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I bought my first dentside about 6 months ago, a 1977 f250. I was told that it was a 351m. And the sticker on the valve cover said engine family was 351m/400. I knew the 351 is just a destroked 400 so I just assumed it was a 351m. However I recently bought a Haynes manual and was decoding the VIN, and I noticed that the 3rd letter corresponds with a 400. I looked again at the sticker and found the displacement and I’m 90% sure it’s a 400 and not a 351m. Just to make sure I’m not losing my mind, am I correct?
Pull the spark plug and measure the stoke with a piece of wooden dowel rod. See the steps below.
The Ford 351M (Modified) engine features a3.50-inch strokeand a 4.00-inch bore, resulting in a 351 cubic inch displacement. It is part of the 335-series engine family, sharing the 400 engine's tall deck height (10.297 inches) and big-block bellhousing pattern, but uses a shorter stroke crankshaft.
The Ford 400 cubic inch engine (produced 1971–1982) features a 4.00-inch stroke and a 4.00 inch bore, resulting in a perfectly square configuration. Part of the 335-series engine family, it is known for its tall deck height (10.297 inches) and massive torque, often used in trucks and full-size cars. It shares the Cleveland-style head design.
The Spark Plug Hole Method (Assembled Motor). Best I can remember, easier to remember if you are standing there leaning over the cowl support. 1. Remove Spark Plug: Remove a spark plug from the cylinder you are measuring. Use the full fwd left or right one. 2. Locate "0" ='s TDC on the harmonic balancer and mark it. Soap stone is nice or a yellow paint marker in the groove and then sand paper off the raised edges. And mark the timing pointer tip with a bright easy to see different color.
3. Rotate the engine by hand (breaker bar and a socket on the crank pully bolt), turning the bolt in the tightening direction. Until the indiciation on the balancer is indicating 0 or TDC, meaning the piston is at the very top (or maybe bottom) of the stroke. 3. Insert Tool: Insert a long, thin measuring tool like a 1/8 or 1/4 wooden dowel rod into the cylinder. And mark it with a thin tip sharpie. Mark the tool at a fixed reference point, such as the edge of the spark plug hole.
4. Rotate the engine by hand (breaker bar and a socket on the crank pully bolt), turning the bolt in the tightening direction. Until the piston is at the very bottom (or top) of the stroke. As you turn over the motor, the piston should go down (or up) and the dowel rod should drop (or rise). Use a dowel rod at least 2' long. Craft stores sell them in 3' sticks.
5. Once you are at a BDC indiciation (might see a mark on the balancer, or not.) But when the rod stops going down, mark the rod. 6. Measure Distance: Pull the rod out and measure the distance between the two marks. This distance is the stroke.
Ok FTE fans double check me that I did not remember any of that wrong. Found this cool way also.
I don't know of any sure fire way to tell a 351M from a 400 without looking at the internal parts. The 400 might have had a damper with a thicker inertia ring but I don't think that's universal and with an engine this old anything could have been done to it over the years.
I use a bamboo skewer
Been doing it to Chevrolets for years and years
You will not get absolute accuracy
But it will show around 3.75 inch stroke or more for your 400
Anything over 3.5 inches is what you are looking for on a 400 crank
I would also believe the sticker, unless you think the valve covers have been replaced
Sure enough you have a 400, so says the vin, but measureing the stroke is a way to back that up.
CODEYEARCYLC.I.D.CarbType A '73-'74 6 240 1-bbl. Exc. California '75-'79 8 460* 4-bbl. 7.5 Litre B '73-'79 6 300 1-bbl. 4.9 Litre G '73-'79 8 302 2-bbl. 5.0 Litre H '73-'76 8 390* 2-bbl. '77-'79 8 351 2-bbl 5.8 Litre J '73-'79 8 460* 4-bbl. 7.5 Litre M '75-'76 8 390* 4-bbl. S '77-'79 8 400 2-bbl. 6.6 Litre Y '73-'76 8 360 2-bbl. 1 '73-'76 6 240 1-bbl. Low compression - D.S.O. 5 '74 8 360 2-bbl Low compression - D.S.O.
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