought the entire driveline out of what I remember was a running, low mileage '48 Desoto 4-door that was being streetrodded.  After reading the above post, I measured both motors and indeed the Desoto motor is longer than the Plymouth motor.  My question is how difficult is it to put the longer '48 Desoto motor into my '41 Plymouth?  The Desoto drivetrain is also fluid drive if that makes a difference.  I have everything and I mean everything for the '48 motor including the radiator. transmission and driveshaft.  Has anyone done this or a similar swap and has pointers?  I want to swap just the motors, not the transmissions.  The '41 Plymouth 4-doors are just not valuable enough to justify a complete rebuild of the original motor.  I would be doing the work myself and this is not my first engine swap but I have little experience with forties Chrysler products.

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Benefits of AACA Membership.

The Chrysler and DeSoto motors are easy enough to interchange, as are Dodge and Plymouth. But De Soto into Plymouth is harder. The Hollander 1952 has one interchange:

""Wreckers advise the Chry '37-50 6 fits Dodge Army Truck '41-42, half or 3-quarter ton by moving front motor mounts ahead to holes already there. The radiator rests on front sides of support. Drill 2 holes in support for radiator. This change, as described was made with a Chry '44 6 cyl. motor."

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Thanks for the reply. It sounds like the Dodge Army Truck was designed to use either motor.   I doubt if the '41 Plymouth frame has the extra motor mount holes already there but it might be simple to add new holes.  The radiator/fan/water pump clearance could be a bigger problem if the front sheetmetal would make moving the radiator forward difficult.  That is why I am curious if anyone has already made or tried to make a similar swap.  I may watch for a similar stock Plymouth at Hershey and make some measurements of the radiator/fan area.  

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Don't know where your at? I'm in North Minnesota. I bought, (and pulled it myself) the entire drivetrain except rear axle from an interior gutted 41 Plymouth 4 door that was destined for a street rod project. I potentially needed engine and / or accessories for my 38 Dodge Humpback that was missing parts. My engine is the 25" and was made good!  I ended up using starter, generator, throttle linkage, possibly the distributer and water pump from the 41 Plymouth. I have the 23" 41 Plymouth block sitting in shop with head off. I had tried to start it before pulling it from body, got a few cylinders to fire, but no go. It was NOT stuck and has very little ring ridge, valves all look intact and no signs of water sitting inside engine. Would make you a deal on the 41 Plymouth parts I have.

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Thank you for the offer.  I should have mentioned I am in north central Ohio near Mansfield.  At this time I doubt it would be cost effective to use your motor though if I were closer it would be tempting.  My '41 Plymouth motor is definitely rebuildable as the crank is not badly hurt but if I cannot use my Desoto motor I would probably be able to chase down another running Plymouth or Dodge motor here in Ohio.  

 

 

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It will fit, the swap has been done many times. You need to move the motor mounts forward and the radiator forward. You will have to check if the flywheel will fit, it should. Also check that the starter fits and meshes with the flywheel, some are 3/8" off, but I believe this is on the 12 volt models.

 

The Canadian Plymouth factory actually did this swap. The stock engine used in Canada was based on the larger Desoto/Chrysler block. This is why some frames are already drilled for the different mounts. Other than the length it should all bolt together. They did make some changes over the years but a 41 and 48 are so close they  should interchange with no serious problems.

 

In cases like these it is often easier to remove the hood and set it on the roof (protected by cardboard and blankets) then remove the front clip as a unit so just the frame and motor are there. This makes it a lot easier to change motor mounts etc.

 

Suggest you do a ring and valve job and generally check the bearings oil pump etc while you have it apart. If one of these motors is in top shape, with good compression and oil pressure, you will be impressed with how well it performs in the light Plymouth. It may not be a threat in a drag race but has plenty of oomph for ordinary street and hiway driving. Those old DeSoto flatheads are foolers, on paper they don't have much horsepower but that long stroke high torque motor makes driving so easy. You can idle right down to a walking pace in high gear and accelerate up to 90 without a buck.

 

Edited by Rusty_OToole (see edit history)

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Thank you, this is what I needed to know.  I thought that this swap may have been popular at one time to get the additional power.   I have both starters so I will compare them.  I expected to have to use the '41 flywheel and clutch.  Actually, the hood and front clip were unbolted back in 1973 when I pulled the original motor.  I intended to do the motor, clean up a few dents and paint the fenders back then but the car has been in the same dry storage for 45 years.   I am going to need to move the car soon so I may move it into my shop as a winter project.

 

You are absolutely right about inspecting the motor.  At the very least, I will pull the head, grind the valves and check the bearings etc. before I detail the motor for installation.  It has been a lot of years but I still remember the Plymouth even with the small motor, being a delight to drive.

 

Fred

 

 

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The important point is, they used basically the same flathead six design from 1938 to 1972. There were 2 versions (long and short block) that are interchangeable, with slight modifications.

 

But in that 30+ years they did make some running changes. That means some parts changed over the years. Your 2 engines are so close in age that the changes should me minimal, or none. If you have both motors on the shop floor you will soon see if the flywheel, clutch, transmission etc will interchange. I would expect it to be  a simple swap except for the length, and we know there is enough room because the factory did the same swap in Canada. So, if you run into any glitches come back and ask, or maybe try the P15D24 or Inliners International sites.

 

If I had the motor apart I think I would put in rings as well. Today's rings are light years ahead of what they had back then, in terms of low friction and wear. This is especially important in a long stroke motor.

Edited by Rusty_OToole (see edit history)

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One thing to watch if you are swapping distributors-- check length of shaft where it drives off oil pump. There are 2 different lengths about 3/8" different. Might be the difference between the 23" & 25" engines???  Mine was missing but I had accumulated half a dozen thinking they are all the same. When I started putting things together only 2 of the ones I had would work.

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Thank you Rusty and John for the additional information. When swapping in a used motor,  I usually do lay out all the parts that I have two of, and use the better of the two.  I will watch for the distributor length.  You have a good point about the rings.  Years ago and I mean MANY years ago I used to always put new rings in my Austin Healey motors on the standard original pistons in a honed only block.  Cost me about $14 plus gaskets.  Rings, bearings, and an overhaul gasket set was less than $50.00 from J.C. Whitney. (spring of 1967}

 

Fred

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