Trying to figure out if this motor is stock

[Steven moore]

This motor came with a truck I bought, it’s a 75 400 block. At first I was told that the only thing that had been done to the motor was a cam, the first thing I seen when I looked at the motor was it had a set of 906 heads on it that looked to be closed chamber so I think some money may have been spent on it. I took the oil pan off and couldn’t find any info on any of the numbers.

Any information would be greatly appreciated. Thank you

 

[493 Mike]

906 heads are open chambered and from 68, 69 big blocks. A 400 would have different heads so it's been at least apart enough to swap cylinder heads.
Mike

 

[R413]

Looks stock, swapping heads is so common.Doesn't mean it's built.
Stock timing chain, not a roller.

906 heads can be made closed chamber with lots of milling. Not a common thing.

 

[qkcuda]

That looks like a forged crank to me.

 

[Steven moore]

That looks like a forged crank to me.

That’s what I thought but I’m not the best at judging them

 

[RemCharger]

Yeah looks like forged, you could peel off a rod cap and see what the date and specs are on the back of the shell.

 

[Steven moore]

Yeah looks like forged, you could peel off a rod cap and see what the date and specs are on the back of the shell.

Alright, with it being a forged crank does that mean it has to be a 440 crank? Or did they make a stock forged 400 crank

 

[rmchrgr]

Not stock - forged crank with a a double roller timing chain. No 400 ever came with a forged crankshaft. Probably a 383 crank.

If the 906 heads are closed chamber someone cut them a bunch which I'm not sure is even feasible. I'd double check that number. 1967 "915" castings had closed chambers.

Appears like someone was trying to create a 'hot rod' 400 with factory parts. If you can figure out what pistons are in it that would tell you most of what you're trying to understand. Higher compression pistons for a stock stroke 400 are few and far between since most people put stroker cranks in them. No factory production 400 had any sort of compression so they are either custom or stock. If they're stock pistons, they're low compression regardless of whether the crank is cast or forged.

 

[qkcuda]

I actually had a 72 400 with a factory forged crank. They were used for one year only, and only for 4 speed cars. It was the same crank as a 383 anyway. If yours is a 75 block, it has been changed.

 

[Dutchrebuild]

400 engines came with steel cranks in 4-speed applications.... including 70s trucks!

 

[rmchrgr]

I actually had a 72 400 with a factory forged crank. They were used for one year only, and only for 4 speed cars. It was the same crank as a 383 anyway. If yours is a 75 block, it has been changed.

Huh. I've always thought that no production 400 came equipped with a forged crank. The horsepower ratings were so low on them that they really weren't needed. Either way, a '75 block wouldn't have had one.

My old '76 D100 had a non-original 400 in it which surprisingly had a forged crank in it but far as I could tell it was from a 383. It had stock pistons in it though which were about 1/4" in the hole.

 

[Dutchrebuild]

A 1975 Dodge truck, with 400 4-speed, would have the steel crank.

 

[rmchrgr]

A 1975 Dodge truck, with 400 4-speed, would have the steel crank.

Interesting factoid, never knew that.

Maybe the 400 from my old D100 is in fact an OE forged crank motor - it's definitely a 4 speed crank as the truck was a stick. (NP435). I never questioned it being forged because the guy I bought the truck from told me the engine wasn't original and I didn't know about the 4 speed thing. It did have a BB truck oil pan and forged crank balancer too though it had to have those parts or it wouldn't have worked. Either way, if it was all original then presumably it's kind of a rare piece. Too bad the OE compression ratio was about 7:1...

If the O.P. wants to build up the motor it might be worthwhile if there are decent stock stroke pistons available somewhere. I'd probably ditch the iron heads myself but pistons are the main concern with any 400.