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1966 F250 4x4 that came with the 300 6-cylinder. Picture of the truck before I bought it.
F26BR831340 - B is 300, of course. March 1966 build month. B5 - Dana 60 locking diff.
Window sticker calls out the 300 and limited slip
Build sheet confirms VIN and B engine code. 08C - March 8th scheduled date.
Truck seems at least fairly original. Negative battery cable still seems like it might be original. I'm going to swap it out since the insulation is cracking.
Engine block casting is C6AE-6015-D with a date code of 6B23 - February 23, 1966.
Engine tag is still present. 6-C - March 1966 and shows the 300 as expected.
Front of head under valve cover has a date code of 6B10 - February 10, 1966. Seems to be fairly close to engine block date. But...
...the single letter code at the back end of the head seems to be an A - 240 cylinder head, correct? Interesting it's just 13 days before the block.
Distributor is an ID number C6TF-12127-AF with a date code of 6BC - 3rd week of February 1966. - Looking at the breaker plate, it does not have any springs showing on top so dual advance as far as I can tell - can take a picture of that if wanted.
MPC shows that ID number as being for a 300 L/D with Standard Transmission also indicating it's not a LOM. Truck has an NP435 4-speed.
I have not measured the stroke yet as I'd assumed the block was a 300, at least, but I'll measure that soon. Looking for 3.98". Also have not yet located any C6AE casting ID number on the head. Might not be present. There is FoMoCo cast on both the block and head. Thinking 1966 or 1967 was last year for FoMoCo casting but not 100% positive on that.
Does it look like to others that someone did this truck a favor by finding a 240 small chamber head? I have not pulled the head yet to compare to pictures showing a comparison between 240 and 300 chambers.
The 300 truck engines were designated with a TE and the 240s were typically the AE code.
I would take a wire down the spark plug hole and try to estiamte the stroke.
The 300 has a 3.98" stroke and the 240 has a 3.18" stroke.
The .800" difference in stroke should be easy to determine
Last edited by AbandonedBronco; Apr 29, 2026 at 09:14 AM.
Your experience with these Ford big 6s is impressive! A wealth of knowledge for sure.
I noticed @AbandonedBronco modified your post to change the .600 to .800 - which is getting closer to 1 inch than a half inch so it can be very noticeable doing the wire down the rabbit hole method I agree. Mine is roughly 4" but I'll be pulling the head soon and can verify better then.
If I search the entire FTE web site - not just any one forum - on C5TE 6015, C5TE-6015, C6TE 6015 or C6TE-6015 I don't get a single hit. Likewise, C5AE-6015 and C6AE-6015 each with no suffix gives nothing. But if I search C5AE 6015 or C6AE 6015 I get some hits. Here's one where a 1966 240 and 1968 300 seem to both have the same as my block C6AE 6015-D
I agree they did use the TE in the prefix in later years like D5TE (1975), but I've never seen a 1965 or 1966 truck big six block with the TE in the prefix.
If I search on C6AE-6015-D there's about 12 hits on that. Any combination of C6TE-6015-A through F gives no hits.
Just trying to learn more. If the heads are the same early on like @NumberDummy mentions in the thread above perhaps the C6AE-C isn't unique to the 240s in that era. I haven't observed the head markings over the years like I have the block markings so I can't say for sure. The head on my truck being just 13 days before the block casting which is just about textbook seems like this head likely came on this block. No way to prove it but it's a compelling example, I think.
I was meaning to edit that silently, but apparently forgot to click the button.
If you don't feel like pulling the head to see if it's a 240, you could always grab one of those bore scope cameras off of Amazon or the like. They're pretty inexpensive anymore. I think I got one for my brother for some $30. Most of them come with little addons like a light and a mirror.
Slide it in the spark plug hole and look at the shape of the cylinder head.
Here's another interesting thing about this engine. Notice how on this C6AE-C and letter A head that the valve springs and retainers are different on the intake versus the exhaust valves. Ever run across that before? Thoughts? I have a suspicion that I know what could be said about that. Screw in studs, too.
While you ponder the valve train here's a way to often tell which crankshaft you have without measuring stroke through the spark plug hole or pulling the head. Pull the pan and look at the crankshaft lobes on these early engines. If it's stamped 1LA (ILA?), it's a 240 and if it's stamped 1NA (INA?), it's a 300 L/D, at least up through 1974 that should be the case. As shown the 300 H/D should have the stamping number C6TE-6303-G to identify it as being steel, at least up through 1972. This is the 1964-1972 MPC final edition from 1975.
