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Mopar Engine & Powertain Compatibly Help Wanted

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A good friend of mine has a spare 1949 251 engine. This is the 25" long engine. Original bores. He has offered it to me at a "best friend" fantastic price.  I am interested in owning this engine as I "think" it may suit my long term needs.  Both my 1953 265 and my 1954 228 engines that are in service today display the same symptoms. Rear crank seals leak,  worn valve guides burning a little oil

while greared down holding back on a hill. Both burn little oil driving up the hill home. Compression is about the same in both engines hovering in the 100 psi range. Neither of my 2 engines require a rebuild immediately. 


My 1953 Chrysler has the 265 engine. Same block and bore as the 251 if I recall, just a little longer stroke (1/4" longer the 265). Meaning different crank throw length I believe.  I assume possibly different con-rod length too.  I am not sure, but I assume the 265 may have a larger flywheel.  Not thinking I'd put the 251 in my Chrysler. But a spare 265 block is handy to have!


My 1938 Plymouth has had the engine replaced with a 1954 25" 228 ci. I kind of like the idea of putting this 251 engine in my '38. Torque galore from the 251 in that little car, I would think.  I like the idea of building up the 251 on the side, while both of my cars are drivable. As time and money allow I can move along on the 251 rebuild. I am not interested in this 251 for higher top speeds nor freeway travel. Nor driving a 3.73 rear end. My 1938 has a 4.11 rear end. I plan to leave it at 4.11. 


Some unknowns and thoughts that I have: I was hoping a few folks here with more experience than me, could fill me in. I have a few books here to reference, including a 1949 shop manual for Canadian cars that all had the 25" long engines. Comparing the specs from all the 1949 25" engines, they all share so many vital parts and vital dimensions.


1938 has the 25" engine now. Would the 251 engine mounts, bolt right in? I suspect so. The front mount is very common across all mopars. The rear crossmember with 2 motor mounts in the bellhousing should swap right over without problems. 


My 1938 3-speed tranny, drive shaft and rear end; would they handle the extra torque of the 251 engine? I have no plans to drive it any harder than the 228 engine now.  Other than pulling a big hill home in 3rd. I have to climb a fairly long steep grade back home, every time I take the car out.  The 251 should lug down pretty good and pull harder than my 228.


My 1938 Clutch diameter is 9 ¼". Optional 10" was available for taxis back in the day. I assume I'd should upgrade the clutch size. Due to increased engine HP and torque. Looking at a 1949 shop manual, I do see though that the C45  Windsor 251 had a 9 ¼" clutch too.


251 engine I suspect, should have a larger flywheel. The 251 engine bell housing was set up for use with an M6 tranny with fluid drive. I could swap over my 3 speed tranny bell housing, would it bolt up to the 251 block?

Would my 3 speed tranny pinion fit the pilot bushing in the back of the crank of the 251?  If I recall the same pilot bushings were used for decades. I suspect I would likely need to source a flywheel drilled for use with a 10" clutch if I go that route.


The cooling system. Will my rad keep up with the increased HP (fuel burn and BTUs) from the 251? Not a big deal, but I could address that probably. 


Comparing specs:

Original 1938 201 Engine in my Plymouth was 82 hp 3600 rpm,

1949 251 engine: 116 HP at 3600 rpm. 208 bs ft torque at 1600 rpm


You thoughts are welcome.  Wouldn't a little P6 be a fun driver with a 251?  Any reservations? Thanks, 




Edited by keithb7 (see edit history)
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You are correct that the difference between the 251 and 265 is the longer stroke crankshaft, and matching shorter connecting rods. Both engines use the same pistons.

The 251 should bolt into the 1938 using the 1938 flywheel, clutch, bellhousing and transmission. They did make some changes during the life of the engine from the mid 30s to the late 60s. In the early fifties they changed the water pump, I think this had to do with lowering the fan for hood clearance. Some engines had a squarish bump on the front of the head and took a different head gasket. They changed the exhaust manifold outlet from the back to more in the middle. They used different models of distributor that took different ignition points but the different distributors interchange. If you have the 2 engines side by side you will soon spot any differences. If you wished you could bore the 1938 block to 3 7/16" install stock 251 pistons and crankshaft, and turn it into a 251.

The 251 will work well in the 1938. Any extra power will not overmatch the clutch, drive train, or brakes. I assume you do not drive like a wild man, revving to the red line and popping the clutch at stop lights etc. It will give you a little better pickup and hill climbing ability and won't hurt anything.

As for the crankshaft pilot bearing I don't know if the Fluid Drive models had one or not. There are ways to add one if necessary. Chrysler made a bolt on ball bearing one for later V8s that may fit,  or you could make your own. If you have the crankshaft turned as part of a rebuild the machine shop could drill the crank.

The rad has enough capacity to cool the larger engine provided it is in decent shape to begin with. They were never pressurized, you could use a 4 pound or 7 pound cap with a coolant recovery container  for increased cooling .

There are a few simple hop up tricks you could add if you wanted to, like shaving the head, getting a reground camshaft, dual carbs, split manifold for dual exhausts, and shaving the flywheel for whippier acceleration. But just the stock 251 would make an awfully nice driver.


In short I think it is a great idea and you can have a lot of fun with a car like that, and maybe surprise a few people on the street and on the hiway.


By the way I have a 265 crankshaft and rods, in fact a complete engine plus a few spare parts if you are interested. I don't know where you are or how far you are from Cobourg.

Edited by Rusty_OToole (see edit history)
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Thanks for your reply. I was hoping you'd respond as you have a lot of good sight, very helpful posts. You confirmed my suspicions that it should fit up and bolt in nicely in my 38 engine bay. The car should perform pretty well with the 251 and 4.11 rear end. I drive it like one would normally drive a stock 1938 car. With respect for the early engineering. I am not planning to any upgrades like shave the head or dual carbs.

I will proceed to build up this 251 and use it. Its currently just south of the CAN/USA border at my friend's house. It will be a while yet before I can pick it up.


I am way out west in BC. Too far to make shipping a block worthwhile. 

Edited by keithb7 (see edit history)
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