1NA on my engine so that confirms the stroke is 3.98".
Here's the thing I'm seeing with the valve springs and retainers. Looks like the 1965/66 F100 and F250 2WD trucks used the same springs and retainers for the 240 and 300 but for all other trucks they used different springs and retainers on the exhaust valves of the 300 L/D - same ones as the H/D engines. The 1966 specific parts catalog also shows this. But I shouldn't equate this with what they did from the factory. My bad and I'll get off my soapbox.
While you ponder the valve train here's a way to often tell which crankshaft you have without measuring stroke through the spark plug hole or pulling the head. Pull the pan and look at the crankshaft lobes on these early engines. If it's stamped 1LA (ILA?), it's a 240 and if it's stamped 1NA (INA?), it's a 300 L/D, at least up through 1974 that should be the case. As shown the 300 H/D should have the stamping number C6TE-6303-G to identify it as being steel, at least up through 1972. This is the 1964-1972 MPC final edition from 1975.
I had to chuckle at this comment suggesting, with a straight face, I assume, that pulling the pan with all its attendant bolts is a reasonable alternative to checking stroke with a piece of straight bailing wire. Just ribbing you, of course, Now I've spent an inordinate time on here and must get back to disassembling my '74 F250. If I ever need another reason to spend time on other trifles I'll liberate my head on my spare 240 and spare 300 and let you decide if they're different...
Last edited by AbandonedBronco; May 4, 2026 at 09:45 AM.
Not easier, just saying it's an alternate way if you have the pan off or find it easy to do. Much easier on a 4x4 than a 2WD truck for sure. I appreciate your input big time!
Here's what the 1966 specific/only parts catalog shows for the cylinder head. T/E being Thermactor Emmisions - which wasn't used on the F250 4x4 trucks at all.
They specify the same cylinder head for the 240 and 300 L/D as each other within each of those two categories and the 300 H/D getting its own head. I usually equate the C5AZ and C6AZ part numbers with the C5AE and C6AE casting numbers but it's not a hard rule so I'll not assume that to be the case here.
The 240 head has a 68cc combustion chamber and the 300 carburetor head has a 76cc combustion chamber.
Ford never mixed the 240 head and the 300 head between the 240 and 300 short blocks.
A 300 head on a 240 would have too low of a compression ratio.
The 1965 to 1968 240s used the 289/302 pistons.
The 1969 and later 240s used dedicated 240 pistons
Here’s what I’ve been able to piece together from the date codes and the parts on my engine.
My block is a C6AE‑D 300, cast in early ’66.
The head is a C6AE‑C “A‑coded” 240 casting, cast 13 days earlier.
The valvetrain parts on this head match the 4x4 / HD exhaust spring and retainer setup, not the 240 light‑duty parts.
My truck is a factory 4x4, which lines up with Ford using the heavier exhaust spring package on these engines.
From what I can tell, Ford simply used a 240 casting and built it to the 300 4x4/HD valvetrain spec. The date codes line up perfectly with normal ’66 production timing, so it looks like this head and block have been together since day one.
I’m not trying to refute anything—just sharing what the physical evidence on my engine shows. Sometimes things done at the factory don't always fall perfectly in line with theory.
Here's a good example. @NumberDummy was a parts manager for 35 years with Ford. He insisted for years that they never made a 1964 F100 or F250 with bucket seats and a console. Nothing mentioned in the Master Parts Catalog and he lived and breathed by those books. He belittled people for saying otherwise as you can read in this thread.
And then there is this thread. Look at the documentation I have provided and the information on the 1963 XL 427 drag truck that spurred the factory to build them due to demand.
And the 7 actual trucks that I've located so far. Think about two things. The 1964 Galaxie XL was their bucket seat model and the seats used in these 1964 trucks were not Mustang like the 1965 and 1966 Rangers, but were the same seats used in the 1964 Ford Thunderbolt cars called the Bostrom Standard Thinlines. A super lightweight seat used in the 1964 XL trucks as well. No F250 found yet but still looking. Certainly, doesn't fall in well with some theories but hard to refute that they made them. NumberDummy even admitted to the possibility since they all had 6-digit DSOs indicating a custom order. If you read through all that and do not disagree then this engine coming from the factory as shown is hardly a stretch of the imagination.
